** SAFETY NOTE : Read entire post for safety as this involves the data bus for your air bags. Dont do ANYTHING with the battery connected, wait 5 minutes before starting.***
Hello from Detroit Metro in Chrysler Country. Sorry this is long winded, but someone will be happy I posted this.
I have a 98 XJ Sport/Classic (Body of Sport, Harness of Classic) & I'm bringing her back to life. Before handling minor repairs like floor pans & welding/frame replacement, I wanted to focus on the vital things. Like 94 ZJ grey leather seats & OHC's :P
I knew I would probably not find an XJ OHC. So I pulled a 98 ZJ OHC last year thinking I'll make it work. This year I found a 94 Laredo in mint condition (got the seats *Huzzah*) with a mint condition dark gray console & 95 Limited ZJ with a tan OHC in mint condition. I wanted to use these in my XJ & was reading through the "Installing a ZJ/XJ overhead console:" thread on the NAXJA forums. Here : https://naxja.org/threads/installing...onsole.949746/
Where someone went into detail on how to use a 96-98 ZJ OHC in an XJ by simply buying a ZJ Body Control Module and piping all the temp & fuel signals into it, then running the BCM CCD bus output data wires to the 96-98 OHC for 100% function & tucking it away in the kick panel or OHC itself as an added appendage to make the OHC work. That 98 ZJ wasn't available for me to get the BCM from anymore & I certainly wasnt paying 100$ to ebay to make my 98 OHC work so I kept reading. He theorized 93-95 should work. So I wanted to finish what he started with what I had. First I grabbed every FSM PDF I could get via torrent without the black helicopters showing up.
(Still cant get a 98 XJ FSM so if anyone wants to help me out in return with one that'd be great)
IF your XJ has an OHC plug its behind the glove box off to the right, remove the rubber stops & swing the glovebox down, you'll see it connected to the dash subframe. The RKE plug is above the fuse panel and has blue wires on it. I THINK classics & sport/classics like mine have them. Sports/SE's do not.
If you have power windows, or cruise control or a classic feature like mine having EXT Idle or something like power seats, & it doesnt say sport or classic on the outside of your jeep, you have a sport/classic. Your VIN will tell all.
Now I bench tested my 93-95 ZJ OHC & could see average MPG 17.7(4 liter ZJ lol), compass, then everything else READ "OC" for open circuit. (Except time and stored trip odometer) But this told me they actually read the PCM for MPG data rather than just compass/temp/time/trip? Couldn't figure out why the CCD bus wires would be at the OHC computer plug without it doing MPG which is what I want.
This could be limited to OHC's from limited/laredo packaged 93-95 ZJ's as thats what my two 93-95 ZJ OHCs came from. With a 13 pin plug and 12 wires coming out of it. Maybe a regular 93-95 ZJ only has compass/temp/time which means the console computer just needs power. So it would be 3 wires if that even exists. Dunno.
The 93-95 ZJ OHC & the 97+ XJ OHC have one key difference but are very similar as they both read PCM data rather than BCM data like the 96-98 ZJ. Because neither have a BCM, which is why this works much easier/cheaper.
NOTE: The US/Metric button on this swap ZJ OHC to XJ wont work. You will be stuck in Freedom units.
**
In a 97+ XJ the analog fuel signal (voltage trim pot or dimmer switch adjusting resistance) goes to the PCM, turns it into a digital 2.5v-5v reference signal & sends the fuel level information on the CCD bus to your instrument cluster. Meaning the fuel gauge in your XJ cluster is digital 2.5v-5v, not analog 8-10v. So the XJ OHC can read fuel level information off the 2.5v-5v CCD bus to get the Distance to empty info.
**
In the 93-95 ZJ, the analog fuel signal goes right to the instrument cluster & that fuel gauge in the cluster is analog. So this 93-95 ZJ OHC has a Fuel Sensor + input pin & that loops into the standard body ground rather than needing its own "sensor ground" to the PCM like the ambient temperature sensor which must return to the logic circuit it came from. The ambient temperature sensor is a passive/unpowered thermistor or glass bulb with a tungsten wire in it. The temperature fluctuations lower & raise the resistance across that tungsten wire. The OHC computer compares that resistance against a pre determined "resistance table" within the firmware and displays the temperature. Which is why if you use anything other than 18 gauge wire for the temp sensor it will probably be inaccurate as that is what the firmware table was tested with based off the harness. But by so little it wont matter unless you use something way too big/small.
**
As time went on Chrysler wanted the CCD bus to do more. But didn't want the PCM to do too much which is why they birthed the BCM. It is also why a 97/98 XJ OHC glove box plug has its own positive & negative wire for the ambient temperature sensors passive signal. But the 99-01 has the ambient temperature sensor run to the PCM spitting that info out also on the CCD bus +/-. (Yes multiple signals can digitally be carried across one +/- wire set via frequency which saved wires, money & weight) The 99-01 XJ OHC computer might have ambient temperature pins but not need them. So the 99-01 harness for the XJ OHC on the passenger kick panel end only has CCD bus +/- wires on one 8 pin plug, fused power with black/yellow grounds on another 3 pin plug & RKE on a 3rd 6 pin plug. Whereas the 97-98 OHC harness has fused power, black/yellow grounds on one 3 pin plug, ambient temp +/-, CCD +/-, run power & ground on a 2nd 8 pin plug & RKE on the third 6 pin plug above the passenger kick panel junction/fuse box. So on a 99-01 you should be able to just power the OHC & run/plugin the CCD bus wires and have everything maybe not temperature. The 8 pin plug is all thats different, 97/98 may be the same, and 99-01 should be the same. With the only difference being the 8 pin plug down at the kick panel, and the computer plug obviously being different.
So this swap will work in ANY XJ with OBD2 96+. As OBD2 has CCD +/- present for you to tap off then everything else can be wired independently.
***
But for running any CCD bus length anywhere you must use twisted pair wire from an ethernet cable. Otherwise data frequencies will be interfered with by radio signals, cell signals, or even close by powered wires & electronics putting off EMF. Making it not work. But one 6ft ethernet cable has 4 twisted pairs inside (Not flat ethernet cables only round ones). So that will give you 24ft to run CCD bus data where ever you'd like. Feel free to re-use the layers of shielding in the ethernet cable if you can cut it open clean with a box cutter. Then electrical tape it back up thats ideal for no interference if you run VHF/UHF/CB under the dash.
EDIT : After examining the harness the twisted pair is a big 18 AWG set of wires vs an ethernets 26-28 AWG its doubtful ethernet cable itself will work. I've twisted up my own twisted pairs with a bench vice & dewalt drill *Shrugs*. Also cutting some out from a JY can work but its time consuming.
***
** THIS WAS THEORETICAL & EDITED WILL NOT WORK WITHOUT ZJ SENDING UNIT AFTER FURTHER TESTING READ FINAL POST **
96 XJ
will just need a custom wiring job as it isn't going to have plugs in the passenger kick panel. No worries, the CCD +/- run to the OBD2 port so with the battery disconnected, after waiting 5 minutes, you can chop ONE WIRE AT A TIME!!!!(Doing both at once can fry a PCM!!!) of the CCD bus cables there and tap into it and run your ethernet twisted pair wires up the drivers side A pillar to the OHC for trip odometer. To get Active/Average MPG or DTE (Distance to empty, not Detroits terrible energy monopoly) in a 96 you DO need a ZJ sending unit added to your XJ unit, or swap your fuel/voltage gauges for ZJ gauges (20$ on ebay) and only use the ZJ sending unit on your fuel pump. Super easy to drop the tank without a skid plate. But it will be calculating based off the ZJ 23 gallon tank (21.8 gallons it multiplys to leave 1.2 gallon reserve) so it will be inaccurate until you do a ZJ gas tank swap anyway. Then obviously running your own 18 gauge wires from the temp sensor (you might need to buy) under the battery to the OHC if you wanted temperature.
*** 97/98 XJ
disconnect the battery, Same thing applies about the ZJ gas tank and sending unit.
The ambient temp sensor wires are already in the classic or sport/classic XJ off to the right behind the glove box in the OHC plug & can be tapped off there with 18 gauge if you dont have an OHC harness along with the CCD bus +/-. (Use twisted pair!)
(In a sport you probably have to run your own ambient temp wires from under the battery where you might need to get a sensor like a 96, all the way to the OHC pig tail.)
Then any XJ has some over head dome light so you will have the 3 pin connector for pink/yellow/black & utilize that harness and chop the plug for the light (ONE WIRE AT A TIME) & splice it into your 93-95 ZJ OHC pigtail. Once you have the fuel sensor, temp sensor, CCD bus, & power wires up there it works 100%.
Just do this with the battery disconnected. Both positive and negative terminals for good measure.
99-01 XJ
You should be able to power the OHC and give it CCD bus +/- and get everything working but average/active MPG , maybe temp. Just like the 96-98 ZJ. You should have the sensor under the battery/bumper location or at least the plug. You should have the 8 pin plug behind the glove box and its just the CCD bus +/-, Use twisted pair wire to jump that to the OHC through the A pillar as described previously. Then use the dome light harness for power at the OHC and plug it into its spot in the passenger kick panel junction/fuse box. Trip odometer, time and compass. It probably wont read temp off the CCD bus as it has its own pins. Dont chop the sensor from the PCM. the 00-01 PCM may use ambient air temp to calculate fuel trims better based on weather. Just use a second sensor, they are generic chrysler ambient sensors and cheap. Then run it to the 93-95 ZJ OHC pigtails +/- ambient temp pins to get temp function. You will need to do the same as the 97/98 XJs to get DTE with the ZJ sending unit added, or only the ZJ sending unit with a ZJ gas/voltage gauge swapped into your XJ cluster somehow.
I'm leaving the PCM pinouts incase people need them for other purposes. But you shouldnt need to tap off your pcm for any reason. Grounding the brown/yellow sensor ground circuits to the body instead of the PCM makes most gauges stop working temporarily.
** DIY Harness **
If you have access to a junk yard, that main 8 pin CCD/ambient temp plug for the OHC behind the glove box is used many times through out the XJ if you dont wanna keep splicing up wires but want an actual plug. Its used for power windows & lock switches. All but 1 pin is used so making a harness out of it if you DO have the plug in your glove box is easy for 97-01 XJs.
The OHC CCD bus/Ambient temp plug on the JEEP side connected to the dash subframe is a Female plug with male pins inside if that makes sense. Same for some of the switch panels for the power locks & windows harness maybe in the switch itself, or used before going into the door for easy door removal. Dunno. You need a 8 pin male plug with female pin connectors 3 pins on the top row 5 on bottom. Get yourself a nice long pigtail for making a harness.
(00-01 are easier as this 8 pin plug only uses 2 pins for the CCD bus to do everything except power and RKE which are separate plugs & if you're not doing RKE its simply 2 plugs with 5 wires)
I found my 8 pin plug in a junkyard XJ already chopped & said "Thats the same as the CCD bus connector. I can use that and sell the harness lol" Yes I said lol.
Point being I dont know where it comes from exactly but has these wires coming out of the back.
** Looking from the backside of the connector **
Not Used Pin | Clip spacing | Brown/ Yellow Stripe | Orange/ Black Stripe Red/ Orange Stripe | Yellow | Green/ Yellow Stripe | Red/ Black Stripe | Pink
You can make this connector pigtail into a 97-01 XJ OHC harness easily saving 150$. As the ZJ OHC you pull or buy should come with its own pigtail. Then just connect the wires correctly. You can cut 18g harness wire from a junkyard. Not all harness wire is 18, just most.
(Bonus points for those who relocate their stock dome light to the rear using only pink/yellow for a good hatch light. I plan to do this and add the under the footwell lights to the bottom interior corners of my tail gate so when it lifts it lights the area where you stand under the gate as well)
I'd use LED lights to reduce the amps on the 18g wire. As the stock bulbs pull .9 amps @ 12v or 10.8 watts each. LEDs will use 1w each. You can even leave a map light on over night & it wont kill your battery. Depending on length, 18g is good for 7-10 amps.
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SAFETY NOTE : The CCD bus also handles SRS airbags so do NOT plug/unplug/chop/tap into the CCD bus while the battery is connected or it can result in airbag deployment. Also NEVER chop positive and negative wires at the same time they will short and damage or destroy sensors and computer modules.
************************************************** *********************************
I will include the needed FSM pinouts you need to make this happen and a few photos as proof this works and you're not wasting your time.
My motivation to do this is to increase the value of plentiful 93-95 ZJ OHC's because they can go into XJs so I can sell mine for more :P
I hope this helped someone & if there is anything I missed feel free to ask away. I modify open sourced firmware for machines here in Detroit Metro & have my own machine building company. (kind of)
If you're wondering why I gave this much effort to make a simple console work. Cause I wanted it, I wanted it cheap, & I wasn't payin for an XJ console.
If I found one in a yard, I'd sell it cause there is currently 48$ in my bank lol. I get some extra money on eBay with WJ/XJ/ZJ parts t(^_^t)
My sellers page is the same as here, DruCanDoIt. Like Rob Schneider in 'The Waterboy' being the racially insensitive Latin man :P
I will post my results in my 98 XJ after it stops raining to show this works.
Keep Jeepin!
97 XJ OHC pinout but use the colors for your glove box plug if you have it. Be warned there are two Wt/Bk wires and two bk/lg wires. One wt/bk is the CCD 2.5v-5v data negative, and the other is a 12v positive. The one in the twisted pair is the CCD negative. The regular one is the 12v hot. BK/LB is your ambient temp sensor ground on the 98 with BK/LG just being a ground.
New photos 3/6/25 After some research the plug on the right in the above photo is NOT from a 99-01 XJ. Its from a 98 classic which did everything on CCD bus.
Well I have good news! It DIDNT work... AT FIRST!
No PCM data updated when plugging it in & Average MPG still said 12. (I bench tested my other 93-95 OHC and it had 12 MPG).
I thought it was just my Jeep as I have a parasitic draw atm I'm chasing & leave the negative battery terminal disconnected. So maybe there isnt any average MPG data on the PCM to give. Maybe if I drive it around active MPG will start to math? NOPE I drove her. No Mathing.
HOWEVER.
I did prove it reads PCM data, over what is a Jeep standardized CCD Bus. <--- *important nerd stuff @ bottom*
How? Through something most people knew already. The Odometer works....... Well uhh..... Hey guy..... That means its reading odometer data from the PCM over the CCD bus! It certainly isn't guessing or getting that info from the power wires. All I had plugged in was Pink/Black for +/- 12v. Then the CCD Bus +/- with the plug I showed previously.
It cant use the vehicle speed sensor to determine odometer data without reading what the PCM is saying.
NOW most people give up here and say, oh well.
But I was wrong, about how the 93-95 ZJ OHC calculates.
It doesnt take stored PCM data like odometer, average MPG from the PCM as I imagined.
I thought this because if you OBD2 most any 2000+ car it will have that live MPG data on whatever Torque app your using. I thought it would just pull whatever was on the PCM.
But this isn't that fancy. The OHC stores that. More than likely the 93-95 ZJ PCM doesnt store Avg MPG. Just fuel trims etc...
The OHC NEEDS the fuel level sensor to do any MPG anything. How would it know when you've used a gallon otherwise? Especially if the fuel sensor goes only to the 93-95 ZJ cluster and not the PCM? The subject nobody touches on is the fuel sensor wire at the PCM on the XJ. Thats where I think I can do this. I'll unplug my PCM & chop the wire to splice into & post results. As I type this I realize why I'm the one doing this to my 1000$ Fred Flintstone XJ :P
I will reply if it works and I get it going. If not I'll come back anyway to tell you what a dummy I am. Either way decent content. Side Note : I did drive about half a mile & my 98 XJ has 29s on it. So its about accurate.
**Why thats important theoretically**
This is great news. It means the 93-95 ZJ & 96+ XJ PCM's 2.5-5v reference signal uses a universal table for resistance/voltage/frequency from the firmware of the PCM. Firmware devs didn't wanna make everything vehicle specific as I originally thought. Because its a ton of extra work with no real benefit other than having vehicle specific sensors/electronics that allowed Chrysler to make more off parts. They weren't doing that yet ;P
So the Odometer data identifiers are the same from the PCM of the 96+ XJ & 93-95 ZJ. The PCM talks in a way any OHC (except 96-98 ZJ) can understand. Its limited by the OHC and what it requires to do its job. This makes the 93-95 ZJ OHC work in any 96+ XJ. Because as I said, it needs 2 things to calculate MPG. Odometer data & fuel level. Engine speed helps improve accuracy with idling but those 2 are all it technically needs.
So the 96-98 ZJ BCM CCD Bus output is probably a different table of resistance/voltage/frequency to avoid PCM interference. Which is why if you hook the XJ PCM CCD Bus to 96-98 ZJ OHC CCD Bus pins they dont work. Different firmware tables between the PCM & BCM which 96-98 OHC needs.
If this is true, this COULD mean the 96-98 ZJ PCM CCD bus output may be the same as the 93-95 ZJ & 96+ XJ. Hunt down those wires & you should be able to make a 93-95 ZJ OHC work in a 96-98 ZJ. I cannot test this without a 96-98 ZJ. I'll pass that one to the next Jeep nerd. I'd start at the OBD2 port and jump THOSE CCD +/- wires to your 93-95 ZJ console (XJ console should work too, but if your reading this you should sell it and get the cheaper one). As you want the CCD bus wires from the PCM not BCM. You'd also need to run the fuel sensor wire. But it also means 96-98 OHC's only understand a 96-98 BCM. Not any PCM data.
I did pull these 2 OHC's from 93-95 ZJ's with 4.0 liters. It shouldnt matter if you pull from a V8 because you get 0 MPG at idle & it just uses that 0 MPG for an elapsed time to drag down your AVG calculated by odometer data & fuel level. Dont know if engine speed/type is used. Would have to see firmware.
**
Ok so it KINDA works. Ran into some hiccups and spent my whole saturday on this. First thing is the OHC ground has to be on the same ground circuit as the fuel level sensor. On the 93-95 ZJ thats a body ground meant for electronics. On a 96+ XJ thats the PCM "Sensor ground" Brown wire with yellow tracer.
Second thing is I jumped to the PCM fuel sensor wire and it relieved too much resistance to get a reading. Anything below 30 ohms (which is empty tank on the ZJ OHC) reads as OC (Open Circuit) even if there isn't a open circuit on the 93-95 ZJ OHC. The resistance ranges are as such
17-130 ohms Empty-Full 93-95 ZJ
75-300 ohms Empty-Full 96+ XJ
Iz Prublem.
The "sensor ground" is in the PCM of the XJ which is the issue. OHC has to be on that ground to compare a clean non resistive line to the sensor resistance. But it having many sensors and possibly termination resistors in the PCM is creating a relief in resistance. Ive tried disconnecting the fuel sensor from the PCM and just running to the OHC and the OHC using the PCM ground. This makes the gas needle go to full then all cluster instruments stop working (except oil temp)
At first I was nervous about grounding the 12v OHC to an unpowered sensor ground on the PCM. But PCs ground numerous voltage rails to the same common ground. So I did it. & at first it worked but showed 00 Distance To Empty instead of OC. Also didn't help I only had a 1/4 tank of gas. Despite what I've read, The DTE function shows more miles with higher resistance and less miles with lower resistance. So the sending unit on the 93-95 ZJ must read high resistance when full and low resistance when low. I've got the range nailed down to 30ohms = empty and 150-160ohm = full for the 93-95 ZJ OHC. According to a YT video its 75-300 ohms for the XJ, 300 being full.
This took some nerding but I got my other OHC on the bench, bench powered it and MADE the DTE function to nail down how it works. I used my Car Senser 10000. High tech state of the art car testing equipment that took me many years to craft & develop. Some call it a ****. Not important.
This highly complicated Car Senser 10000 should not be shared with foreign nations & I expect Jeep people to keep this on the hush :P
(Also can be used to see if a new sensor would make a problem go away without buying that daggum sensor 1KiliOhm-1000Ohms is best)
I dialed the Car Senser 10000 to about 45 ohms. Tied one end of the car senser 10000 to the violet/white fuel level sensor pin on the 93-95 ZJ OHC. The other end went to the same ground the bench supply was grounding the power for the OHC. This made it show miles.
this was at 44-45 ohms from the car senser 10000
This was with the car senser 10000 dialed to 148-150 ohms. 386 mi divided by the 17.7 MPG of this particular OHC = 21.8 gallons. Almost 23 gallons for full. Id imagine 160 ohms is full. I imagine they are different because of the difference in capacity. If 300 = full XJ @ 20 gallons, how would you get 23? Halfing it somewhere.
Side Note : I previously said you'd be stuck in freedom units, thats wrong. You'd be stuck in miles on your cluster. The OHC switches to KM fine. It just has an output thats connected to ground pins on the cluster to get it and the VIC to switch to KM too. Other side of KM switch is grounded.
Now you can see the problem. The ZJ sensor has its grounded resistance loop K102/G44. Then an EXTRA body ground. The BK/OR ground eventually ties into the BK/LG ground your OHC & instrument cluster ground to on the 93-95 ZJ.
Here is where the BK/OR and BK/LG tie together.
You can see on the XJ it ONLY has its resistive loop going to the PCM.
Not outta the woods yet boys. Few ways to handle this.
I can A) Keep my XJ pump, drop the tank, Isolate the sensor ground to the body ground, then run 18/19 awg to half the resistance of the 30-300 ohm XJ sensor to 15-150 ohms. Which would mean when I'm down to the last few gallons it would read "00" or "OC". This will make it so the PCM gets zero fuel data and my gas needle wont work, but the DTE and MPG will.
B) Get a ZJ 93-95 Pump and sending unit for 60$, drop'r in, Use the resistive loop for the PCM, and the body ground would let me tap off the fuel sense at the PCM like I am now, except you ground the OHC to the body ground rather than the PCM. Dont know if ZJ pump sending unit has high enough resistance for the XJ PCM to spit the data out correctly on the CCD bus. A full tank may only register as half, & a half tank read empty.
Or my favorite option
C) Get a ZJ sending unit, screw it onto an XJ pump.... The pump body is much smaller than the hole in the tank unlike the ZJ pump. I'm betting I can make 2 sending units stacked on each other fit. XJ sending unit will go to the PCM normally, and the ZJ sending unit can go to the OHC normally PK/BK to ground, VT/GY to OHC, OHC grounds to same place as PK/BK. If you have an old jeep the fuel pump & filter need replacing anyhow. Also if you're doing this you want the ZJ tank so its accurate to 23 gallons not 20 gallons. I can get a ZJ tank with a hole in it for 12$. So I might pull it with the sending unit to try this on the cheap. But this means the camrys engine block needs to get welded and back running before I mess with this further. I need to have a running car lol. My Bafang eBike isnt that bad now that its a warm 40 degrees out. Swimmin weather.
ZJ Pump
XJ Pump
SIDENOTE : This is my 18 wheeler. I plan on swapping from U-joints to CV joints and towing or driving cross country with this once a settlement comes in. The extra 3 gallons and this OHC would be ideal. It already has the Tow Package, Upcountry package & Tracloc. So its D35c has 3.73s for towing, 10 inch finned drums for towing, & the HP D30 on the 98 XJ has 3.73s too from the factree. Pretty rare. I plan on moving to Alaska from Detroit.
Dont know what I'm gonna do. Open to suggestions. What do you all think? A, B or C?
Update : I put my meter across the Brown wire with the yellow tracer and the blue wire with the green tracer. The resistive ground loop for the fuel sensor on the XJ. I got 170 ohms with a 1/4ish tank. gauge/sending unit always sticks down low. So when I grounded it to the body ground and it said 00 for a bit then said OC its because 170 is out of its resistance range. I opened up infinite resistance by grounding the sensor PCM ground to the body creating an open loop. Which is why my tach, speedometer and gas gauge stopped working. First the gas gauge slowly went to full, then they all went off when. Did the same thing when grounded to the Br/yl PCM sensor ground loop.
Update : 3/10 I went to the Junkyard and made a quick realization. The ZJ sending unit sensor DOES put out 75-300 ohms, split to either side of the sensor! Duh. Also all gauges make SOME resistance. So the side of the sensor that goes to the cluster then back to the fuel sensor ground experiences some resistance. Same for the side going to the OHC. So all I'd have to do is measure a jeep gas gauges resistance between the fuel sensor input wire on the cluster and the electronics ground pin Black/Green tracer. Then get a resistor that matches that resistance value. Then do Option C, with one side of the sensor going to the 93-95 OHC, the orange/black body ground going to a chosen ground in the rear of the truck, ground the OHC at that same ground, and run the pink/black (cluster side) of the sensor to that ground as well, placing the resistor in between the pk/bk side of the sensor and said body ground. I know its confusing, so I made this paint image to illustrate. You will need the ZJ sending unit stacked on the XJ one. Once I have it figured out I'll reply again.
After some research I am going to pull the ZJ tank tomorrow for 12$ with the hole and plasti-weld it as these tanks are High Density Poly Ethylene and welds last 20 years. So a 12$ ZJ tank sounds good. Ill take the ZJ sending unit with it as I dont wanna buy a new one for this. I'll just clean it up.
Getting the weld sticks & dropping this tank will take a while so this will be my last update until April when I have this all sorted out. Gotta move this summer (Not Alaska). I hope something here helped someone. I will prove this is possible. I promise :P
Update 3/15 : The answer is 130 ohms! I measured the gas gauge on a 95 ZJ (Also bought the cluster for resale) and it read 130 ohms on the nose. Which explains why the 93-95 ZJ OHC also works up to 130 ohms. and 1-2 ohms being empty. So I just need to buy a set of ten 1% tolerance 1 watt 130 ohm resistors for 5$ on ebay. Have it emulate the gas gauge, then just get the sending unit and tank. Went to the JY yesterday and the 94 Laredo was gone. Reason that stinks is the 95 Limited ZJ thats there they sat the whole truck on welded rims stuck under the tank. So its bowed in. Probably fine but I wanted the other one because they used 4 sets of rims to mount it and the tank was fine. So Now that I know I need a 130 ohm resistor, I'll just need the ZJ sending unit and tank. Tomorrow I'm going to the JY and using the A frame lift to lift that jeep up and see if that tanks usable. If so I'll get it, order the resistors/HDPE weld sticks from Maven & will post a 4th and final result post when its done by April-ish.
Update 3/19 : Got the ZJ tank, & 130 ohm resistors on the way. Havent ordered weld rods yet. I moved the ZJ off the welded rims the tank was sitting on and let it sit a day. Today went there and it retained its original shape popping back out. The knuckle draggers working at the yard drilled 2 holes though >_<. Sending unit in tact! I left the pump. (Should have measured the height it sat at probably will tomorrow) But that gas tank is SUPER easy to drop with out the skid plate. Just need a 9/16 box end wrench & the shortest flat head you have for the hose clamps.. Couple turns and the rest came out by hand.
From the sending unit, Blue wire turns into the pink wire going to the instrument cluster and gets the resistor. Yellow wire turns into the violet wire with the white stripe that goes to the OHC. Black turns into the BK/OR body ground. Resistance can ONLY be measured between a yellow or blue wire and the ground. Testing resistance from blue to yellow just gives 130 ohms. BUT going from yellow to black or blue to black gives you the active resistance of what the sending unit says.
This is because the black wire grounds to the float arm side of the wiper, both the yellow and blue are on the panel side of the wiper with all the contact pads. So an XJ sending unit cannot work with the OHC due to the lower resistance of the ZJ sending unit due to its dual outputs doing 130 ohms. The XJ unit only has 1 wire coming off the same wiper panel with contacts (probably) and creates more resistance. So a ZJ sending unit cannot send to XJ PCM because its too low resistance, & the ground goes to the body not the PCM creating another ground wire for the pcm off the sending unit just opens the PCM ground up to infinite resistance making the gauges not work. The XJ sending unit cant be used with this OHC because even if you added another ground wire too, you'd open up infinite resistance to the sensor ground and all gauges will shut off. So you need 2 sending units to make this happen. I dont know if a pre 96 XJ cluster reads 130 as they didn't have OHC's that did MPG I dont think. If it did read 130 ohms you could probably swap a pre 96 cluster and just use the ZJ sending unit. I can check that tomorrow too. I'm guessing it will read 300. Another possibility is just taking the fuel & voltage gauges out of a 93-95 ZJ cluster. They do come out. Then see if you can replace/isolate the 96+ digital gas/voltage gauges. I'd sacrifice mine but its currently listed for 90$ lol. I'd rather go the 2 sending units route now, & maybe add those gauges later. But id imagine thats possible.
The ZJ sending unit ground makes the two sensing wires independent of each other despite what I previously thought. I probably dont need the 130 ohm resistors and should have waited to order them lol. We will see as later today I'm hooking up the sending unit to the OHC to see if the resistor is needed to get it to read correctly. Then I will bring the entire setup into the XJ to make sure the CCD bus Odometer data I proved it can read is used in concert with this fuel data to give active MPG while driving. Thats a really important feature to me for gauging usage.
** Please forgive my learning curve in this thread, I am dumb. Follow me at your own risk. That should be a T-shirt **
Update 3/22 : Ok after some testing Active/Average MPG do not work even with the ZJ sending unit reading correctly while driving. So its not as simple as odometer data and fuel level. So I was wrong!What a dummy I am lol!
Maybe I'm using the OHC wrong and need to reset it. Maybe its cause it needs my battery to stay connected. As I fix my XJs other problems I may try leaving a computer plugged in long term to see if anything changes. If so I'll update sooner than sept. Thats just my estimation of how long it will take to find another 93-95 ZJ. What I proved :
If you
1) Wire ambient temp sensor
2) Wire the XJ CCD bus to 93-95 ZJ OHC computer (Use 18awg twisted pair) You get : Temp/Compass
Elapsed Time
Trip Odometer
50% function from a ZJ OHC in a 97-01 XJ. I think it has a clock feature too wasn't paying attention.
Definitely not what I wanted. I wanted the accurate MPG feature with the 23 gallon tank. If you think about it. Using the XJ OHC computer and ZJ 23 gallon tank wont work accurately. The OHC computer will always think I have less gas than I do, skewing MPG by +15%. When the float drops in the XJ tank lets say 1/2 inch, thats a gallon. 1/2 inch drop in the 23 gallon tank will be some 1.150 gallon nonsense. So when the XJ computer thinks I went 17 miles off 1 gallon. I really went 17 miles off 1.15 gallons and really got 14.5 MPG. I can figure 15% fairly quickly like anyone in my head. No big deal. I just dont wanna.
BUT I'm not done yet its just going to take longer and cost more to continue testing. I thought my Tan 93-95 ZJ OHC wasnt working cause it came from a V8 & I was reading that this OHC MPG calculation isn't so simple. It does use the TPS, IAC, MAP, VSS etc to gauge usage as well. (According to some guy on a forum) So then I tried my ZJ OHC from a 94 Laredo with the 4 liter. No change. My only and last chance is a 93-95 ZJ 4.0 liter PCM/TCM. Might not need the TCM, but Ill get it for testing. The VSS is supposed to be similar to an ABS sensor, just a magnetic pulse from a tone ring I think. But none of that should matter because I proved the odometer data works from the XJ PCM. So thats universal. So the PCM CCD bus VSS signal output remained the same in the firmware from the 93-95 ZJ and 97-01 XJ. Which is the ONLY reason I'm dragging this out & spending more of my money on it. I'm just assuming now they re-arranged the CCD bus output frequency/voltage/resistance tables between 93-95 ZJ and 97-01 XJ to add more things to it. But left odometer data or PCM VSS CCD bus output alone for some reason. Since the 93-95 ZJ with the 4 liter & select trac maybe command trac too? has the same VSS, IAC, MAP, & TPS as the 4 liter XJ. Sensors should input to the 93-95 ZJ PCM correctly as long as I use thinner than 18awg wire to maintain resistance readings. THEN output those ZJ PCM CCD bus wires to the 93-95 ZJ OHC computer. It SHOULD work. As the OHC read the XJ PCM odometer data. Since both vehicles had the 242 Tcase. I dont know if a ZJ with Select trac's VSS works in an XJ with select trac. It might. So I will see how many ZJ computers I need to duct tape into my XJ to let me have the 23 gallon tank, with accurate MPG data. If not I'll repair the 23 gallon tank, grab a JY pump and use it as a generic garage gas tank with casters for putting on the trailer to fill equipment or my XJ.
I copped out & saw someone on eBay selling just the 97-01 XJ OHC computer for 50$ and bought one. Looks like it comes from the 00-01 with just the CCD bus and a bunch of grounds. He wanted more for the pigtail so I hedged on it. This isn't good. Cause now I have to leave my 20 gal XJ tank for the MPG to be accurate. As I also proved the ZJ OHC computer multiplies average MPG by 21.8 when reading a full signal. I did order the Maven made in USA HDPE weld rods (4.5mm 7/32 triangle rod). I also found 2 more holes in the 23 gal ZJ tank.... FOUR HOLES! Now it needs mesh reinforcing for sure. Maybe even on both sides.
So for now I'll use the XJ computer I ordered in the 93-95 ZJ dark grey over head console. When another 93-95 ZJ with the 4 liter over heats and ends up in the JY this summer, I'll grab a 93-95 4.0 liter ZJ PCM/TCM and continue this test.
End solution for 23 gal theoretically is to swap out my XJ voltage/gas gauge to the 93-95 ZJ voltage/gas gauge, use the ZJ 23 gallon sending unit. Then pipe over the TPS, IAC, MAP, VSS to the 93-95 ZJ 4.0l PCM, then send the 93-95 4.0l ZJ PCM CCD bus output to the 93-95 ZJ OHC computer. Then Active/Average MPG SHOULD work. If not I got parts to sell on fleabay.
Update 4/1 : Ok I lied about not updating til sept. I wont reply again til its done. But I must have been mistaken when thinking the laredo or limited ZJ had a 4 liter. They come with v8s. So I went to a new JY & they had another 96 ohc computer, 96 ZJ BCM & a OHC computer in a 94 that was a base model with the 4 liter. I just sold a 93-95 computer cause I had the two from the laredo/limited. Both the 93-95 ZJ laredo/limited OHC computer had a black plastic body, I get this third 93-95 ZJ base model computer and it has a gray plastic body. I hope that means they are different as they talk to the PCM and the PCM's would be different between models. Maybe this one will work? We will see. I cant seem to get a PCM for any Jeep out here its the first thing people take. So I will test this last gray 93-95 ZJ model, then test the 96-98 and post it all here. I now have 5 computers after just selling one. I think I got a problem. (x2) 93-95 ZJ 1 limited/1 base (x2) 96-98 ZJ (x1) XJ computer.
Update 4/11 : 4 liter ZJ OHC computer didnt work either. But the Jeep Gods like me. I found an XJ OHC with no fabric and a cracked off fabric covered lip from someone pulling it down. It had the harness & computer for 20$ at the JY. I cut that same cracked off piece off a ratty OHC in the yard to plastiweld on. ALSO Got a 95 ZJ 4.0 PCM! (And another VIC to repair) @ this point I've had 4 93-95 ZJ ohc computers. I have 2 96-98 ZJ ohc computers and 2 XJ ohc computers. Getting an EVIC for dedicated TPMS screen in the XJ, apparently that works too. Also I found a dealership service manual procedure for "Customer programming method" of any 99-02 Jeep FOB to any RKE from that year without DRB. I know thats random & will cover it in another writeup with less "learning" posted. Doing really well at the junkyards idk if people here are following me and buying my stuff, but thank you to anyone who has. Really helps me have more time for this stuff.
