First off lets get the Jeeps details out there
4.0
AW4
4x4 231 J

Last month I was on a road trip and the jeep all of a sudden wouldn't get out of first leaving the campsite. A day before that happened I was wheeling in Sand Hollow and the transfer case started popping like crazy assuming my chain is skipping. The next day we drive 700 miles and then the transmission problem came up. The speedo meter is jumping all over the place and saying i'm going 150 mph! I disconnected the TCU and that allowed us to get home safe n sound. The speedo meter is still out of this world wrong, yet it is installed correctly and the correct tooth count for my tire size. Now I am back and need to solve this issue (s) haha
Things I have checked I have 5 volts going to the speedo gear sensor and cleaned the three prong pigtail. I have 12 volts going to the TCU. My next plan of attack is to replace the shift solenoid #2 and replace the speedo sensor to see if that clears things up. If those don't clear things up I'm guess the TCU has gone bye bye. Awe and I checked all fuses to rule out the simple fix. Also the NSS is working correctly and have been cleaned out, TPS is also new and working as should
I'm checking in to see if anyone has had these two problems together and how they fixed it and to make sure my problem solving is sound. My guess is that the jeep isn't shifting right due to the speedo meter acting wacky and confusing the TCU? I'm also guessing the speedo meter might not be reading right cause there is something wrong in the transfer case since 4wd makes this large popping noise grubbles when driving. I'm not sure if that would cause the speedo sensor to fail or not. Could be two different problems.

Thanks for the love ya'll are always so helpful !

 

have a read of the FSM, but I thought the VSS is on the TX case, i could be wrong, but I believe it informs the PCM of vehicle speed

 

The vehicle speed sensor (VSS) is in the transfer case and is used for the speedometer. The TCU has it's own sensor in the rear of the transmission, left side roughly under the bracket for the xfer case linkage. The two problems aren't directly related.

It's very rare that the TCU is bad. Look at the wiring for damage, including under and clamp where it might have worn through. Measure the solenoid resistance at the connector under the hood.

 

CRUISER'S MOSTLY RENIX TIPS

TRANSMISSION CONNECTOR REFRESHING

OCTOBER 30, 2015 SALAD 22 COMMENTS



Over near the transmission dipstick tube are 2 rather large connectors. One is black and goes to the NSS and the gray connector goes to the transmission itself
. These 2 connectors carry all the info between TPS, TCU, NSS, speed sensor, and transmission solenoids.

Unplug each one, visually inspect for corrosion or bent pins, spray them out with electrical contact cleaner and plug them back in.

Additionally, if your Jeep is an ’87 to ’90 Renix, it’s always a good idea to reach up under the glovebox area and unplug the connector to the TCU and spray it out along with the receptacle of the TCU. While you’re there, find the fuse right in that area for the TCU. Remove it and spray out it’s receptacle and clean any corrosion from the fuse.

 

Updated, I’ve cleaned out the connections to no avail in solving anything. While I was there I tested the resistance for solenoids all where in spec hovering around 15.1-15.3 but on the high side. After which on todays drive I finally got a check engine light! Codes were 12 which has to do with memory to computer being cleared from 50-100 miles not sure how that relates to anything. The other was 15 which is on the money. Not signal form speed sensor during normal driving conditions. Since my speedo is wacky that makes sense. I’ve tested the volts at the plug and all my grounds are refreshed so I’m guessing the sensor has gone bad. Plan is to replace the sensor and then drop the transmission pan to check resistance at the shift solenoids and also see if they click when energized since they are on the high side of tolerances.

 

Those resistance numbers look pretty typical. Speedo reading wildly high smells like damaged wiring to me.

 

Best way to fix that would be to pull the harness and look for damage ? Is there a good guide on how to wire up pigtails ?

 

I'd look the harness very carefully, starting back at the speedo. Remove any clamps and see if the wire has been worn through.

 

Update: I pulled the harness for the VSS someone had been in there before which makes sense because it’s a 2x4 to 4x4 conversion by the PO. Added new wires soldered, shrink wrapped and sealed with a new sleeve and low and behold my VSS is back to normal ! Speedometer no longer wacky. Part one is solved, now however the ol girl will still not shift out of first with the TCU connected..
my only lead is the solenoids..

 

You could also test the output speed sensor. On a 1995, it's a magnetic switch that closes once per revolution of the transmission output shaft, pulling 12-volts to ground once per revolution (at least I think it's still 12-volts on a 95). You could measure resistance across the sensor while turning the output by hand (trans in neutral and tire lifted). Or back probe the output speed sensor while you put the trans in D and the xfer case in neutral and it should look like a/c voltage.

Could still be a bad solenoid. Definitely more likely than the speed sensor itself or the TCU

 

Another update: I pulled the transmission pan and tested the solenoids. Resistance was good but.. when I tested mechanic function on the battery to see if they click I had one completely dead and another was spotty. I replaced with known working solenoids and I still can’t shift out of first !!
At lest I know I’m solving one problem at a time. Tomorrow I’m going to test the internal trans speed sensor pull that harness and check the wiring. Probably going to pull the TCU harness as well and just give them a good look.
If that doesn’t pan out off to the jy for a TCU.
By tomorrow night I should have a better idea what’s going on or at lest eliminated some possibilities

 

with the TCU disconnected it think it should be able to be shifted out of first manually, unless its the solenoids, so I would probably try that before replacing the TCU

 

I think you meant "unless it's not the solenoids". Disconnecting the TCU and shifting manually leaves the solenoids unpowered, and is a good initial check of the mechanical health of the trans.

He already disconnected the TCU when the problem first started to get home. I'm betting on wiring or the output speed sensor, with a failed TCU still a possibility.

 

Think I found the problem when I went to pull the output speed sensor the thing came apart ! The rubber is now stuck in the housing arrg. Any tips to get the rubber out ?

Went to the jy and pulled a TCU they are having a huge sale only cost me 10 bucks but no luck on the jy TCU. So I went digging to pull the output sensor and now I feel super stuck..

If/when I get that out are newer model output speed sensors okay to replace they look different ? I've got one on hand from a 98 aw4 for a 2wd trans.

 

No the 98 sensor will not work. It changed in 98 and is too short.