jeffasmi

I don't know the difference between the Camshaft Position Sensor and Crankshaft P.S.. My Laredo doesn't want to start, just turns over. I fiddled with the connector of what I thought Was the CPS and it started for a while but now it's doing it again.
Where are the two sensors and which is the typical problem.
Also, when it did run, my 1990 laredo would bog down at take off. I had to baby it and the idle seemed to be low, so I had to drive with two feet at stops and add a little gas to the idle. Are there any pics of the IPS, and CPS's. I'd like to work on it. I'm mechanical enough but my experience is limited and I'm not good with electrical testing of parts.

cruiser54

Crankshaft Position Sensor is located behind the intake manifold on the bell housing and can cause no-start issues even intermittently.

You don't have a Cam Position Sensor. The sensor in your distributor is a sync generator. All it does is tell the ECU which cylinder is about to fire so the ECU can fire off the injectors just before that, or sequentially. If that sensor fails, the ECU defaults to batch firing the injectors. Chances are nobody knows when a sync generator is bad. I've only found out by scanning the sensors with the factory tester.

Do you have a multimeter? If so, you can test your CPS. The fact that it started after fiddling with the connector is a good indication that teh signal is weak.

jeffasmi

no multimeter. Is the sensor in the harness or the part going into the housing?

cruiser54

no multimeter Get one. $10

Is the sensor in the harness or the part going into the housing? Sensor is in the housing.

DFlintstone

Just a quick question. If it's batch firing with a bad sync generator wouldn't your mileage tank? If so I wonder how much? Converter temp peg? Smell gas/high HC? Might a guy diagnose that by plugging in a (7th) injector and measuring the flow rate (running on 5), and comparing that to a known correct rate?

And..pasting I like to make it clear like "If so I wonder how much?"
.............................................Or >>If so I wonder how much?<<

jeffasmi

well I got the sensor loose, where does the harness end?

DFlintstone

If you've taken the distributor apart you may want to carefully put it back. I think Cruiser has said>>>Do you have a multimeter? If so, you can test your CPS. <<<. and>>>no multimeter Get one. $10<<<. My question about a sync generator didn't mean you should start taking things apart. The CPS plug is back by the firewall on the drivers side. The CPS can be checked there with a $5 meter from Harbor Freight.
...........................V Sorry, I know next to nothing about the sync generator business V

jeffasmi

I didn't take the distributor apart. I got the sensor loose but do not know where it connects.

5-90

First, you have "RENIX" - which troubleshoots (and functions) a bit differently than OBD. Nothing wrong with that - I'm a big RENIX fan anyhow! I prefer it.

The two sensors you're referring are the CKP (CranKshaft Position sensor) and CMP (CaMshaft Position sensor.)

The CKP is located at 11:00 on the bellhousing, looking forward along the crankshaft axis (if you stand on the driver's side of the vehicle and look down along the firewall, you can see it behind the #6 exhaust primary.) The wiring termination is at the rear of the fuel rail, it's the three-pole Weatherpack connector with only two wires in it (I believe they're red and white.)

The CKP is typically good for a service life of 150Kmiles or so, before it wants replacing. Note that the later (1991-up) CKP will not work, even though it will mount in place! The operating principles and generated signals are different.

The CMP is inside the distributor, and require distributor removal and disassembly to replace. However, failure of the CMP is rare.

The purpose of the CKP is to tell the ECU (Engine/Electronic Control Unit) how fast the engine is turning, and that a pair of cylinders is coming up toward TDC (Top Dead Centre.)

The function of the CMP is to tell the ECU when the #1 cylinder is coming up to TDC, firing position.

Quirks of RENIX:
- The ECU will not send out pulses to fire fuel injection/delivery or ignition until it reads 300rpm at the CKP. Period. Given reasons on why this is so vary, but this means that it is vital to keep your battery well-charged and make sure your primary grounds are in good condition! CKP failure will give you a "Crank, No Start" condition - the ECU will not run without that signal.

- The ECU will look for the CMP signal to establish base timing, but RENIX (unlike OBD) can and will start & run without it! However, you can spot a CMP failure by a "hard start" condition, it will act like an old-style engine with the distributor timing a bit off until it starts, then smooth right out. (I have independently verified this by disconnecting the CKP then starting the engine. I have also cut the ear off of a distributor and turned it 180* each way with the engine running with no adverse effect - another detail unique to RENIX!)

Therefore, one of the first places to check for a "Crank, No Start" condition is the CKP - which is best accessed from under the vehicle. The screws take a 7/16" or 11m/m socket, and I was lucky enough to find a 30" extension for a 3/8" drive socket wrench that works neatly. I do suggest using a "swivel socket" (you can get them at Sears and most tool suppliers - it's a shallow socket with an inbuilt universal joint) and I suggest using something sticky or tacky inside the socket to hold on to the screw as you remove it (if you drop it in the bellhousing, you're probably going to have to remove the transmission to get the damned thing out.) RTV and chewing gum are typical. (RTV may be allowed to cure and be peeled out, chewing gum removes easily once frozen.) I believe that RENIX CKP units can be had as aftermarket parts, but I usually end up getting them from Morris (I order two or three at a time, and keep spares on hand. It's easier that way - but I've had five RENIX rigs to date.)

While you're under the hood, recall I mentioned grounds? Here are a few things to consider:
- The engine main ground attaches above and behind the distributor - make sure this is clean. This is the primary ground for the entire vehicle, and is therefore critical!
- The firewall ground strap (rearmost driver's side cylinder head stud to firewall) is the primary ground for the chassis. Since the ECU grounds directly to the chassis (it's under the IP, down by your right knee,) this ground is also critical! If you replace it, use min. 8AWG wire. Use a 3/8" ring on the chassis end, and you can run it forward to the rearmost fuel rail mounting screw without incident (this is what I usually do,) which means a 3/8" ring there. If the screws are corroded, use a 5/16"-18x3/4" for the fuel rail and an M8-1.25x20m/m for the chassis. I also suggest cleaning the contact patch on the chassis down to bare metal (about the size of a postage stamp) and apply corrosion inhibitor on reassembly, that will help.
- There is no direct chassis ground from the battery. Adding one often helps. Again, use min. 8AWG wire, and clean the chassis as previously described. Use a sheetmetal or tapping screw to attach to the chassis, attach the other end of the lead directly to the battery negative terminal. (For both leads, larger is better - but the thumb rule for grounds is that it should be the same size or the next size larger than the supply. OEM supply leads are 6AWG. However, you can get away with using the next size smaller for these two leads because they're not full-current grounds - the one from the battery to the engine has to ground full starting current, or about 200A. The chassis generally needs to ground 50-80A, worst case, unless you get silly with accessories. And, you'll have two ground paths for the chassis if you add the secondary lead from the battery, which will add up to more than a 6AWG lead anyhow.)

As far as bogging when running, I'd check the MAP sensor and the TPS.

The MAP sensor (Manifold Absolute Pressure) is on the firewall on its own little bracket. It is a standard GM part of the vintage (as are a large number of other sensors underhood,) and readily available. However, the most common MAP problem I've run across is that the vacuum line comes loose form the throttle body - it attaches to the passenger side. There is a small oval rubber dingus at the end that has two bungs on it - leave the one for the vacuum line alone. Put a bit of RTV black on the other "blind" one and press it back into place - they can get loose as they age, and need the help. If this line comes loose, it not only causes a vacuum leak but it also deprives the ECU of an important signal for basic fuel metering!

The TPS (Throttle Position Sensor) is also on the throttle body proper - follow the accelerator linkage to the thing, picture the shaft running through the throttle body, and follow it. I seem to recall that the TPS is on the front of the throttle body, as mounted. You should be able to turn up how to verify adjustment and function with a brief search - I've covered both topics on several different boards, and I'm sure this one has been included. You'll have an easier time verifying adjustment if you make your own "backprobe pins" - go to your wife/mother/significant other's sewing kit, pull out the two biggest straight pins you can find, and round off the points on a stone. Description for use is in the relevant posts.

The IAT (Intake Air Temperature) sensor can also become coated with carbon, which insulates it against the air in the intake and causes sluggish readings. This is also a standard GM part, you'll find the sensor proper in the intake manifold plenum behind the throttle body (two wires, takes about a 9/16" wrench.) Keep the old sensor when you install the new one - you may have to adapt the wiring, and it's easier to do so by cutting the tail off of the old sensor than to modify the vehicle's wiring... However, the MAP sensor is more important to base fuel metering than the IAT.

Good luck!

cruiser54

I think it was more for emissions. I never saw mileage tanking. Like I said, we only found it if we were scanning for a problem and stumbled upon the fact it was batch firing instead of firing sequentially. Never a customer complaint of driveability or poor fuel economy.

5-90

It doesn't seem to batch fire - it just "guesses" until the #1 cylinder comes up and everything falls into place (takes 5-10 sec longer to start than normal, and seems to stumble,) then it runs rather normally after that.

After I'd established that stating without a CMP signal was entirely possible, I then left it DX'd for a week and drove it around. Except for difficulty starting, I didn't note any problems. RENIX is probably the most adaptable production electronic engine control setup I've run across in thirty years...

DFlintstone

Cruiser, 5-90, Much thanks for taking the time.

BigzXJ

Subscribing to this thread as reference. Good stuff about a renix. Trying to learn as much as I can about them.

jeffasmi

Hey, I replaced the CKP and it started fine and I ran out to do a few errands and it did fine. At my last stop the jeep didn't want to start but after about 5 min and 6 or so attempts it finally started. I'll try to scope out the vacuum lines and other sensors that you mentioned if I can. It is still bogging down when I stop. It idles fine for a min and then just boggs out mainly at take off. Sounds kinda like a vacuum prob?

P.S. Thanks for all the info. Luckily I have a 96' classic to back up this 90' laredo. I thought about selling one but every time I do, I need it.. ha.