Problem solved I replaced the PCM and it fired right up.

Original post: I have a 1997 jeep grand Cherokee limited v8 5.2l that will crank but will not start. There is no voltage at injectors or at the ignition coil. I have tested both the cam sensor and the crank sensor and both work. Also, no check engine light illuminates at all when turning the key to the run position and the gas gauge is showing empty along with a gaslight even though the jeep has gas (I'm not sure if this has anything to do with it). By grounding the ASD relay with a jumper wire from battery negative I am able to get 12 volts to the coil and to the injectors however the jeep will not start because although there are 12 volts at the coil, the coil will not function because there is no on/off signal from the PCM. Also, when I disconnect fuse 20 which is for the pcm and the asd I get 2.5 volts to the injectors. Do I need to replace the PCM or could the problem be somewhere else? I was also wondering if the problem could be the fuse box.

 

How did you test the Crankshaft Position Sensor (CkPS)? Are you able to confidently say the CkPS is okay? The CkPS is such a highly likely suspect to cause exactly those symptoms.

Without having recieved an input signal from the CkPS to the computer, there will not be sparks. If there are no sparks being signaled.. then there will be no fuel injector pulses. A symptom very specific to having a failing or failed CkPS is losing the fuel gauge function ('wonky' fuel and/or voltage gauges are a common symptom). All of this is exactly what you have described as happening with your Jeep.

There is a pigtail connection for the CkPS near the firewall located within the engine compartment. Disconnect the CkPS here at this connection, turn the ignition key to run, and look back at your fuel gauge. The fuel gauge will read normal when you have the CkPS disconnected.. if the CkPS is the culprit.

In what ways have you tested the Crankshaft Position Sensor?

 

I was also highly suspect of the CkPS so I actually bought a new one and installed it and using a multimeter I made sure that the sensor was grounded, that sensor had 5 volts going to it and, and that the sensor was producing a crank signal.

 

The no check engine light (CEL) had me a little worried. If I remember it correctly? The CEL is supposed to light up and then stays on with the key in the run position. When there is a crank-no start problem happening, the CEL should stay lit up while cranking if the CkPS has all out failed and is faulty. The CEL light would normally temporarily go out while cranking if the CkPS is okay during a crank-no start condition, right?

I would have to find out more on why this sometimes happens with the CEL totally disappearing like that? I doubt the CEL bulb is burnt out but, it could be?

The no CEL does make me wonder about the PCM.

If you have not done it yet? I would test each fuse with a test light before moving on any further.

 

Same situation on my 96 limited 5.2 and I had thought it was a bad fuel pump but verified the pump is OK and produces 50 PSI when the relay is replaced with a jumper wire. However it cranks but does not fire. Today I went to check for codes and neither of my scanners (icarsoft i930 and Ultragauge) can find a system with key off or key on. I moved enough stuff to get to the PCM connectors and sprayed the pins and sockets with contact cleaner, but still no OBDII system is found. I had not noticed the lack of the CEL until I read this thread, but I also have no CEL and no gas gauge reading besides the low fuel light. When I turn the ignition to on the ABS, brake, air bag, seatbelt, brights, & check gages lights come on temporarily but no CEL. They all go out leaving just the low fuel light on.

I have not yet checked the crank sensor, but did check all the spark plugs and they were all a nice light tan and not wet and all fuses in the PDC and the fuse block have continuity. SO...is it likely a dead PCM? Is there any way to check it? I saw one video that said it should have 12 volts on pins 2 & 22 in the black connector and that pins 31 & 32 should be ground, but I will have to wait for the rain to let up to check those. Anyone know what OBDII system a 96 has?

 

Sounds like we are having the same problems and may possibly have the same solutions so keep me updated with what you find and I will do the same. Heres a diagram of the PCM connectors

 

 

Here are two wiring diagrams that may help as well ^^^^

 

Thanks for the diagrams, Nick. I'll check for 12V at the PCM connector tomorrow and try disconnecting the CPS as Noah suggested to see if the fuel gauge shows any level with it disconnected. Have you checked for codes or checked to see if a code reader sees your OBD system? I also read a thread that suggested checking the grounds, especially at the dip stick mounting stud. I'll check that too.

I also have a 96 Cherokee Country, but I doubt the PCMs are interchangeable - that would be too easy. Anyone know if any other year ZJ PCMs would work? Or am I at the mercy of the rebuilders if I need a "new" one?

 

I tried two different obd2 scanners and both lit up but neither made a connection. I dont think the pcms can be swapped but you can see if both of the part numbers are the same and if both pcms look the same.

 

I was reading a thread on another forum with two guys talking about both of them having the same symptoms. I took a screenshot of a part of their thread (added below). For one guy it was the CkPS. For this other guy it was the computer.

You could try it to see if the scanner will be able to communicate and work normally after disconnecting the CkPS? Doing this it could allow the scanner to communicate and read codes when having the CkPS disconnected.. if the CkPS is a part of the problem?

 

Thanks, Nick. So your PCM is "off-line" too.

And thanks Noah, for that info. Interesting that either the PCM or the Crank sensor can cause a lack of buss communication. I'm not getting my head around how a bad sensor could cause the PCM to fail to communicate, but I'll definitely disconnect mine and see what happens if we get a break in the rain today.

 

OK, guys. I disconnected the Crank Position Sensor (CPS) as suggested and the icarsoft i930 still did not find any OBD system (the last one it looks for is ISO 9141). I checked the black PCM connector pins 31, 32, 22, & 2; pin 31 had 0.0 ohms to ground, pin 32 had 0.1 ohms to ground, pin 22 had 12.49 volts with key off or on, and pin 2 had 0 volts key off and 12.26 volts with key on.

I reconnected the black PCM connector and checked the CPS connector’s white/black stripe wire: key off 0.082 volts, key on 0.63 volts. The CPS connector’s black/light blue stripe wire checked 8.5 ohms to ground key off and 21.5 ohms to ground key on. I assume this should be getting 5 volts from the PCM pin A18 on the white/black wire and should be grounded through the PCM pin A4 on the black/Lt blue wire (along with a bunch of other sensors)? I wonder what might happen if I grounded the black/Lt Blue wire externally, but if the CPS is not getting 5 volts from the PCM it probably wouldn’t make things work and might let the smoke out somewhere.

Any other things I might check before I declare the PCM brain dead? I did read that to check the CPS you need to turn the engine by hand and see if the signal goes on and off, but if it isn't getting its 5 volts or ground, will it still have an on/off signal? There is also a video on re-soldering the PCM's inputs at that looks like a last ditch effort.

 

When I tested my crank sensor it had ground, 5 volts of constant current and 5 to 0 volts of the crank sensor sending the signal. I didn't turn the engine by hand instead I just bumped the key so the engine would slightly turn over and I did this until I got a reading of 0 volts and then back to 5 volts which means my sensor was working properly.

 

I pulled the PCM thinking I might try checking the solder joints inside, but my PCM is much different than the one(s) in the PCM repair videos. The case is a wrap around case and does not have a top plate to take off, so I am not going to try to open it yet.





I did find a series of 3 videos checking PCM and sensor 5v supplies on a Cherokee with no computer communication that eventually found a blown #20 fuse due to shorted wires to an O2 sensor at

I will try some of these things tomorrow with the PCM back in and see if I can find any shorted 5V supply wires. (assuming we don't blow away in the storm tonight).

EDIT: BubbasgarageTV video destroying a PCM that looks like mine: I am definitely not going to try this....At least not yet.