Another crank but not start problem

[dllama]

Hi all, my 1st post and it is a technical request.. sorry about that.

I have a 1994 ZJ, 5.2, 4x4, LTD with ~80k miles. The Jeep is in primo condition and until just the other day had no problems mechanically. I parked it in the driveway and later tried to start it but no-go. It cranks but will not fire.

I've done a LOT of searching and reading! This forum, others, even downloaded the FSM and went throught that. All this has brought me to one of two possibilities; either my CKP sensor/CPS has failed or my PCM has.

Here are the clues and actions:
Crank no start
Does not pressurize the fuel rail in "on" position nor during cranking
No spark coming from coil
ASD and Fuel relay have been replaced (1 new and 1 swap from AC relay)
I can feel the ASD engage wen put in "on" position
I'm unsure if the fuel relay engages, doesn't feel like it
I can jumper the fuel relay to engage the fuel pump and get pressure
No MIL at all, none at "on" none during crank
No MIL = no codes, it won't flash
CKP sensor continuity test passed (I understand this isn't a 100% test)
PCM connector cover is broken, will explore that more today


Okay gurus, sages of the Jeep world, what do you think? Just replace the CKP sensor and eliminate that before the PCM?

Is there another test for the CKP sensor? Is there any test for the PCM?

Thanks in advance

[Fraser_Brown]

Have you tried to pull codes using the on off on off on off on method with the key?

[dllama]

Originally Posted by Fraser_Brown

Have you tried to pull codes using the on off on off on off on method with the key?

I tried but as I said the MIL doesn't come on at all.. so no codes. I actually think that is one of the stronger clues pointing to PCM but I can't find a conclusive answer.

[SouthZJ]

First check for codes with the key trick, then check the basics 1. Do you have fuel presure when cranking if not and you can hear the pump it could be the fule presure regulator 2. Do you have spark when you crank the engine

if that dose not help and you dont have any codes it gets more difficult

A PCM actualy just works or it dose not at all but it is not very often that the PCM gose bad its more likly to be something else

[dllama]

Originally Posted by SouthZJ

First check for codes with the key trick, then check the basics 1. Do you have fuel presure when cranking if not and you can hear the pump it could be the fule presure regulator 2. Do you have spark when you crank the engine

if that dose not help and you dont have any codes it gets more difficult

A PCM actualy just works or it dose not at all but it is not very often that the PCM gose bad its more likly to be something else

Thanks for the reply, but as I said, No MIL at all so no codes.. Short of finding someone with a DRB II scan tool. Also in my original post I point out no fuel and no spark as a clue.

[gomes512]

I say pull the computer and crack it in half to check for exploded capacitors. I believe there are 3. I made a writeup somewhere about it.

[jlindseyjr]

Originally Posted by gomes512

I say pull the computer and crack it in half to check for exploded capacitors. I believe there are 3. I made a writeup somewhere about it.

That was a good writeup.

Fixing lazy PCM. New caps writeup.

[dllama]

Originally Posted by gomes512

I say pull the computer and crack it in half to check for exploded capacitors. I believe there are 3. I made a writeup somewhere about it.

Good advice. I was planning to remove it today anyway but now i have something to look for and a plan of action if it was failed.

[dllama]

Okay, the PCM is removed and opened. The 3 capacitors look to be in good condition. The gel coating has no burned areas or brittle areas.

Just out of curiosity I tried to start it without the PCM connected. I got the exact same result as with it connect (see original post)! In my simple mind this is another clue the PCM is dead.

[dllama]

I've removed the gel around each of the 3 capacitors and they all seem to be in perfect physical condition.. Should I go ahead and replace them?

[dllama]

Okay, removed the 3 capacitors and they did look like they were bulging a bit on the bottom. I replaced them and reconnected the PCM, same situation. I took the PCM back off, double checked my soldering work, resoldered a connection that seems to have a little play in it, reconnected and same.

Seems I am back to my original question; Is there a way to test the PCM or CPS other than the basic continuity test?

What should I do next, buy the CPS and try?

[jlindseyjr]

You really should try to find someone with an OBD II scanner before you start throwing parts at it. It's possible for codes to be stored with no MIL.

ETA: I realized right after I posted this, you have a 1994.

[dllama]

Originally Posted by jlindseyjr

You really should try to find someone with an OBD II scanner before you start throwing parts at it. It's possible for codes to be stored with no MIL.

OBD II would be easier to find than the DRB II that is required for the 94. They cost about $3k

[dllama]

Seems to me the PCM would engage the ASD and Fuel Relay in the "on" position even if the CPS was faulty. Also would a CPS failure keep the check engine light from coming on or displaying codes?

Really looking like PCM..

[jlindseyjr]

Originally Posted by dllama

Seems to me the PCM would engage the ASD and Fuel Relay in the "on" position even if the CPS was faulty. Also would a CPS failure keep the check engine light from coming on or displaying codes?

Really looking like PCM..

The crank and cam sensors can both cause you problems without throwing a code.