Topic: YJ PCM question

[neale_rs]

 Does the '95 YJ PCM wait for any sensor signal before it provides ground to turn on the check engine light, ASD relay, and fuel pump relay?  The FSM is not explicit about this fine point.

Thanks

[Jeffy]

I doubt it waits for any sensors at startup.  You can have a clear memory then start up the engine and as soon as something fails that has a code associated with it will turn the light on.  Then when you shut it down it will hold the code and if the code is in the memory at start up it will turn the CEL on.  If you turn on and off the ignition several dozen times it will reset the memory.

[neale_rs]

Thanks.  In that case then I think my computer or at least its connector is messed up.  Today I checked the ground (all 3) and 12v (the one that is always at 12v and the one from the ignition switch) cavities on the connector and they are perfect.  I can't find any reason why the PCM would have a delay of 16 seconds before it turns on the check engine and relays with the key on run.  The check engine light and the relays are on different wires from the PCM so they have nothing in common except the PCM itself. 

Would a scanner reveal anything beyond what the codes provide (only 12 and 33 come up using the check engine light method)?

[Jeffy]

Thanks.  In that case then I think my computer or at least its connector is messed up.  Today I checked the ground (all 3) and 12v (the one that is always at 12v and the one from the ignition switch) cavities on the connector and they are perfect.  I can't find any reason why the PCM would have a delay of 16 seconds before it turns on the check engine and relays with the key on run.  The check engine light and the relays are on different wires from the PCM so they have nothing in common except the PCM itself. 

Would a scanner reveal anything beyond what the codes provide (only 12 and 33 come up using the check engine light method)?

Hmm, I guess it could be waiting BUT I haven't seen anything send the CEL on before starting unless it's already started up once.  The O2 takes a while to heat up.  I've had and O2 issue when it's sent intermittent signals to the PCM but that was only while driving.  I've never seen it turn on before starting up from a clear memory.  Same goes for TPS.

So what issues are you having other then that?  Does the engine start or do you have to wait?

[neale_rs]

The way it has always worked is that if I turn the key to run, the check engine light will come on right away along with the shift up indicator for 2 or 3 seconds while the fuel pump runs, then it goes out and no CEL is flagged.  The FSM says it does this as a bulb check. Now when I turn it to run it has a long delay before the check engine light comes on and the fuel pump runs.  As it gets colder outside, the delay gets longer.  If it gets too cold outside it does not start at all.  If I try to start (cranking) without confirming that the check engine light has come on, the jeep will just not start at all, only after this delay. The check engine light seems to be serving as an indication that the computer is ready to let the jeep start.

[aw12345]

From what I have read this is a somewhat common problem onyj's and points to the ecm going bad. you could try to warm the ecm to 75 or 80 degrees fharenheid and see if it starts right up. This seems to be a fairly common problem in colder weather

[neale_rs]

That's what I was afraid of.  I pretty much narrowed it down to the PCM, its connector, or some delay for sensor information but this makes it MUCH more likely it is the PCM.  In warm weather the problem decreases quite a bit and even goes away on some but not all very warm days.  Will a scanner tell me for sure?  Thanks

[Jeffy]

Yeah, start buy checking all the cheap stuff.  Reseat the connector at the PCM and see if that does anything.  It should start once the lights go out and the pump stops priming.  The blow dryer is a good idea too.

[neale_rs]

Once the lights go out and pump stops priming it starts right up and runs as well as it ever did. It's getting it to that point that gets harder as it gets colder.

This is what I have done so far:

1) cleaned clamps and installed new battery (it was old anyway).
2) cleaned the engine ground connection behind the dipstick bracket
3) cleaned the ground connection on the firewall just to the side of
the battery.
4) cleaned the positive connection to the power distribution box
5) put a test light on pin 5 on the computer data link
connector to see if that point was getting 12v as soon as the
ingnition switch is turned to run.  It checked out fine.
6) Verified power and ground to the PCM at connector, all fine
7) installed new ASD and fuel pump relays
reset PCM at least twice
9) taken out, cleaned and reseated PCM cnnector

It's getting narrowed down.  The blow dryer would be a good test too.

[neale_rs]

I took it to the dealership today.  Let's see what they say. I'm pretty sure it is the computer at this point so anything else will be good news.  Thanks for all the help, I'll post the final resolution.

[aw12345]

a bit more work but you can stick the ecm in the fridge if it will not start you got her nailed

[neale_rs]

Dealership confirmed it is the PCM.  I'll try looking for a cheap used PCM now.  Thanks for all the help.

[aw12345]

In case you have a really hard time finding a pcm you could rewire the fuel pump and asd relais like they used too on fords with a electric fuel pump and a carb. On those they wires the relais to a oil pressure switch and the I terminal on a ford style remote starter solenoid should not be that hard to do. This way the pump will prime when you crank and when it starts the oil pressure switch will continue to give it power

[Jeffy]

In case you have a really hard time finding a pcm you could rewire the fuel pump and asd relais like they used too on fords with a electric fuel pump and a carb. On those they wires the relais to a oil pressure switch and the I terminal on a ford style remote starter solenoid should not be that hard to do. This way the pump will prime when you crank and when it starts the oil pressure switch will continue to give it power

What's the fun in that?    It should run to it's own switch on the dash.  Flip the switch and you're ready to start...  Like in a race car! 

[neale_rs]

Actually, we tried that up on the mountain where it completely failed to start due to the cold.  ASD and fuel relays were jumped and we confirmed the pump was working.  It still would not start.  It's like the computer does not know it has been turned on and it won't provide ground signals to the fuel injectors or anything else it controls.