Cranks but no ECU/PCM

Hi all,

Would be ever grateful for any help concerning a "slightly out of my leage problem" I'm having with my 96 4.0 AW4 XJ. Search gave me a few threads with similar issues but all of them ended before a solution was presented.

As far as I can tell I have no PCM activity whatsoever. No gauges or dash warning lights with ignition on, no OBD connectivity with scan tool or key trick, no 5 volt reference at sensors no sound of fuel pump priming and needless to say engine won't run.
What I do have is cranking all day long, volts to cigar lighter, interior and exterior lights, stereo and so on.

So far I've tried disconnecting the CPS (still no priming or gauges), checked all fuses and relays (you guessed it, all's fine and dandy) and reset the PCM (no dice).

Never fiddles with the PCM/ECU before so would love opinions on the next logical step.

1. The no gauges, dash warning lights and dead OBD excludes the ASD circuit as the culprit right?

2. Pinout readings of the PCM (if I can find a chart) would be the next step right? The logic would be to find out if the problem is before, in or after the PCM.
I figure there must be a 12V in, a few 5V outs and a number of ground?

3. Basically at this point I'm thinking the problem could be a broken wire somewhere, a faulty sensor, ignition switch or the PCM itself. Am I missing something crucial?

Any help hugely appreciated!

A PCM pinout test is really the next step if you have confirmed batt voltage at all hot all time and Acc/Run fuses. Have you checked spark as well?

A quick check is to unplug your sensors, CPS and/or cam position sensors first and see if your check engine light comes back on with the key in the run position. Then some other sensors, like the MAP, TPS, speed sensor.

If you have a shorted sensor the computer will/may shut down. No CEL when the key is initially turned to the run position is an indicator. Either the bulb is burnt out (unlikely) or you may have a sensor short that is shutting down your PCM.

When that happened to me it was the CPS. I didn't get a five volt reference voltage I got 3 volts, so mine was a little different than what you are describing. But worth a look and easy enough to do before you start swapping parts on a hunch.

Thanks for the great replies!

Actually didn't check for spark since I took for granted that there wouldn't be one. Will do now to know for sure.

Also will test to unplug every single sensor I can find relating to the PCM.

After that it will be time to break out the multimeter and check PCM pinouts.

Anyone know for sure if the ASD circuit is already ruled out from what I've checked so far?

Do you have the Euro model? Does it have the anti theft module that works with the auto door openers? Blinking red diode when the anti theft is armed? Many of the Euro XJ's have this module, at one time the Russians were stealing them, a dozen at a time, every night. The module is a dealer add on.

The ASD relay circuit is likely fuse 20 30A power in and the dark green and orange wire out, going to your ignition coil, injectors, alternator and the relay position (signal) back to your PCM. Maybe another fuse, at least three different PDC configurations I know of.

Unplug your senors one at a time and see when or if the CEL comes back on.

8Mud said:

Do you have the Euro model? Does it have the anti theft module that works with the auto door openers? Blinking red diode when the anti theft is armed? Many of the Euro XJ's have this module, at one time the Russians were stealing them, a dozen at a time, every night. The module is a dealer add on.

The ASD relay circuit is likely fuse 20 30A power in and the dark green and orange wire out, going to your ignition coil, injectors, alternator and the relay position (signal) back to your PCM. Maybe another fuse, at least three different PDC configurations I know of.

Unplug your senors one at a time and see when or if the CEL comes back on.

Click to expand...

No anti theft module as far as I know. But I've been dealing with a lot of aftermarket wire nests so it very well might be in there. Do you have an idea where the diode would be and if it could be connected to immobilizer?

They usually put the red lighted blinking diode on top of the steering column. But I've seen them coiled up under the dash. My module was upper right above the gas pedal. Like I said, it was a retro dealer install, part of an anti theft TSB. Most of the XJ's in Germany had it. Part of an insurance rebate plan. XJ's were real popular in Russia.

If yours does have the module, there are no replacements. I finally figured out how to bypass mine after it failed.

Worth a look before you start buying parts.

When you check power and grounds, always use a multimeter AND a test light to load the circuits. (12v circuits)

Darn, mine was manufactured and originally sold in Germany. I'll have a look up there and maybe snap a pic if something looks out of place.

Subzero temps and stuck outside. Not sure if I'll be able to go through all the tests because of the cold. If you don't hear back from me I froze to death cussing under a Jeep XJ. Not a bad way to go.

No dice with disconnecting any sensors. Couldn't find any trace of alarm system either.

One odd thing which I might or might not be thinking the wrong way about. I disconnected the negative battery cable and then took a volt reading between that and the negative terminal on the battery. The reading I got was 13 volts.
That would indicate a short somewhere right as the reading with ignition off should be 0 volts?

Pinout readings are next.

did you get this jeep this way or is it yours and then just stopped starting? maybe the ignition switch is bad?

Make sure the battery is fully charged. The engine can still crank, but the battery be low enough not to allow the ECU to fire up.

DannyD said:

No dice with disconnecting any sensors. Couldn't find any trace of alarm system either.

One odd thing which I might or might not be thinking the wrong way about. I disconnected the negative battery cable and then took a volt reading between that and the negative terminal on the battery. The reading I got was 13 volts.
That would indicate a short somewhere right as the reading with ignition off should be 0 volts?

Pinout readings are next.

Click to expand...

Because you were measuring voltage negative-to-negative, You basically performed a voltage-drop test. The meter was showing you a 13v drop between the terminal and the clamp. This would be normal, given that they weren't in contact. If they were touching, and clean, the meter reading would be close to zero (a tiny amount of voltage drop). But having them disconnected is basically like having a very bad connection, so all of the voltage is being dropped.

jordan96xj said:

Because you were measuring voltage negative-to-negative, You basically performed a voltage-drop test. The meter was showing you a 13v drop between the terminal and the clamp. This would be normal, given that they weren't in contact. If they were touching, and clean, the meter reading would be close to zero (a tiny amount of voltage drop). But having them disconnected is basically like having a very bad connection, so all of the voltage is being dropped.

Click to expand...

Gotcha! I was confusing volts and amps basically. Frustrating to be barking up the wrong tree but I'm learning a lot of new stuff as I go.
Did an amp test and it's not pulling any when off.

90xj06 said:

did you get this jeep this way or is it yours and then just stopped starting? maybe the ignition switch is bad?

Click to expand...

Having spent way too much time at work today doing research I'm kind of leaning towards the ignition switch as well.
Going to perform this check tonight which I gather will give fairly straight answers.
http://www.justanswer.com/jeep/5yl2x-looking-1996-jeep-sports-cheroke-1-auto-shutdown.html

I've had the car for more than five years so this really came out of the blue.

Oh, and the battery is checked and charged!

Thanks for all the input guys!

Just did the test and it failed the last stage of the diagnostic. I've got no 12 volts in cavity A2 of the Black PCM connector. Meaning the ignition switch is bad, there's a bad fuse which I already ruled out or I've got a broken wire from switch to PCM.

For anyone referencing this in the future. Don't crawl around aimlessly disconnecting sensors and turning the ignition back and forth to run. Much easier to just break out the multimeter and take readings straight away. That way you'll narrow the problem down.

Now for troubleshooting the switch!

Mystery solved!

Way back when I got the Jeep I more or less straight away ran into burning wires in the stearing column caused by the heater blower. Replaced the circuit with relays bypassing the problem and spliced the cables in the column.
http://www.jeepforum.com/forum/f11/...blown-melted-resistor-switch-wire-fix-580627/
Unfortunately for me I was really bad at crimping back then and the one for the dark blue ignition switch output cable separated. Probably due to the cold weather.

Lessons learned.

1. Buy a ratchetting cable crimper and don't waste time on those combined function crap crimpers.
2. If a problem looks to possibly be related to the PCM look up and perform an appropriate dignostic test before doing anything else. It will save you a lot of time.

Case closed.