Do I have a PCM Coil Driver Circuit issue?

[Doug Ellis]

I have a ’98 Jeep Grand Cherokee, 4.0L – 6 Cylinder with over 150,000 miles. The ignition system is a single coil with a distributor, and the fuel system is fuel injected. The truck runs great for 2 or 3 days but then has a day of random stalls and sometimes a no-start condition. Sometimes a fuel smell is present, and sometimes there’s a front-fire through the exhaust (gas igniting in the muffler). This can happen at idle, highway speeds, acceleration, and deceleration. It can happen with a cold engine or a hot engine. From time to time, a P0351 code will be set. Typically, switching the car off and then restarting will solve the problem. It is my understanding that the ’98 PCMs do not suffer connection and capacitor problems found in earlier model years.

Here’s some testing results:

• Fuel pressure is good at all times, including stalls and no-starts
• Using an old scope, the Camshaft looks to be about 3-4 degrees out of synch with the Crankshaft … again, it runs very well so this could just be an estimation problem with the antique scope
• The 5-volt supply to all critical sensors such as the Crankshaft Position Sensor is not disrupted during the stalls
  [li]The 12-volt B+ to the coil does not appear to be disrupted during the intermittent stalls, but I’m not 100% sure of this.
• One day during a no-start, I was able to put a meter on the B+ of the coil and attach a spark light on a plug – during crank, there was a fairly steady 11.9 V at the coil and no activity from the light. Then I would see the spark light come one, almost a start … light off / no start … light on for a few pulses, almost a start … light off / no start … steady light and full start.
• Cable wiggling does not create a stall. Connector wiggling at the PCM and Coil do not create a stall. Light rapping on the PCM does not create a stall

I’ve replaced the coil and all secondary ignition parts including the distributor cap & rotor, plugs and wires (they all needed it anyway). I also replaced the Crankcase Position Sensor with a new Mopar CKPS. I have switched the ASD relay with the horn relay. All engine compartment grounds, including the ground to the PCM, have been cleaned.

Since I am seeing B+ voltage at the coil during no-starts and stalls AND I noticed no spark during that one no-start condition, my assumption at this point is that this is a spark problem where the Coil Driver is becoming ungrounded OR I have some sort of wiring problem on my Coil Driver wire between the PCM and the Coil. Because the problem is intermittent, I’m assuming the Coil Driver in the PCM is good.

My questions:
1. Am I correct in assuming that a coil driver in the PCM is good … or that the PCM is good?
2. Since the wiggle test hasn’t caused a failure, there is a good chance that the wire between the PCM’s coil driver and the coil is indeed good. If this is the caase, what conditions might cause the PCM Coil Driver to unground intermittantly … is there a sensor that might cause it?

Topic

[dave]

You might have a timing issue. You might have a broken crankshaft pulley key.

[Doug Ellis]

Thanks for the reply. I’ve had a timing issue concern for a while, but when I saw the no-spark condition, I don’t think timing is the problem because there is still spark with poor timing (correct?).

As for the crankshaft pulley key, assuming a Jeep had one, that would mess up the car significantly wouldn’t it? This is a random, occasional, problem … and again, there is no spark.

[dave]

I would get a timing gun, set your ignition system in “base timing mode” (varies per vehicle) and then shoot the timing & see if it is within specification.

Also, there is a possibility that a distributor ignition system can damage the computer, but I can’t remember what condition can cause this to happen off the top of my head… I’ll have to break out the book and see.

[Doug Ellis]

Again, how does a timing issue have a no-spark condition? If timing were off, I would still have spark. Right or wrong?

I thought that my diagnostic steps had isolated this down to a problem with the Coil Driver connection or circuit … even the random P0351 code backs up that there is a problem there.

Thus it seems that my problems are (a) a second defective coil, (b) wiring between the coil and the PCM, (c) something wrong with the PCM that causes an erratic behavior, or (d) some sensor or other condition of the car that would cause the Coil Driver to unground/disconnect.

[freefalcon]

Have you tried getting under the hood while the vehicle is running on a day while it’s running good and just do a wiggle test? I mean just grabbing the engine harness in various locations and wiggling it also at the connectors. I know the PCM I believe is located on the firewall. It could be a bad solder joint or wiring connection causing it to be intermittent. It may stumble while your doing this when you get close to the bad area.

[Doug Ellis]

[quote=”freefalcon” post=62629]Have you tried getting under the hood while the vehicle is running on a day while it’s running good and just do a wiggle test? I mean just grabbing the engine harness in various locations and wiggling it also at the connectors. I know the PCM I believe is located on the firewall. It could be a bad solder joint or wiring connection causing it to be intermittent. It may stumble while your doing this when you get close to the bad area.[/quote]

Yep, several times. See the 6th bullet in my original post.

[dave]

perhaps the ignition module or computer is getting an intermittent signal from the distributor pick-up coil resulting in an intermittent no-spark condition.

the fun starts at 2:45
http://www.youtube.com/watch?v=MA4dYnCWbg0

[video]http://www.youtube.com/watch?v=MA4dYnCWbg0[/video]

[Doug Ellis]

My jeep isn’t that old. 😛

It doesn’t have a pick-up coil or vacuum advance … it uses a Camshaft Position Sensor (which is good) that sends a signal to the computer.

[college man]

see if this helps. when you replaced the crank sensor
did you install the card board spacer?

http://www.justanswer.com/jeep/3m3wt-1997-jeep-grand-cherokee-code-p0351-set.html

[dave]

If you have variable valve timing, perhaps the mechanisms that advance or retard the timing might not be operating effectively.

[Doug Ellis]

[quote=”college man” post=62666]see if this helps. when you replaced the crank sensor
did you install the card board spacer?

http://www.justanswer.com/jeep/3m3wt-1997-jeep-grand-cherokee-code-p0351-set.html%5B/quote%5D
Yes, the spacer was on the new Mopar Crank Sensor, and installed with the sensor.

Regarding the justanswer.com link, the ASD relay is still good given that I have B+ voltage at the coil. I do recognize that the Coil Driver signal wire could be shorting or opening, but I haven’t reproduced the problem with wire wiggling … I suppose I could replace the whole wire.

[quote=”Wrench Turner” post=62670]If you have variable valve timing, perhaps the mechanisms that advance or retard the timing might not be operating effectively.[/quote]
The computer does control the timing on this vehicle, but I’m still trying to understand why you suspect this is a timing issue.

By the way, I was able to see two more events today and I can confirm that the B+ voltage is not being cut to the coil … at least, it isn’t dropping much with a digital voltmeter. I suppose my next step is to monitor the Coil Driver side of the Coil (probably DC voltage) to see what is happening on it.

[dave]

Wrench Turner wrote:
If you have variable valve timing, perhaps the mechanisms that advance or retard the timing might not be operating effectively.

The computer does control the timing on this vehicle, but I’m still trying to understand why you suspect this is a timing issue.

This could be the result of incorrect timing:

and sometimes there’s a front-fire through the exhaust (gas igniting in the muffler).

[college man]

Check your pcm connections. also check that the ecm is not
hot.If it is you may have an pcm issue.

[Doug Ellis]

[quote=”college man” post=62714]Check your pcm connections. also check that the ecm is not
hot.If it is you may have an pcm issue.[/quote]

I had pulled the PCM connections earlier, cleaned and put dielectric grease on them … no change.

The PCM is always hot simply because it is in the engine compartment.

I know that earlier Jeep models had problems with broken solder joints on the circuit board that host the PCM connectors, but this problem is typically telegraphed by wiggling the connectors.

So, yes, the PCM and the wire between the Coil and the PCM Coil Driver are potential problems. I’m just trying to see if there is wisdom out there that it might be something else (since wiggling seems to rule out the wire).

[Author]

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