ECM suddenly died? - JeepForum.com

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post #1 of 21 Old 07-13-2013, 11:33 AM Thread Starter
ZJMatt
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ECM suddenly died?

1998 ZJ, 5.2 V8, Cali emissions.

My jeep stalled and will not start again. It just keeps cranking over.

Symptoms:
No spark.
Gauges are not functional aside from dummy lights & odometer (always been an intermittent problem that could be fixed by shutting off the jeep and starting it again)
Communication Error through OBDII port

Diagnosing the no spark issue, I found there to be no power to the connector for the Crank Position Sensor. The connector to the distributor has 2.5v with the ignition ON, and drops to 2.1v when I try cranking it over.

All under-hood fuses are good.

This leads me to believe the issue is either with the wiring between the battery and the CPS connector, or within the ECM. Judging by the other issues, I'm putting my money on ECM.

Does it seem likely that the ECM would just die all of a sudden?


The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #2 of 21 Old 07-13-2013, 11:35 AM
MeanGreenZJ
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I would put my money on the Crank Position Sensor. Unplug it and see if your pcm communication is restored.
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post #3 of 21 Old 07-13-2013, 11:48 AM Thread Starter
ZJMatt
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Quote:
Originally Posted by MeanGreenZJ View Post
I would put my money on the Crank Position Sensor. Unplug it and see if your pcm communication is restored.
There is no power coming in to the CPS (with it unplugged) so how could the CPS possibly be the problem?

Unless this sensor is different for some reason, it's a 3 prong connector so 1 wire should be a 5v constant, one ground, and one signal.

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #4 of 21 Old 07-13-2013, 11:57 AM
Oldfrog
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https://www.youtube.com/watch?v=yp_kK6U8a64

2007 5.7 Hemi, Ltd. QD II -
1997 ZJ, 4,0 select trac, Up country, track lok.
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post #5 of 21 Old 07-13-2013, 11:59 AM
ZeeJay1997
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the crank sensor or o2 sensor can short out and cause problems with the reference and supply circuits inside the PCM. Many times the PCM is disabled by the shorted sensor causing loss of com and no CEL for 3 seconds when the switch is in the ON position.

So if you remove these from the equation and the PCM comes back to life, there you go.
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post #6 of 21 Old 07-13-2013, 12:11 PM Thread Starter
ZJMatt
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The rear o2 sensor got caught in the drive shaft and tore the wires out of the connector but it has been like that since I bought it with no problem

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #7 of 21 Old 07-13-2013, 12:15 PM
Oldfrog
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Did it blow a fuse when it did that? It usually does.

2007 5.7 Hemi, Ltd. QD II -
1997 ZJ, 4,0 select trac, Up country, track lok.
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post #8 of 21 Old 07-13-2013, 01:33 PM Thread Starter
ZJMatt
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Like I said all the under hood fuses are good. Is there an ECM fuse somewhere else?

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #9 of 21 Old 07-13-2013, 05:54 PM Thread Starter
ZJMatt
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One more symptom I just remembered. When the key is ON the fuel pump stays running until you shut the key off. It doesmt shut off after a few seconds like it should.

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #10 of 21 Old 07-13-2013, 10:16 PM
dellis
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Quote:
Originally Posted by ZJMatt View Post
Like I said all the under hood fuses are good. Is there an ECM fuse somewhere else?
What ZeeJay was describing does not go through your fuses. The ECM has a small power supply in it that creates a 5V reference from the 12V battery voltage, and this reference powers all your sensors. If it is shorting, then it could cause the PCM to completely shut down.

But, this is not where I first probe to see if my PCM were dead. The test points I would go though are shown in the diagram below.

(1) I would be first interested to see if the PCM starts to go through some of its initiation, and in particular I would be interested to see if the PCM is activating the ASD relay when it first starts up. It should for about 2 seconds. To test, you can disconnect any of your fuel injectors and place your meter between the Dark Green / Orange wire of a fuel injector connection and a ground point. When the key is turned to the "on" position, you should see 12V present for about 2 seconds.

(5) If the PCM is staying on, then 5V should be at the TPS or the Camshaft Position Sensor (on your distributor). If not, then I would test points (2) and (3) to see if I have voltage going to my PCM.

(2) I would check the voltage on both sides of fuse 20 (powers PCM)

(3) Then, I would disconnect my PCM connector #1 to see if I have voltage on pin 22.

Also, check the ground points on the engine.

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post #11 of 21 Old 07-14-2013, 06:36 AM Thread Starter
ZJMatt
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Thanks. Which is fuse 20 and pin 22?

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #12 of 21 Old 07-14-2013, 06:56 AM
dellis
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The fuse identification should be on the lid of your engine compartment's Power Distribution Center (between battery and coolant reservoir).

For your PCM's connectors, the Black one is connector 1, White is connector 2, and Gray is connector 3.


When you unplug a connector (by grabbing the tabs on top and bottom), you will see numbers, like the image below:

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post #13 of 21 Old 07-14-2013, 07:27 AM Thread Starter
ZJMatt
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I figured that out. Thanks . I only have 3.7v coming in pin 22. And only 2.5 at the injector like you said (also only had 2.5 at the distributor).

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #14 of 21 Old 07-14-2013, 08:18 AM Thread Starter
ZJMatt
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Sorry I think that was a bad reading. Tested again and got 12v.

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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post #15 of 21 Old 07-14-2013, 12:41 PM Thread Starter
ZJMatt
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Replaced the ECM and no luck... waste of $100

The "Mall Crawler" - 1998 ZJ Grand Cherokee 5.2 V8, 5.5" LongArms, 32s, 12000lb winch & more.
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