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Unread 03-25-2012, 01:17 PM   #1
Nick_S
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1996 ZJ 
 
Join Date: Mar 2012
Location: Los Angeles, CA
Posts: 51
Is it my CPS or PCM?

Since I’m one of those few lucky Jeep owners that has that quirky engine stalling and restarting problem, you know that problem where you drive long enough for the engine to get warm or hot, then it sporadically dies out on you in mid-drive (without warning) and where it can take anywhere from 1 to 5 minutes or up to almost 3 hours, before it restarts?

Well I’m working through a list of many options that some of the members have provided to me from another thread of mine. Yet upon doing more reading on the Jeep’s stalling and restarting problems, I keep seeing 2 things that stick out the most about this problem. It seems that many people that have this problem, always have it when the engine is warm or hot . . . but never while it is cold. Then people that have replaced either their CPS or PCM (sometimes they have replaced both), have reported that this fixed the problem completely. Now with that being said, some people said that when they replaced their CPS, that “did” or “did not” fix the problem. Then some people have said that when they replaced their PCM, that “did” or “did not” fix the problem.

Is there any way to check to see if a CPS AND PCM is bad, BEFORE one start yanking them out? Because I would be interested in knowing if either one of them is bad, to begin with, before attempting to remove them. Yet if one has to remove one or both of them completely, is there any way to see or test them to see if they are bad BEFORE buying a new one?

Nick

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Unread 03-25-2012, 01:31 PM   #2
RazorbackZJ
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1994 ZJ 
 
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Personally mine did exactly the same thing. Replaced the crank sensor 3 years ago and haven't had the problem since.
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Unread 03-25-2012, 01:44 PM   #3
BigRed4x4
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I know how frustrating a problem like this can be. The crankshaft position sensor will either be a magnetic pulse generator (2 wires) or a hall effect sensor (3 wires).

If two, then the sensor will produce weak AC voltage signal during the engine cranking or running. No chance of a DVOM picking up the signal while the engine is running but while cranking the engine is turning slow enough that there's a good chance of the multimeter picking it up. Using an oscilliscope would be the best option but a DVOM is more commonly found. The way the sensor works is it has a small coil of wires in the sensor are held very near to a reluctor wheel on the crankshaft. Everytime a tooth on that wheel comes near the coil (pickup coil) it induces a small AC voltage in the coil which translates to crankshaft position. This is relayed to the PCM.

If three, the sensor will have one wire that is ground, one wire that is a 5 volt reference voltage from the PCM, and one wire that is the signal back to the PCM. With the key on you should have no or very little voltage drop across the ground, a reference voltage of close to 5 volts, and 0 volts on the signal wire. You should take one lead from your multimeter and ground it, then take the other lead and read the voltage from the signal wire (backprobe the connector or tap into the wire). While cranking the engine you should see the voltage go from 0 to somewhere between 2 and 4.5 volts. This works almost the same as the magnetic pulse generator except it modifies a reference voltage and sends that to the PCM. Also this sensor works on DC voltage.

This is all while on the vehicle. If you test them off all you have to do for the magnetic pulse generator is replicate the reluctor wheel passing close to the coil by using a knife blade and sweeping is passed the crank side of the sensor. The hall effect sensor has to have a reference voltage and a ground so it's a bit harder to replicate conditions.

Hope this helps,
Seth

P.S. Not to dismiss the PCM idea, but a sensor is much more common to malfunction than the control module. Crank sensors are bad for letting oil get into the connector plug area.
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Unread 03-25-2012, 02:21 PM   #4
cootertwo
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Good stuff there Seth
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Unread 03-25-2012, 02:44 PM   #5
Oldfrog
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If it's a 4.0. ( please fill out your profile), why not just wiggle the PCM connector plugs while it's running? If it stalls....that's your problem. ....and if it is NOT running, wiggle them and try to start it. If it starts.....it's still your problem. The last time mine stalled, I didnt even consider the CPS. ( it's a 3 year old Mopar CPS). I merely wiggled the PCM connectors and it started right up. Replaced it ( again)....and I'll drive it until it falls apart...which it will eventually do.
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Words of wisdom, Posted by Zeejay:
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Unread 03-25-2012, 03:01 PM   #6
BigRed4x4
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I agree with Oldfrog too. The wiggle test is tried and true. Especially if your problem is a result of corrosion, water in sensors/connectors, or just a loose connection.

Also, the PCM contains very small and inaccessible transistors that act as switching devices for all sorts of components including the ignition system. Transistors are notorious for not failing until they get hot. In the old, and even newer, ignition control modules, if they failed or were failing you could take a look at the backside of them and in the insulation (hard silicone like material) there would be small spots where melting had occured. This is where the transistors had gotten too hot and started to melt the insulation.

This is why when you tell someone you have an intermitent starting problem or a random dying problem they will inquire about the ICM (Ignition Control Module). The PCM in your Jeep (if a ZJ) acts as the ICM, so the transistors inside can fail and cause an intermitent problem. However, in almost any dealership/shop the recommended method for diagnosing a bad PCM is to rule out all other components or possibilities related to it.

Seth
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Unread 03-26-2012, 11:45 AM   #7
Nick_S
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Oh sorry, Everyone, I forgot to put the specs on my Jeep. Here they are.

I have a 1996 Jeep Grand Cherokee Limited, with a 4.0 6 cylinder engine, with 2 wheel drive, and with about 165,000 miles.

Seth, what type of CPS would most likely be on my model?

Oldfrog, thank you very much for your suggestions. I tried that one day, when the engine would not start after I parked . . . that was the day that it had to sit for 3 hours before it would restart. GRRRRR!!! I lightly banged on the PCM and wiggled the wires, but the engine would not start. Yet I will try as you suggest, but when it is running and when the engine is warm or hot, because the engine starts up right away and quickly whenever it is cold. The engine always runs fine fine when it is cold . . . yet when the engine gets warm or hot, it is totally unpredictable. Yet maybe it could be a transistor in the PCM.

One last question, since I am a sub mechanic novice, is there any way that the average mechanic can tell if a CPS and PCM is bad, without removing them from the engine? Since I’m trying to do what I can on my own and try to keep my repair bill down, I would like to just take my Jeep to a mechanic, have him test both units, then hopefully he will be able to tell me which one “might” be bad, then I can have the bad one replaced, then keep my fingers crossed, and hope that fixes the problem. I would try to do the recommended testing myself (based on what has been suggested here), but that is a bit too advanced for me with my ultra limited mechanic skills. Yet I still have other things I can check out on my own, based on the great feedback and suggestions in another thread I have out there. So I’m hoping that “something” will fix this problem.

Thanks again, Everyone!

Nick
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Unread 03-26-2012, 12:11 PM   #8
JeepCares
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not sure if applicable or not - it's from the service manual

CRANKSHAFT POSITION SENSOR - 4.0L ENGINE
To perform a complete test of this sensor and its circuitry, refer to the DRB scan tool. Also refer to the appropriate Powertrain Diagnostics Procedures manual. To test the sensor only, refer to the following:

The sensor is located on the transmission bellhousing at the left/rear side of the engine block Crankshaft Position Sensor Location - 4.0L 6-Cyl. Engine



Near the rear of the intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
Place an ohmmeter across terminals B and C Crankshaft Position Sensor Connector Ohmmeter should be set to 1K-to-10K scale for this test. The meter reading should be open (no resistance) Replace sensor if a low resistance is indicated.
cps-location.gif   view-cps.gif  
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Unread 03-26-2012, 01:08 PM   #9
Nick_S
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Thank you, JeepCares. I appreciate your help and the diagram. This is a great help. Thanks much.

Nick
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Unread 06-20-2012, 04:05 PM   #10
Nick_S
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Okay, I have returned with a final verdict. After going to a (3rd) third mechanic, he found nothing wrong with my PCM, however he did find that the new crankshaft position sensor that I had installed in April was “bad”. So he ended up replacing it with a Mopar crankshaft position sensor, since the mechanic that put in the “bad” crankshaft position sensor is no longer in business, so I can’t go back to get a refund. Now my Jeep runs GREAT without any other problems.

Nick
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Unread 06-20-2012, 06:09 PM   #11
KoreaZJ
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Tried a non-OEM sensor once, wouldn't even fire, waste of $$ and time. Tossed in a Mopar sensor, good as new.
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Unread 06-20-2012, 06:41 PM   #12
Flyin
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Quote:
Originally Posted by JeepCares
not sure if applicable or not - it's from the service manual

CRANKSHAFT POSITION SENSOR - 4.0L ENGINE
To perform a complete test of this sensor and its circuitry, refer to the DRB scan tool. Also refer to the appropriate Powertrain Diagnostics Procedures manual. To test the sensor only, refer to the following:

The sensor is located on the transmission bellhousing at the left/rear side of the engine block Crankshaft Position Sensor Location - 4.0L 6-Cyl. Engine

Near the rear of the intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
Place an ohmmeter across terminals B and C Crankshaft Position Sensor Connector Ohmmeter should be set to 1K-to-10K scale for this test. The meter reading should be open (no resistance) Replace sensor if a low resistance is indicated.
Just an FYI. I have a '96 ZJ 4.0 & FSM, my CKS sensor connector is on the passenger side of the engine, above the oil filter. Not the intake side like the FSM says.
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Unread 06-21-2012, 07:09 PM   #13
Nick_S
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Quote:
Originally Posted by KoreaZJ View Post
Tried a non-OEM sensor once, wouldn't even fire, waste of $$ and time. Tossed in a Mopar sensor, good as new.
KoreaZJ, since I first bought my 1996 JGC new, I had always taken it to the Jeep dealer for preventative maintenance and repairs. Then when it got 100,000 miles on it, I started going to my local neighborhood mechanics and have never had a problem until I started having the normal problems associated with a bad crankshaft position sensor (CPS) (yet I did not know for sure what was wrong with the engine at the time), however I never had the CPS replaced before. Though I have not had to do many repairs on my Jeep since it hit 100,000 miles (I have about 165,000 miles on it now), I am sure that the neighborhood mechanics did not use OEM parts on my Jeep in many cases. Yet while trying to find out what my problem was, I had read in some of the Jeep forums, that when it comes to replacing any type of electrical parts on a Jeep, always use OEM parts. Now I have learned my lesson, so from now on, I will only use or require the mechanic to use OEM parts whenever it comes to replacing any electrical parts on my Jeep.

Though at the time, before I took my Jeep to the 1st mechanic, I did not know what might be wrong with my Jeep. Based on my readings from these Jeep forums, the usual suspects (based on all of my engine symptoms), it was most likely either the “CPS” or “ignition coil” or “PCM” or the “fuel pump”.

Yet this time around, what had sold me on a possible non-OEM part was, that the mechanic that installed it gave me a 1 year warranty on it (part and labor). So I figured if the part did not work (for any reason), that I could go back to the mechanic . . . yet I never figured the mechanic would go out of business with less than 2 weeks of him putting in the part.

Yet I had read, in some cases with Jeep owners that had my same exact problem (when it was determined that the CPS was bad), "sometimes" a non-OEM CPS worked, other times, the non-OEM part DID NOT totally fix the problem (like in my case), BUT once they swapped the non-OEM part for a OEM part, the problem was totally fixed (like what eventually & finally happened in my case, to fix my problem). Yet the 3rd mechanic did tell me that even though the new CPS that the 1st mechanic had installed, it was more than likely bad out of the box (which can happen I have been often told, even if it is a new OEM part).

Nick
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