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Unread 04-20-2011, 08:52 PM   #1
Oxenhandler
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PCM aka "computer" symptoms

I take it when the mechanic says he thinks it's a computer problem, he's referring to the PCM (power train control module). So, what are the symptoms of a shot - needs to be replaced "computer"? My only symptom is no spark.

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Unread 04-20-2011, 09:02 PM   #2
ZeeJay1997
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Computer = PCM. PCM rarely causes a no spark condition on its own. It usually has something causing it to misbehave. Most common suspects are shorted O2 sensor and crank sensor.

Actually on 95, it's an ECM.
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Unread 04-20-2011, 09:05 PM   #3
ZeeJay1997
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After further thought, the 93-95 ECM is more apt to cause a no spark than a 96-98.
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Unread 04-20-2011, 09:06 PM   #4
Oxenhandler
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I paid a mechanic to replace the Crank Sensor. Turned out, the original one was still good. Cha Ching. Haven't considered the 02 sensor. How can this be tested?

ECM = PCM?

Last edited by Oxenhandler; 04-20-2011 at 09:57 PM..
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Unread 04-21-2011, 05:19 AM   #5
ZeeJay1997
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Quote:
Originally Posted by Oxenhandler View Post
I paid a mechanic to replace the Crank Sensor. Turned out, the original one was still good. Cha Ching. Haven't considered the 02 sensor. How can this be tested?

ECM = PCM?
Disconnect the O2 sensor and see if it runs.

The 93-95 has a separate transmission and engine control module. ECM + TCM.

The 96-97 has a combined transmission and engine control module. Powertrain Control Module PCM.
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Unread 04-21-2011, 08:26 AM   #6
JS97ZJ
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The PCM (ECU) in the 93-95 does have the problem with providing spark because the ASD relay is not energizing. "lazy caps"
Other things can be suspect, but if you and your "machanic" have determined it's the ECU you can try this if you feel like getting hands on......

http://www.jeepforum.com/forum/f13/f...riteup-501951/
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Unread 04-22-2011, 12:19 PM   #7
Oxenhandler
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I was told the ASD relay was checked and is operating properly. I have not had a lazy start or intermittent start. The jeep was starting easy. I brought my Jeep back to the auto-center to get the coolant temp gauge on the dash working again. It had stopped working after a tune-up in October with coolant exchange. They wanted me to pay for the service. I told them I wanted it covered under the warranty for the tune-up. But what the auto-center did was they changed the CTS (coolant temp sensor) on the T-stat. It was then, in their garage, that the no spark condition began. Causation? Correlation without causation? Fill in the blank?

Where can I find pictures or diagrams showing the location of the two computers in the '95? When I'm standing in front of my Jeep looking under the hood; is one in the front, right corner, driver's side and the other in the rear, left, passenger's side? And which is the TCM (transmission control module) and which is the ECM (engine control module)? And of these two computers, which is the one most likely to cause a no spark condition? The igniton cranks fine. And the relay's click off.

Also, where is the O2 sensor, what does it look like and how do I remove it to see if the engine will start with out it?

Thank you.

Last edited by Oxenhandler; 04-22-2011 at 12:42 PM..
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Unread 04-22-2011, 01:21 PM   #8
ZeeJay1997
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The o2 sensor is mounted though the exhaust pipe just behind the exhaust manifold. The wiring runs forward for a few feet and there is a connector on the drivers side lower engine block. Unplug it there.

The ECM is mounted on the firewall behind the coolant reservoir.

When you first turn the key on, do you get a check engine light for three seconds?

Try this: Disconnected the battery. Then turn the key on and turn the headlight switch on. Be sure to turn the key and lights off and reconnect battery. This resets the ECM.
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Unread 04-23-2011, 04:28 PM   #9
Oxenhandler
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Thanks Zee Jay. But right now, I don't have access to my Jeep. And believe it or not, I don't recall if it has a check engine light. I think it does but I'm not sure. It is still at the auto-center, I am waiting to see if they are going to pay someone to repair it. I do not want to sell them my Jeep for blue book (not that they've offered). I want it running again the way it was when I brought it in to them April 1st. I am not a mechanic and have no background in mechanics. But I am willing to learn. I want my Jeep. These guys at the auto-center, I can't even talk to. I would ask them to try the "disconnect the 02 sensor" and see if it starts but I don't think they'd give me the time of day. I can't blame them since I understand they're on commission. $7/hr base pay. $3.50 commission on an oil change. I have a pretty long list now of trouble shooting things to try.
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Unread 04-23-2011, 06:51 PM   #10
leadfootny
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Tow it to a different shop. If a mechanic won't listen to my thoughts on how to repair my vehicle, I bring it to someone who will. Do you have a friend who might be mechanically inclined? Plus, while it could be coincidental, I find it suspect if it was running fine and while in their hands a major problem pops up. Especially, since its not the first time things have failed while in their hands....
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Unread 04-23-2011, 08:36 PM   #11
mrfister11
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why the hell would the o2 sensor cause no spark???? absolutely no relation.. if you dont have the jeep then there is nothing we can do really.. i would say crank it and see if the tack moves or get a computer and check the rpm signal.. if it doesnt have one then the wires/ crank sensor is bad.. u said they replaced the sensor but wires might be messed up.. it could be just bad luck that it happened when they changed the ect.. could even be coil..
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Unread 04-23-2011, 09:10 PM   #12
leadfootny
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Quote:
Originally Posted by mrfister11 View Post
why the hell would the o2 sensor cause no spark???? absolutely no relation..
I don't know about in the 95, but in the 96-98 the o2 sensor can short out on the exhaust causing a no spark condition. It is essentially shorting the PCM.
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Unread 04-23-2011, 09:11 PM   #13
mrfister11
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now that makes sense.. not the actual o2 sensor
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Unread 04-24-2011, 08:59 AM   #14
Oxenhandler
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The regional manager of the auto-center is on vacation and will be back Monday. So, for the time being, I'm leaving it there. I already brought it to an independent diagnostic mechanic and he found nothing. He said he had a wiring diagram for my Jeep and he checked that. But he is also the guy who replaced the CPS (crank positioning sensor) and then told me the one he took off was fine after all. When he tested it, his ohm meter didn't get a reading but his buddy's did. Go figure. He demonstrated it to me on his bench with me holding the CPS and him touching the pins to the leads on the meter. I don't know what all else he tested. He made out he put hours into diagnosing my Jeep and found nothing.

I have one friend who could help but he's in FL.

As for the coil, I saw the mechanic I hired test the coil using a test like this: http://autorepair.about.com/od/troub...est-manual.htm There was no spark.

What "wires might be messed up"? Does this mean corrosion at all or any wire connections, frayed insulation, other?

What do I look for to see if "the tack (tachometer) moves" when I turn on the igniton?

"A tachometer measures the speed at which a mechanical device is rotating. A common example is the tachometer found on automobile dashboards. In this application, the tachometer measures the revolutions per minute (RPMs) of the engine drive shaft."

My jeep sounds like this. Funny this guy just changed his crank sensor too!


Last edited by Oxenhandler; 04-24-2011 at 07:02 PM..
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Unread 04-24-2011, 08:36 PM   #15
98mopar
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If it happened at their shop they should be responsible. Unless they can tell you how and why it is not there fault. My honest opinion is they did the work and did not disconnect the battery and spilled coolant on your ecu. Just an opinion, but none the less they should be able to explain how this just "happened" in their shop while working with fluids in the engine bay and now you have an electrical problem. IMO
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