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.

 

After further thought, the 93-95 ECM is more apt to cause a no spark than a 96-98.

 

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?

 

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.

 

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/fixing-lazy-pcm-new-caps-writeup-501951/

 

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.

 

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.

 

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.

 

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....

 

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..

 

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.

 

now that makes sense.. not the actual o2 sensor

 

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/troubleshooting/ss/coiltest-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!

 

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

 

x2 on what 98 mopar said. I would raise hell if I drove my Jeep to the shop and can't drive it home. The spark check that was done doesn't really test the coil just makes sure spark is present. There are ways to test resistance of the primary and secondary coils that are better indicators of a faulty unit. Assuming the 95 is similar to my 96, if the ASD is not tripped, the only things that can cause a no start are crankshaft sensor, camshaft sensor, coil and fuel pump. Anything else, it will start, but run like crap. Also, if the ASD is supplying power, the ECM is not faulty (I think). I kind of sounds like you don't have a mechanic that you can truly trust....can you try something like looking on Angie's list for ratings on shops? Or ask around here...im sure there is another jeeper from your area that could put you in touch with someone trustworthy..or even check it out for you for some money/beer/etc....good luck!

 

That's a wicked interesting theory, bro. I hope you're wrong. When I first got my Jeep away from the auto-center, three days had elapsed but the battery was almost dead. My diagnostic mechanic had to charge it. And he commented that the battery clamp nuts were loose. He said, "That tells me something right there." But he didn't tell me what. He also said he had a couple of friends at the auto-center. Do these things corroborate your theory?

Here's another thing. When I got my Jeep to the diagnostic mechanic's and he opened the hood everything was covered with a thin layer of dust, except the black box in the rear left of the engine compartment in front of the passenger's seat. This box had two fresh clean finger smudges on it, one was definitely a thumb, like someone had very recently grasped this cover. I now know this is the ECM (engine control module) in the '95 Jeep.

When my Jeep was back at the auto-center, I asked the mechanic there if this black box was the computer and he said no, it was just a harness holding wires. He said the computer was in the front right driver's side. Now I figure that's the other computer, the TCM (transmission control module) computer in the '95 Jeep. My point is, someone, not me and not the diagnostic mechanic, for some reason was at the ECM in my Jeep.

 

Anyone tried disconnecting the coolant sensor that was just replaced? A shorted sensor can bring down the ECM, producing a no-spark scenario. Usually that's the crank or O2, but there's no reason why it can't be another sensor that's shorted out.

Is there a Camshaft position sensor? On my TJ, it's a pigtail on the underside of the distributor. My cam and crank sensors failed simultaneously last year. With either one of them connected, I couldn't get a code reader to connect to my computer; the computer was down.

 

Anyone tried disconnecting the coolant sensor that was just replaced? A shorted sensor can bring down the ECM, producing a no-spark scenario. Usually that's the crank or O2, but there's no reason why it can't be another sensor that's shorted out.

If it's the pigtail/connector on the CTS (coolant temp sensor) on the T-stat that's shorted, would the Jeep start with the CTS unplugged? Remember, the '95 Jeep has two CTS, one for the gauge and one to the PCM. It's the one to the PCM that was changed out (and never needed to be).

Just called the manager at the auto-center (because he didn't call me). He said he'd spoken to his boss and they had an outside mechanic look at it and tomorrow they are going to change the ECM at no cost to me. He'll call me tomorrow.

 

When I get the Jeep back to the house here, if she's still not sparking, I'd appreciate any help anyone local has to offer. I hope to come to a resolution with the auto-center tomorrow, which is Monday.

As for the ASD sensor, I brought it to the attention of the diagnostic mechanic and he said he tested it and it was good. I'm not sure how he tested it. I read you can exchange the ASD with the A/C relay if you think the ASD might be malfunctioning. Both he and his neighbor mechanic said they tested it along with the other fusses and they were good. I understood him to say something is sending a signal through the ASD to prevent spark. His final conclusion was, "I think it's the computer". But when he changed the CPS, then told me the original one tested good, then charged me for the new one plus labor - I lost faith in him. Besides, he's got friends at the auto-center. Cha Ching.

 

Yeah, that's why I brought the Jeep back to the auto-center from the diagnostic mechanic's. I convinced the auto-center, based upon things I had read, that the Coolant Temp Sensor they replaced was, number one, the wrong sensor to replace to correct the problem that i brought the jeep to them for in the first place: dash gauge not registering anything. Secondly, that the sensor they replaced was not a Jeep/Chrysler, i.e. Mopar part and that the CTS they put in might in itself be bad or not communicating with the computer. They took my Jeep back (I've got to hand it to them for that) and installed a Mopar CTS and it still wouldn't spark! I asked the mechanic at the auto-center if the pigtail for the sensor was cracked or broken and he said, No, it was not. I looked at it and it did not appear to be broken to my eyes either. The part man at the Jeep dealership told me they are easy to break.

Does the temp sensor for the gauge communicate with the temp sensor on the T-stat through the ECM for instance, or are they entirely independent of one another?

 

PCM
Is Power Control Module, Distribution of voltage, it is the primary connection with relays, and the first line of fuse's Generally very near the battery.

ECM Engine Control Module This would be the brains, of engine management

 

PCM is "Powertrain Control Module" PCM, ECM, Computer - these are all terms for the same thing, the silicon-based device whose primary purpose is to control spark.

What you were referring to is generally called the "Power Distribution Center" or the engine compartment fuse/relay box.

 

Monday: 5:30 pm, called auto-center. They said they are replacing the ECU with a refurbished ECU at no cost to me. Should be done tonight.

Tuesday, Update: Auto-center didn't call. Finally, after 4pm I called them. The computer store was closed when they got there so they couldn't get the ECU to change it out. They'll have it done by 10 am tomorrow.

Wednesday, Update: The auto-center didn't call. Finally, after 4pm I called them. They changed out my ECU and had my Jeep towed to the nearest Jeep dealership to have it "flashed". They expect to hear from the dealership tomorrow.

 

Monday a week after the Monday above - I got my Jeep back today. The auto center came through, they paid the Jeep dealership to make it happen. 31 days of hell. I didn't appreciate her until she was almost taken away from me. Never again. Now she's sparking and I am too!

 

Ecu diagnosis is a process of elimination there really isn't a single test that can be performed to rule purge other caused of s no start condition beyond parts swapping (and testing the individual parts in succession)

As you've discovered, the 93-95 computers actually go bad quite a bit more often than the later jtec obdii units.

 

Update: One month later...

My jeep is now dieing out while driving - just shuts down - engine turns itself off. I pull over and wait half an hour or more and eventually, she'll turn over, no problem. Now, when she decides to start, she'll go five minutes before shutting down and won't start again until she wants to.

Check engine diagnostic flash code was 12 = battery, but mechanic says, when she starts, she starts smooth and easy so doesn't think it's a bad battery. He's gonna get an OBD scan on her later today.

 

heat related coil failure or crank sensor. neither tends to throw a code.
the computer in the front driver's side is the abs computer fyi.

 

You were right, ratmonkey. Mechanic rented an obd 1 scanner from a neighboring shop; but my Jeep threw no codes. So, knowing what you know, mechanic replaced the coil. $124.00. Problem solved.

 

Ok, i have a starting issue on my 97 GC laredo v8, i did get the o2 sensor ripped off by my driveshaft, if it did short out would the computer be ok or fried? i untangled the wires from the o2 and it still does not start, also the guages dont move, doesnt show gas, rpm, temp, battery, but theres power to the cluster, and theres no check engine light that come on when i turn key to on position.

 

if it did short out would the computer be ok or fried?

Sounds like the fuse is (and usually is) blown. Check the small fuses in the box under the hood, should be a blue 15A.