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I think my computer is toast

xj88superjeep said:
2. If the ASD test ok, and I am still scratching my head, I have read that bad O2 sensors can cause low voltage at the ASD circuit causing the no start, but I do NOT want to throw O2 sensors at this thing unless someone can verify that they could be bad without giving me a code saying they are bad. Any thoughts?
Click to expand...
you don't have to get new o2 sensors to do your test! just unplug both and see if she starts! when you disconnect them the computer will just stay in open loop mode. mike
 
xj88superjeep said:
Well, still no start!

Here is the latest update. I knew the temp gauge was acting funny, and found out that the coolant temp sensor is part of the startup loop, so I replaced that, and got the same exact results. I am tired of throwing parts at the thing, so I started researching more instead. I spoke to a master tech, and he gave me a clue to look at the A142 circuit which is the circuit from the ASD relay to the PCM during start up loop. He pointed me to the complete starting schematic, and I found the diagnostic troubleshooting procedure for the ASD circuit, so I am primed and ready to diagnose the entire ASD circuit tomorrow morning.

Questions:
1. According to the master tech, Circuit A142 goes from fuse 18 in the PDC to Cavity C12 of the PCM. If that IS open, does that require a new harness? Is it something that I can hardwire? Is it something only a tech can fix? I've never delt with wiring before. Does anyone know of a common fault in that circuit that I should look for?

2. If the ASD test ok, and I am still scratching my head, I have read that bad O2 sensors can cause low voltage at the ASD circuit causing the no start, but I do NOT want to throw O2 sensors at this thing unless someone can verify that they could be bad without giving me a code saying they are bad. Any thoughts?
Click to expand...

Wow, I need to speak with your "master tech"... this is from my thread on jeepforum...

I have been having two problems, now I have a third...

Problem one: the thermostat won’t turn the electric fan. I bandaged this problem by straight wiring the fan to an accessory lead. I’ve drove it like that for the last two years. I’m not too worried about this problem but it caused the third problem so I wanted to explain…

Problem two: intermittent dying: when the truck is cold, I’ll start it and it will run for 10~20 seconds and die. No chugging or anything, just flat dead. It’ll do nothing but crank for a minute or two and then it’ll fire back up. Might run for hours or it might run for another 10 seconds or so and die again. Sometimes it’ll die 3 or 4 times. It’s never done this after it’s gotten warmed up, only when it’s cold. When it won’t start it isn’t getting any signal to the coil. There is a two pin connector that plugs into the coil (green striped wire, should be ground and black wire, should be hot) neither will light a test light.

This brings me to my third: I was to the end of my knowledge on problem two for the time being and decided to work on problem one. I was trying to test the Temp sensor (the one that runs the fan, not the one that runs the temp gauge) connector to see if one of the two pins was sending a current. I accidently grabbed the wrong test lead and touched a hot wire to the hot wire on the connector. As soon as I touched them the engine died (a lot like it does when it’s cold and has the intermittent problem, but the engine was already up to temp). That was Saturday and it hasn’t started since.

So any idea how to solve any of these three things? I’ve been told I might have fried my computer. So you’re telling me there is only one computer on an XJ? If so, where is it?

I built this Jeep from a blown motor 2wd so I know the engine bay by heart. Just point me in the right direction.
confused.gif


I had no idea that the coolent temp sensor was in the loop...
 
Well, I found out that the coolant temp sensor was in the startup loop through a google link to a document called "solving the mysteries of Chrysler ignitions."
http://findarticles.com/p/articles/mi_qa3828/is_200105/ai_n8942314/pg_1?tag=artBody;col1

After finding it again just now, I see that the coolant temp sensor was only in the loop for jeeps prior to 1997, when the PCM referenced the voltage of all sensors during startup, and anything other than a 5 volt reference could cause a no-start. GOOD LUCK!
 
I tested the A142 ckt yesterday with a test light at the coil connector, and I got nothing which shows an open circuit. When I hooked it to a volt meter I got a constant 8 volts during engine crank, but it never lit the test light which I was told should flash rapidly while cranking. Anyone have any idea what might be the cause? This should be fuse #8 in the PDC, but none of the fuses are nubered or labeled in the PDC!!! I changed out the plugs because they looked a little burnt, and I wanted to be precautionary.
 
xj88superjeep said:
No they are not.
Click to expand...

Yes, they are... j/k

Yeah, my lid shows them also. Can someone snap a picture of thiers and post it?



Well I pulled my ECU this weekend and now I am going to go to the pullapart place and look for anther one of two. Does anyone have any experiece with doing this? Does the new ECU have to be from an identical vehicle (ie: Sport, Country, SE, ect). I assume that I need one from a vehicle with the same engine and transmission... but what other considerations should I make?

Another questions, would I be better off to get a new ECU? Any suggestion on where to get one if not the junkyard?
 
Somehow, my PDC cover only tells me how many amps each ckt is, not what fuse # it is or which component it is for! :p stupid PDC cover!
 
Well, after testing for voltage at the coil connector, I have an open circuit between the ASD relay, and the ignition coil. I have to rewire terminal 2 of the coil connector to the PDC. Just to be clear, the cover on my PDC does label the MAXI fuses, however it does not label OR identify the MINI fuses. It only shows the amps of the mini fuses. I did find out everything I needed to know from the FSM. I will let you guys know when I finally get a start.
 
I have not ruled out a damaged or fried link in the PDC itself. Both battery terminals were shorting out when I got the Jeep, so I won't be surprised no matter what I find. It was just a pain in the ass tracking down the actual cause of the no-start, the location of the circuits involved, and the testing procedure for each part of the circuit. I am seeing wiring schematics in my sleep! Test this for voltage, test that for continuity, etc... The whole time I was second guessing myself wondering if it could be something simpler. Whatever else I find, I will post up here. Thanks.
 
Well, After making sure the ASD circuit was complete, and cleaning the corrosion from positive feed to the PDC, STILL NO START....... I finally gave in and took it to the tech. Three hours of diagnostics, and he said everything I did was right, but he suspected the new cam sensor was bad, so I warranteed it for a new one, but still no start, so he did a scratch test at the wiring harness at the cam sensor connecter, showing a wiring fault between the cam sensor and the PCM. He is going to rewire the CPS connector tomorrow, and hopefully I wont have to pay too much to get her back.
 
What a journey.....but look at the bright side; you learned a lot about your XJ in the process! We've all been there at one time or another. Post back when it finally has "fire in the hole". Hopefully the shops labor rates aren't too outrageous. Good Luck!
 
Naturally, mine can't be something simple like a stupid sensor! The shop just called, because they cant get a cam signal at all and they think it's the PCM, and I told them it was doing the exact same thing with the old PCM. Luckily I still have the old PCM to prove it, but now I am highly pissed because I worked on it for weeks, and the tech has had it since yesterday morning, and there is still no cam signal!!!! UGH!
 
OK, I have my Jeep back, and it's running!!!!! :roll:

The mechanic was going crazy because the wiring all checked out OK, but he still could not get a cam signal. The reason ended up being because whenever you cranked it, the engine would rock and short the wire, then when you stopped cranking, and tested the wire again, it would be fine... Until you cranked it again, shorting the connection losing the cam signal again. He said he was a half hour away from giving up, but he wanted to get paid so he stuck it out and finally tracked down the short.

One new distributor + cap + rotor + 4 relays = $175

One remanufactured PCM flashed with all updates, and programmed with all my vehicle's info = $250

3 hours labor at $75 per hour = $225

Getting my Jeep back on the road, and regaining my sanity = PRICELESS!!
 
According to the mechanic, the short was the ground circuit from the PCM to the cam sensor, and the actual break was in the wire harness at the back of the block near the fuel rail bracket. Whenever the engine would rock, it would disturb the wiring in that part of the conduit. He fixed the wire, and prevented it from happening again by simply securing the wire harness where it would not be contacted when the engine moves. It was dark and late when I picked up the truck, so I didn't even actually look at it yet, I was just too excited to get behind the wheel! I'll take a good look at it today and let you know if it looks like anything special.
 
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