I think my computer is toast

I have a 97 sport that had been bucking at highway speeds, and an intermittent no start. The truck already has a new CRANKshaft pos sensor. It was throwing codes for ASD relay circuit, so I replaced that, and still same. Then a few days later would not start at all The code was CAMshaft position sensor, so I did that, and did cap and rotor while I was at it. It still would crank but not fire! Well, right before the battery died, it did start, and backfire like a beast. I returned the core CPS, and now NO START AGAIN!!! I keep getting ASD and CPS (CAMshaft) codes, and no spark at all after all new relays and new CPS! I did check all fuses and replace ALL relays. I am convinced that my computer is fried! Any thoughts?

This Jeep runs like new when its running properly. It's not a trail rig or a beater, it's like new. Could the timing set be bad? 175,000 on the ticker.

Some months ago my 96xj died. As I was trying to figure out what the deal was, I kept hearing the ASD relay click. I checked all sensors/connections and found out that my o2 sensor heater wire was shorting to the header(borla has bung facing rear, so the harness has to do a 180* to go back up front and my zip didn't hold). I blew my pcm--it was giving multiple sensor high voltage codes. I ran a 98 auto pcm for a while and then up til yesterday was running a 97 ax15 pcm. I just got a 96 ax15 pcm and so I have the 97 if yours is an ax15.

It's an AW4, and I've heard it will not run unless it is programmed with the proper VIN and Mileage. I tried a PCM out of a 98 limited, but I don't think it was even communicating with the vehicle because my mileage still popped up on the OD, and it would not give me any codes at all, so I swapped back in the original, and still get same codes.

I thought the same thing about the computers having to be programmed. Not true. I listed to Gradon and bought a computer from him just recently. I didn't program it and the Jeep runs fine now.

Did yours start with the limited 98 pcm? I haven't tried using a pcm from an XJ with the factory security system but if yours didn't have it and that limited 98 had it, that could be why it didn't start.

No, I don't think it would even communicate, and I think you're right, the PCM would have tripped the ASD because the security system would recognize conflicting VINs. I took the $40 dollar gamble just hoping to drive it home, but ended up towing it anyway. Was sad to see that guy carted off the field! LOL

Don't be too quick to condemn your computer. From all the problems I've been involved in troubleshooting, VERY few have been computer related. Sure they can go bad but it's pretty rare. There is evidence that way more than half of the old computers that are returned test out good; the problem turned out to be something else. These problems can be frustrating to be sure but stay on it; eventually you'll locate the root cause. I would start by unplugging and cleaning connectors to some of the major sensors. Freshen all electrical connections including grounds. Simple stuff first. If you get a no-start condition again pull a plug and ground it and check for a good strong blue spark. Or spray a shot of starter fluid and see if that makes a difference. If it does, then it's fuel delivery related. You need to start the process of elimination and determine if it's spark or fuel that is missing when symptomatic.
P.S. Base timing on the 4.0 is not adjustable; programmed by the computer.

jeepster_259 said:

I thought the same thing about the computers having to be programmed. Not true. I listed to Gradon and bought a computer from him just recently. I didn't program it and the Jeep runs fine now.

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Then it must only be true with PCMs from anti-theft Cherokees, because I know of guys who had to get their PCMs reprogrammed because of ani-theft malfunctions.

birchlakeXJ said:

Don't be too quick to condemn your computer. From all the problems I've been involved in troubleshooting, VERY few have been computer related. Sure they can go bad but it's pretty rare. There is evidence that way more than half of the old computers that are returned test out good; the problem turned out to be something else. These problems can be frustrating to be sure but stay on it; eventually you'll locate the root cause. I would start by unplugging and cleaning connectors to some of the major sensors. Freshen all electrical connections including grounds. Simple stuff first. If you get a no-start condition again pull a plug and ground it and check for a good strong blue spark. Or spray a shot of starter fluid and see if that makes a difference. If it does, then it's fuel delivery related. You need to start the process of elimination and determine if it's spark or fuel that is missing when symptomatic.
P.S. Base timing on the 4.0 is not adjustable; programmed by the computer.

Click to expand...

I've already determined no spark, and the trouble codes confirm that. And I've been reading more and more posts around the net saying that a new PCM solved all these issues that people waste time throwing parts at. I figure I can't rule out what I haven't tried, so try the PCM. That is the same cost and easier than a timing set, which will throw off timing if it jumps a tooth, but I am not sure if that would cause my issue.

I think I will be much more confident knowing the PCM has all the newest updates, and a lifetime warranty. Even if it does not solve the no start, at least I will know that it's not the problem. The PO had neglected the battery terminals, and when I fixed them, I could see arcing damage between loose ground wires and the negetive terminal. That's one of the things that is pushing me towards fried PCM.

SORRY that may have been critical discussion material there! LOL

Well, I have a 98 PCM from a Limited if anyone needs one. Mine still has not started, and I am ordering a new PCM today.

Let us know when you get this resolved. Always interesting to see the followups on these threads. Good Luck!

Yes, I will keep everyone updated. I hope it's with the resolution and not more questions! LOL

Just a quick status update...
Ordered the new computer last Friday and I'm waiting for it to ship. Meanwhile, in the back of my mind, the idea that it might be a fuel pump issue just kept nagging and nagging so I gave in and bought a used pump just to rule that out. I have not gotten a chance to drop the tank yet, but when I do get to this, I will tell the results before and after the fuel pump, then again after the PCM.

Well, I put the new PCM in before the fuel pump since it was the easiest job. Still no start, but I definitely have fuel and NO SPARK. The new CPM still gave me the same codes, and they are in this order:

12
Battery or computer recently disconnected

54
No sync pickup signal during engine rotation (turbo only)
or Internal logic module fault ('84 turbo only)
orcamshaft sensor/distributor timing

42
Automatic shutdown relay circuit open or shorted
or Fuel pump relay control circuit
or Fuel level unit - no change over miles
or Z1 voltage missing when autoshutdown circuit energized

55
End of codes

I've already replaced the Camshaft position sensor and ALL the relays, but no spark. And I smell the fuel. What would you check next? UGH!

Voltage before and after coil?

I was afraid that the new computer wouldn't change your no-start symptoms. If I were you, I'd first do a very thorough inspection of the crankshaft position sensor connector and wiring harness. If you have a bad connector or a bad/burnt wire, that signal isn't going to make it to the computer. There is a connector on the firewall near the #6 injector I believe that is part of the CPS harness. DEFINITELY unplug and clean and reseat that connector. You make no mention of doing so; I have had first hand experience with this being a cause of a no-start. How handy are you with a meter? If you get stuck, I'd try to trace the crank signal all the way through the wiring harness to ensure it is making it to your computer. Have you re-done all the engine grounds yet? Another must-do, ESPECIALLY for your vintage XJ which have been known to have ground problems. My bet is you're going to find something real simple that is causing your no-start. Ask me how I know

Thanks, I did check ALL the connectors including the CPS going down to the bell housing... Originally, the ground wire at the neg batt terminal was a short causing arcing at the neg terminal from the PO. I thought that was what toasted my PCM after the new CAM sensor did not fix it. It's raining now, but as soon as I can, I am going to double check the cam sensor install (again) and after that, I am thinking about replacing the fuse block in case that NEG short fried something in the fuse block. It is my 97, and the truck is in "like new" condition, unlike my 88 LOL, and the only ground that looks shaky is that stupid braided strap, but I am going to redo all grounds because obviously I am running out of ideas.

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?