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Unread 04-21-2013, 02:43 AM   #1
Fenix85182
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ECM issues? Shut off while driving

Hey guys--

Quick background, I have an '89 I've dropped a 4.0 into. I drowned the first computer during hurricane Irene, and replaced it with a used ECM out of a Grand Cherokee. That ECM intermittently shut off (maybe once a week), so after a few annoying weeks, I replaced it with an ECM from a Wrangler, and it drove fine for a year and a half. It died during the snow-pocalyse back in February. I spent an hour scavenging a junkyard for another ECM, put it in...had issues with corroding starter cables, fixed those with marine terminals and welding wire, and it has been running fine since. Until today.

It randomly died (read: not able to be pop-started, fuel pump shut off), so I pulled to the side of the road. After rolling the key a few times, I got it to prime, started it, drove it for another 1/2 mile and it died again. Luckily, I have all the old computers (plus another new-to-me, used computer) in the Jeep. I put a different one in, it primed, then immediately died and hasn't primed since.

Is it just that each of the computers are from the 91-95 era and are just old and have dried out capacitors, or is there something else at play here? When I fixed the battery cables in February, I brushed and tested all the major grounds and didn't seem to have any issues. I want to believe it's the computers, but is there something else I'm missing?

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1989 Jeep Wrangler - 4.0 swap, 32" BFG M/Ts, RC lift
1997 Acura 2.2CL
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Unread 04-21-2013, 06:57 AM   #2
Mark05059
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are you getting any codes? Is the CEL light coming on when you turn the key forward (should come on for a few seconds and go off) The starting and then dieing isnt usually the ECM. Its usually the crank sensor.
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Unread 04-21-2013, 09:50 AM   #3
Fenix85182
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I'm going to troubleshoot today, as it's sitting on the side of the road.

I'll be sure to check the resistance of the CPS, though, thanks.
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Unread 04-21-2013, 01:44 PM   #4
Que89YJ
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Usually a coil or crank position sensor if you hear the fuel pump prime che k for spark at the plugs.
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Unread 04-21-2013, 03:07 PM   #5
Fenix85182
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Isn't the crank sensor tied into the ASD relay, so no signal=no fuel pump prime?

Anyway, I bought a CKP on my way to the Jeep this morning. Before putting it in I ran engine codes on two different computers I had: 12,33,55 on one, 12,33,35,36,73,55 on the other. I couldn't get the fuel pump to prime. When I put the new CKP in, it fired right up.

That old CKP was purchased in December of 2010 when I did my swap and has less than 10K miles. I think I'm going to find a good deal on 5 of the fuel injection sensors I put in for the swap as genuine Mopar parts. I didn't drop in the 4.0 to keep breaking down due to poor quality parts.
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Unread 04-21-2013, 05:56 PM   #6
Que89YJ
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The cps inputs to the ecu for can crank sync. without it the ecu will not fire the plugs.
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Unread 04-26-2013, 01:27 PM   #7
Fenix85182
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So, it ran fine for a week and died again. Luckily I was close to home and got someone to pull me with a truck.

I pulled the CPS, so now I have three in my pile. The latest one from the parts store, the one I pulled out last week from the parts store (has 2 years and 10,000 miles on it), and a CPS from a 4.0 with an automatic transmission I bought in error many years ago. Anyway, trying to test them with my multimeter, these are the readings I get:

Auto: 37.81 M ohms
Old: 4.80 M ohms
New: 26.26 M ohms

Of course, my old multimeter died and this is with a new auto-ranging multi from Craftsmen. The reading is supposed to be 200 +/- 75 ohms, but none of these seem to read in that same range. It doesn't seem likely the readings are actually in mega ohms.

Otherwise, I tried pulling engine codes on the Jeep when it first died, as the CEL is on steady. When trying to pull the codes, the CEL doesn't flash, it just stays on steady. What does this mean? Also, rolling the key to the on position, I can't get the fuel pump to prime. When the Jeep has died before, once the fuel pump primes, it definitely starts up.
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Unread 04-26-2013, 02:34 PM   #8
Que89YJ
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You need to check the light and see if it is bad wiring. If it isn't then the ecu is probably bad. Disconnect the battery, remove the cluster and disconnect it. Read the pin on the cluster connector for the cel to ground and see what the resistance is. It should read high. If it reads low then you need to disconnect the ecu and recheck. Fix the cel first. If it is on constant it indicates a bad ecu micro.
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Unread 04-26-2013, 03:34 PM   #9
Fenix85182
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I know the CEL is good--I only hard-wired it a few weeks ago when I first started having this issue. I'll check it again, but it's been reliable the last few months and nothing has changed.

Otherwise, I initially thought it was the ECM, but when I first swapped in the new CPS last week, I plugged in three different ECUs I have to see which would work and pulled the codes to see what each said. One is seemingly dead (no CEL lights at all, wouldn't prime), another had codes 12,33,55,76, but the fuel pump did prime and motor did start. The last ECM I put in (and left in) bad codes 12,33,55 and the fuel pump primed and motor started. When I was waiting to get towed back, I swapped the other computer in to see what would happen, but both show the same symptoms--no fuel pump priming, steady-on CEL and not able to pull codes.

I find it hard to believe I'd have toasted two different ECMs in one week.
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Unread 04-26-2013, 04:03 PM   #10
Que89YJ
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It sounds like you have the Cel wired wrong. You need to check and make sure the ecu is giving it ground when it needs it. The cluster provides power and the ecu provides ground for the light. Pull the ecu connector and the light should go out. If it doesnt then the wiring or the cluster is bad. You have 3 ecu's? You know all the ecu's were good?
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Unread 04-26-2013, 04:26 PM   #11
Fenix85182
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Haha I've actually had 5.
First: died in hurricane Irene
Second: died intermittently due to bad caps
Third: seemingly died during the snowpocalypse in February, but has since been alright
Fourth/fifth: I bought as replacement for the third. One was ordered online and when it didn't arrive in a few days I went to a junkyard and pulled another.

The CEL is hard-wired with the switched ignition wired for the radio also powering an indicator LED I got from parts store. The ground is the black/pink wire on the ECM. As of this morning, it was working fine ...came on for five seconds when keying on, stayed off during driving.

I'll scope it out again tomorrow.
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Unread 04-28-2013, 09:10 PM   #12
Fenix85182
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I haven't touched the Jeep yet, but I did download a FSM for a '94 and have been reading about the open/closed loop modes of the PCM.

When the ignition it keyed on (not start), the PCM does a few things:
1: pre-position the IAC
2: uses MAP to determine atmospheric pressure
3: monitors engine coolant temp
4: monitors intake manifold pressure
5: monitor TPS
6: the ASD is energized for 3 seconds
7: fuel pump is energized for 1 second through the fuel pump relay
8: the O2 sensor is energized through the fuel pump relay, but the information is not used by the PCM during this open loop

So, the no-start I'm experiencing has the fuel pump not priming and doesn't involve the CPS. That means I bought and threw a useless part at the Jeep, and the old CPS is probably still in good shape.

I'm going to hook up one of my supposed functional PCMs and test the different voltage feeds to see if they are supplying the necessary 8v and 5v. I'm following your logic, Que, about the CEL being on steady--either it's wired wrong, or the PCM is fried. I was certain when I swapped the 4.0 in that I wired up the PDC correctly so the ECM wouldn't get any bizarre grounds/surges and everything came through the PDC and was thus behind the appropriate fuses.

I wish the FSM has information on testing a PCM, or even explained the 'always on' CEL I've been experiencing. Maybe it means it's fried, or maybe it means there's a bad ground. I suppose I would need DRB scan tool to actually read the data points, but would it show anything if the PCM was fried? I'm nervous about buying another one, because if it I did just kill two functional PCMs, I would end up just killing a third.
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Unread 04-28-2013, 10:14 PM   #13
Que89YJ
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The ECM is bad if you have a solid light and power at pins 3 and 9. There is no way you are responsible for the dead ecu's. I think you are starting out with bad units. I would try and repair the bad caps in the one ecu and try it out. That is the easiest and cheapest way.
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Unread 05-04-2013, 01:13 PM   #14
Fenix85182
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After talking with Kolak/Nick (Kolak@aol.com) who seems to be revered on Jeep sites for getting reliable PCMs, I've decided to buy a Mopar rebuilt PCM through him. He has been great to do business with and I'd highly recommend him. I'll be getting the PCM in the coming week. In the meantime, I'm going to clean up the ignition spaghetti under the dash and make sure everything is wired clean and through the bulkhead. After having radio issues, I got lazy and ran wires through the firewall and I'd like to rule all those out as an issue before I install a new PCM. I'll post back when I've made headway with the Jeep.
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Unread 06-16-2013, 03:44 PM   #15
Fenix85182
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Ok, so after weeks of waiting for the Mopar refurbished PCM, I received it in the mail today and the Jeep started right up. I think I'm going to reroute the PCM harness to inside the firewall to take care of $400 expense.
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