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Bad Brains (PCM / power control module) & my 1995 F-250 4x4, 5.8, E40D

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Bad Brains (PCM / power control module) & my 1995 F-250 4x4, 5.8, E40D

 
  #1  
Old 04-18-2019, 06:24 PM
Christian Riggs
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Bad Brains (PCM / power control module) & my 1995 F-250 4x4, 5.8, E40D

I've had a nagging issue with the PCM on my truck.

I bought the truck last December (2018). PO had blown the head gasket, so he had the heads redone (resurfaced, new valves, etc.). The truck ran fine for him until one day it stopped firing on 2/3/6/7 cylinders. He put a new MAP sensor and EGR valve on, and got frustrated after a while and sold it to me as-is.

I replaced plugs, wires, distributor cap, rotor. Tried a new ICM (injector control module), which made no difference, so I returned it. At that point, I took it to a reputable local mechanic and paid them $200 to diagnose the issue. They came back to me that the PCM (power control module) was the problem. So I ordered a remanufactured one from O'Reilly. Installed it and no fix to the issue. Returned it. Called the mechanic who swore up and down that the PCM was the issue, and they asked me to order another one (said remanufactured PCMs have been known to arrive DOA). So I ordered one from Autozone (different remanufacture vendor) and popped it in the truck and she fired right up and ran great. I drove the truck the next day a few hundred miles, did some offroading, drove home a couple of days later - NO PROBLEMS AT ALL. Later that week, I drove it again (in a rainstorm) about 30 miles. The truck was running great.

A few days later, once the rain had cleared out, I went to run an errand and 3 miles into the drive, the 2/3/6/7 cylinders stopped firing again. Same exact symptoms to the tee.

I assumed that the PCM went bad again, so I ordered another one from Autozone. Installed it and no difference. Same misfire on 2/3/6/7. ZERO improvement.

I returned both the (presumed to be) bad PCMs, and picked up a used one from a local wrecking yard, same condition. Misfire on 2/3/6/7.

I did notice after installing the wrecking yard PCM, that the wire leading out of PIN 58 (which is responsible for the ground signal to 2/3/6/7 cylinders) was a little loose.

I'm now wondering if I have a wiring issue (shorted wire?), but don't really know how to properly diagnose what's wrong. I do have a multimeter and a tone tracker, and am hoping someone could provide a little guidance on how to properly check things out from here.

Another question that I have is this: Assuming that I might have a broken or weak wire somewhere in the loom, would that potentially ruin the PCM? Or is it more likely that my PCM is fine, but the wire is causing the misfire issue?


I really really want this sweet truck to be roadworthy and useful again. Can anyone help me figure this out?

Thanks so much for taking the time to read this.
 
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Old 04-18-2019, 06:39 PM
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Given the multiple "failures" of PCMs with the same symptom you are looking at a bad connection on Pin 58 as you theorized. I would crack apart the PCM connector and inspect that pin for signs of corrosion and/or wear.

How the EEC-IV Connector Comes Apart

If that checks out you can meter the circuit with a DVM then wiggle the wiring and watch for the resistance to change significantly. I would think the compromised wire/circuit is near the PCM connector.
 
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Old 04-18-2019, 06:43 PM
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Originally Posted by rla2005 View Post
you can meter the circuit with a DVM then wiggle the wiring and watch for the resistance to change significantly.
Can you tell me how best to meter the circuit? Since this wire is sending pulsed ground signals, I'm not quite sure what to hook up where.

Also, do you think that this has fried my PCM?

Oh, and THANK YOU for taking the time to provide some help, @rla2005. Much appreciated!
 
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Old 04-18-2019, 07:34 PM
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Put one meter lead on Pin 58, the other on one of the affected injectors "negative side". Wiggle the wire bundle and watch for varying resistance (implies setting the meter to Ohms). I really expect an open reading when wiggling the bundle.

If nothing conclusive turns up then put one meter lead on ground (negative terminal of the battery) and the other on the return side of the affected injector and repeat the wiggle test. I would also keep the PCM connector disconnected from the PCM. FYI..PCM is the computer (Powertrain Control Module). If you see a very low resistance value pop up you have a short to ground. Potentially that could damage the driver transistor(s) in the PCM and cause the repeated failures.
 
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Old 04-18-2019, 07:48 PM
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Originally Posted by rla2005 View Post
If you see a very low resistance value pop up you have a short to ground. Potentially that could damage the driver transistor(s) in the PCM and cause the repeated failures.
To damage the driver transistor(s) in the PCM it would be a shorted injector or the red #361 power wire in the injector harness shorted to the Pin 58 wire giving full battery power to the driver transistor. A short to ground of the pin 58 wiring would not harm the driver transistor as that is what it does.
 
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Old 04-18-2019, 10:26 PM
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After you check the Pin to Wire at the EECM connector, and the Injector to Pin 58 continuity checks without resolution;
Follow the Pin 58 Tan wire from the EECM Connector, you will come upon a splice or two.
Check the integrity of the splice/s.
 
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Old 04-19-2019, 06:38 AM
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Originally Posted by subford View Post
A short to ground of the pin 58 wiring would not harm the driver transistor as that is what it does.
But now you would have two sides of the transistor at ground potential. When the transistor is triggered on isn't there potential to damage the transistor?

Using the above scenario, shorted wire on Pin 58, is a moot point anyway since Fuse O would most likely open so none of the injectors would function.
 
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Old 04-19-2019, 06:56 AM
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Originally Posted by rla2005 View Post
But now you would have two sides of the transistor at ground potential. When the transistor is triggered on isn't there potential to damage the transistor?
A transistor gets damaged by too much current through it or when hooked to a higher voltage then it can take. A transistor does not have current pass through it from the base. The base just biases it on to conduct from the collector to the emitter in an NPN or vice verses if it is an PNP.

But it is a moot point anyway because the PCM does not use a transistor to ground the fuel injectors. It uses an 5 pin IC.



/
 
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Old 04-19-2019, 08:10 AM
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Thanks so much for all of you contributing thoughts here. I'll be checking the wiring today and will let you know what I learn. You guys are great!
 
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Old 04-19-2019, 10:23 AM
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On your issue I am wondering if my theory about my second ECM failure may apply. I mention it because it is a quick visual and stands to reason by my logic. How do your core support mounts look? Is the ground strap from the core support to the frame intact? My strap had been broken for a while and the core support mounts had given up the ghost since Christmas, the last bit of rusty metal finally broke away. Your computer is grounded to the core support up by the coolant reservoir. I know that proper mechanics and technicians trouble shoot and swap parts and hacks like me just replace whatever is on the list that may cause a failure. However, I imagine like my truck and most others your battery cables are likely original and the grounds could use cleaning and inspection.
 
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Old 04-19-2019, 10:28 AM
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Originally Posted by rla2005 View Post
Given the multiple "failures" of PCMs with the same symptom you are looking at a bad connection on Pin 58 as you theorized. I would crack apart the PCM connector and inspect that pin for signs of corrosion and/or wear.

How the EEC-IV Connector Comes Apart

If that checks out you can meter the circuit with a DVM then wiggle the wiring and watch for the resistance to change significantly. I would think the compromised wire/circuit is near the PCM connector.
That's interesting, I wish you had seen my thread. How in the hell do you get these 26 year old plastic tabs and connectors appart with out breaking everything?
 
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Old 04-19-2019, 11:59 AM
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NOTE: This comment is in response to @CamoF-250. I haven't yet had a chance to test the wires with my multimeter. Will update the thread when I've completed those tests.

Originally Posted by CamoF-250 View Post
How do your core support mounts look?
I'm not sure how to answer the question. I'm assuming you're referring to the PCM rubber gasket and two mounting bolts/nuts. See attached photo. I'm interested to learn by any of you if something doesn't look right or needs repair/replacement. See photos.

Originally Posted by CamoF-250 View Post
Is the ground strap from the core support to the frame intact?
I don't see a ground strap connected anywhere to the PCM mount area. See photos. Is something missing?

Randomly, I do have a ground strap coming out of the passenger cab firewall that has no home. See photo, and I'd love to know where that should be connected and if it's causing some sort of harm or problem by not being connected.


PCM pigtail with pins 58/59 removed. I was able to remove them easily by using an eyeglass screwdriver to first gently pry out the retaining clip (not shown), and then used the eyeglass screwdriver to gently lift the pin retaining tab while simultaneously pulling the pin/cable out from the back. Worked great, no damage to pigtail.

PCM installed in cab firewall. No ground strap to be seen. Is this a problem? The PCM mount has some signs of rust when the PCM is removed. Gasket looked good enough I think. Thoughts?

Pins 58/59 removed.

More of pins 58/59.

More of pins 58/59.

PCM pigtail with pins 58/59 removed.

Passenger side ground strap without a home. Where should this be connected to? Any damage being done by it not being connected?

Battery terminals - are they in good enough shape? Should they be wirebrushed or replaced?
 
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Old 04-19-2019, 12:29 PM
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Your problem I would guess is that the pin #58 does not make good contact with the pin in the PCM Computer.
That radio ground strap just bolts to the upper intake manifold.
Your PCM Computer is grounded at the body ground point at the passenger fender near the battery.



/
 
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Old 04-19-2019, 01:10 PM
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Originally Posted by subford View Post
Your problem I would guess is that the pin #58 does not make good contact with the pin in the PCM Computer.
I'll cramp these two pins so it fits a bit more snugly. Also, it appears that pin #59 (the white wire) runs cylinders 2/3/6/7 from the attached diagrams (hopefully this helps others in the future).

Question: Would adding a dielectric grease to the female pin connectors in this pigtail be a wise or useful idea?



 
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Old 04-19-2019, 03:17 PM
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Be sure to clean those pins and inspect the others closely. I think I see green corrosion inside of Pins 58 and 59 in your photo. Some state to use a little dielectric grease when reconnecting the PCM connector. If you do, use a very light coating.

Did you ohm from Pin 59 to the injectors?
 

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