Hi All,

Dreaded issue I'm sure everyone has heard about. I am hours in to diagnosis and unsure if I am even on the correct track. Please excuse any ignorance or missed steps, I am doing the best I can with the limited skills I have.

Backstory - Truck sat idle for a few years. Runs great. Tachometer is the only gauge that works. I have the "Wait to Start" light. PCM was sent out and rebuilt, signs of over-voltage. New alternator was put in, and the in line fuse protection from this forum was installed. Went to start the truck and I briefly saw the fuel gauge work with key on power. Grabbed a new battery as the old wasn't enough to crank and I am back to the original issue again. Alternator is not charging.

I have checked and wiggled all PCM Connections, did the dash test sweep, checked passenger side grounds, unplugged abs, oil pressure, ac, and every other sensor I can find on the same system. I checked the white connection box under the clutch pedal. I am lost.

OBD shows a code for the communication error. Dash has 920,921,999 codes. I don't even know what to check for the next steps, but I am not giving up on the thing. Could I have blown my rebuilt PCM on the initial key on start?

Thank you in advance for any advise or guidance.

Jack

Need more detail on the truck history and sequence of occurring events.

Was the truck running fine prior to being parked for a few years? Was the PCM repaired before or after the truck was parked? You mentioned, "PCM was sent out and rebuilt, signs of over-voltage." What was the over voltage value?

Was the truck parked inside or outside for the years it was parked? If outside, rodents could have been chewing on wires.

Do you have a repair manual to use for wiring diagram information?

Please provide as much detail as you can.

• John

Edited March 21Mar 21 by Tractorman

Thank you so much for the response.

Truck was running fine when parked. Originally parked as it was needing paint, never for a mechanical issue. It has been parked in the garage/driveway for the past couple of years as life got away from me. Last started and ran (with gauges) roughly Sept.2024. Before then, arguably the most reliable vehicle I own.

Over-voltage is my novice speculation as some signs were alleged by the PCM rebuilder. Their notes were as follows:

"We found your computer had an old power supply which was not creating the correct voltages.There was also some small physical board damage that needed to be repaired.The tech replaced the power supply, capacitors and voltage regulators. The computer is now fully functional and responding properly.Common to fail circuits were also replaced while we had the computer open on the bench to prevent future failures."

I have been trying to trace off of a PCM diagram from AllData that a friend who works at a shop provided me. Other online forums have directed me to try unplugging different sensors. I found mopar1975man had replied to just about every forum around, and figured this would be the place to try. I have checked every loom and wire I can see. How well I checked them is hard to say. I did not find any signs of rodents, and no obvious rubbing.

Truck is fairly clean, no blown fuses, and little to no corrosion issues. Never driven in winter or snow, no salt. I have some of the interior apart for a separate project, but have no clue if or how that could be related.

Please let me know if I am missing any other helpful details.

Kindly,

Jack

You listed your truck as 1998 2500. A 1998 truck has a 12 valve engine (P7100 injection pump), a 1998.5 has a 24 valve engine (Bosch VP44 injection pump). Is your truck a 24 valve?

• 2 wheel drive?

• 4 wheel drive?

So, the truck ran fine when it was parked and ran fine in September of 2024. What prompted you to send out the PCM for repairs? I am trying get a picture in my mind of the time line of what was wrong and when it was wrong, and what you did and why did it, as you progressed.

4 hours ago, Jacksbrokengarage said:

Grabbed a new battery as the old wasn't enough to crank and I am back to the original issue again. Alternator is not charging.

As in,"a new battery", not "a different battery"? Are both batteries connected? Are both batteries the same model and the same age?

You can download this 1999 service manual (assuming your truck is a 1998.5 year).

• John

John,

First and foremost, thank you for even taking the time to read and help. I appreciate it. I apologize for leaving out relevant details inadvertently.

Truck is a 1998.5 24 Valve. It does have 4 wheel drive, and is a 5-Speed. Went to fire up the truck in October of 25 to finally get another thing done. This is when i noticed the issue. The PCM getting sent in was prompted by dead dash, no charging, and inexperience. Forums pointed towards PCM controlling alternator charge and dash, and I chose that solution which in retrospect was likely a bit hopeful and premature. Then it sat again over the cold winter, and today was the first nice day to get out there and try to get it going.

The new batteries were out of another truck of mine that is currently my daily. Same age, same size, both with 4/25 date codes. I will review the service manual tonight.

Kindly,

Jack

Thank you for that information and the quick response. There should be some very helpful wiring diagrams in the manual.

When you are checking connectors, be sure to inspect that there are no pins pushed partially back into the connector body. This sometimes happens and it is easily overlooked.

Not sure of what work you are doing inside the cab, but there are a couple of locations for ground connections for dash components. You will want to find those grounds (shown in the manual) and ensure there is good electrical connection at those locations.

You also should do a thorough inspection of all the main battery cables and connections, including the positive crossover cable. The alternator charges the driver side battery, but senses voltage (state of charge) from the passenger side battery.

• John

John,

Spent some time going through the manuals today. Still scratching my head. Before I dive down another rabbit hole, let me know your thoughts on the following process.

1. Replace all battery terminals.

2. Pop in two brand new batteries

3. Focus in on the passenger side ground. I believe that one leads to the connector in the fender by the battery tray and up to the negative terminal.

I didn't identify anything from the wiring diagrams that was damaged. With my belief that I saw something work when I put the PCM in yesterday, the only thing I can think of that would explain the brief moment of the dash working is I bumped or moved something wiring related that is not working when in resting position.

I also just realized I no longer have a grounding strap between the cab and the bed. I will replace that as well and report back. Its supposed to get cold up here again so we will see when I can get back out to keep investigating.

Jack

Steps 1 through 3 are good. When checking battery cables, make sure there is no corrosion creeping into the cable under the wire insulation. This corrosion is difficult to see and it commonly happens with aged crossover cables.

There a wiring modification called the "W-T Ground Reference" mod. If you have not done that, it is a good thing to do. It takes care of some potential ground issues on the driver side of the engine. This will not likely correct your issue but it is a prudent modification to do as quality ground circuits are necessary for reliable computerized engine control components.

3 hours ago, Jacksbrokengarage said:

With my belief that I saw something work when I put the PCM in yesterday, the only thing I can think of that would explain the brief moment of the dash working is I bumped or moved something wiring related that is not working when in resting position.

This could be a beneficial test for diagnosing. What you accidentally did is called a "wiggle test" in the automotive industry. Perform this test in various locations with a helper watching the dash while the ignition key is turned on, or while the engine is running. You just may quickly find your issue.

• John

Update:

We finally had some ok weather. I got 2 fresh batteries again to be safe. New terminals on all wires to batteries. Checked grounds, and did the wiggle test. Still no dash, but OBD port actually gave more than one code this time.

P1698 - No trans bus (same as before)

P0237 - Boost pressure sensor low (new)

Wouldn't this mean some communication is coming from the PCM if a new code has shown up? I have checked all fuses again, the white harness connector box under the clutch pedal, and am continuing to chase down every bit of wiring I can find. The local dealer wont even look at it so I don't even have the option to give up.

Any other procedural steps would be appreciated. Weather is supposed to turn here again thanks to a lovely Minnesota spring, but I'm going to keep trying to figure it out. Attached is an image of lights on when the truck is running in case this helps anything.

Jack

On 3/22/2026 at 2:19 PM, Jacksbrokengarage said:

Focus in on the passenger side ground. I believe that one leads to the connector in the fender by the battery tray and up to the negative terminal.

Have you seen this article about the PCM grounds?

1. 24 Valve 2nd Generation - Electrical PCM Ground Splice Repair

It says "NOTE: Do this after the other modifications have been done or you will lose the ECM, VP and grid heater grounds. "

What other modifications do I need to do prior to trying this?

Jack

8 hours ago, Jacksbrokengarage said:

What other modifications do I need to do prior to trying this?

The "W-T ground wire mod". This moves ECM, VP and grid heater grounds from the left battery to the right battery.

2 hours ago, IBMobile said:

The "W-T ground wire mod". This moves ECM, VP and grid heater grounds from the left battery to the right battery.

Correction - This moves ECM, VP and grid heater relay grounds from the right battery to the left battery.

From W-T:

• John

Thank you gentlemen. I will try and get these knocked out over the next few days and report back.