Engine/Transmission (1994 - 1998) Blown PCM?

[Vaughn MacKenzie]

I think I have a blown PCM, but thought I'd post this and see what thoughts you guys had.



I had just gotten a SBC installed & NV4500 upgraded and was on my may home when I noticed the alternator charging to max voltage and GEN light coming on.



When I arrived back at work the battery was leaking from overcharging. I had to run some errands and fearing I would damage the battery I decided to try and remove it from the circuit after starting the engine. So I started the engine and disconnected the battery then took off. Well the voltage instantly slammed max voltage and about 1 block after taking off the engine suddenly died. I pulled over and when checking underhood I found the cause of the problem. . . the control wire to the alternator was rubbing on the bracket and shorted to it, making the alternator go to max charge.



After eliminating the short I fired up again and took off, but noticed none of my gauges were working and some idiot lights were illuminated (wait to start, water in fuel). Also after driving awhile I noticed the AC compressor was running constantly, no matter where the dash control was set--even turned off.



What I think happened was when I removed the battery from the circuit it removed the buffer for the alternator going to max charge, causing a huge voltage spike and killed the PCM.



I have a new PCM for a spendy $460 but hate to take it out of the box and risk it not being the problem and the dealership not taking it back. I cannot find a used one either, after having 4-5 wrecking yards do a nationwide search. They go for $100 - $175 used, IF I can find one. The PCM part number is P56040 940. They have lots of P56040 908s but none of my PN.



Oh I already replaced the alternator, still NG. Others have suggested the tack pickup, but I doubt that was it. . . the tach and alternator were working fine (other than overcharging) then suddenly this issue.



Thanks all, Vaughn

 

[Bob Wagner]

Vaughn it took me less than 10 minutes to find this :-laf







1996

Engine Computer

Dodge Truck 2500 Series DSL,56040940 04696T $125 H and H Auto Salvage USA-NE(Cozad) E-mail 1-800-627-3006

 

[Vaughn MacKenzie]

Thanks Bob! I'm giving them a call.



Guess I should check back after starting a new thread huh?

 

[Vaughn MacKenzie]

Vaughn it took me less than 10 minutes to find this

1996

Engine Computer

Dodge Truck 2500 Series DSL,56040940 04696T $125 H and H Auto Salvage USA-NE(Cozad) E-mail 1-800-627-3006

Bob, H and H don't have one anymore, and they can't find one. Guess I wasn't quick enough. Got any more "connections"?

 

[Bob Wagner]

Bob, H and H don't have one anymore, and they can't find one. Guess I wasn't quick enough. Got any more "connections"?

I will have 3 or 4 more name by monday.

 

[Vaughn MacKenzie]

Bob I was talking with someone and they're telling me that PCM number means it's California emissions! Not sure if that's true, sticker on my truck says Federal, nothing about California.



Latest plan is I have someone to swap PCMs with for troubleshooting on Sunday so I'll know for sure. Then if for sure it's bad I'm going to order the P56040 854 and see if it works for giggles, since they're readily available, and see if it works OK.



Vaughn

 

[Bob Wagner]

I have 2 pcms here 56040 414ac 56040 199ac if it will help and the price is right :-laf

 

[Vaughn MacKenzie]

Well kicking this old post up again because someone emailed me about it, so I thought I'd finish the story. I did end up having to pay top dollar for the special Cal emissions PCM for my non-California truck and installed it, and wallah, all electrical problems solved. Started charging again, gauges came back, AC compressor resumed normal operation, no more randomly illuminated idiot lights. It cost over $600 for the darned thing back then (October 2005) but I have driven the truck about 75,000 miles since then and have had zero alternator or electrical issues (except for a starter, but it was worn out).

While I was waiting for the PCM to arrive I disconnected the power to the AC clutch and drove the truck for a week with no gauges. I only had to charge the battery once (had one battery), the nice thing about a 12-valve is low electrical draw (try driving a modern gas vehicle any distance with a dead alternator!!).

The thing that sucked the most about this experience (other than having to pay $600 for the PCM) is this happened just before heading out on a trip with friends to Yellowstone (and the plan was to take this truck). Since it was out of order we ended up taking my buddy's '82 Blazer diesel. That was miserable. Especially wheezing up Western Montana grades at 40-50 MPH and having many Dodge Cummins pickups blast past you like you're sitting still.

Vaughn