Well, my PCM fried about six months ago and I'm still too poor to replace it; $600 from dodge and it's not rebuildable. I've got my lock-up AND overdrive on toggle in the console, haven't hotwired my AC (yet), and my charging system is being handled by an old school breaker point type voltage regulator (soon to be electronic).

Now what I need to know, since my tach and speedo are gone, is if the PCM samples the signals from the sensors or if it changes them completely before sending the signals to the dash. I have the service manuals that tell me what pins do what on the harness that plugs into the PCM (dad had an auto shop in Florida till 96 when we moved up to Wisconsin and started workin on tractors so those 6" thick manuals are all sittin in the corner of our new shop). If I knew it would work I'd take the time to go dig out the book so i could hotwire those too.

 

The PCM converts pulses from the engine speed sensor and vehicle speed sensor to varying voltage to run the gauges.

 

****... Thought so... I'll just make a dash then with all my guages in there... Cable drive speedo's are cheap enough, I'll thow my pyro, boost and trany temp guages in the dash while I'm at it. Know of any electronic tachs a guy can find? Or if there's a way to set up an Autometer tach?

EDIT: Oh and do you know the resistance range of the temp guage; like maybe an autometer emp guage will work?

 

Not answering your question, but a new PCM can be had from Rockauto.com for $218. (Wish I knew that when I bought one off ebay!) (Which, by the way, doesn't work once they're programmed for another truck.. )

I'm finally getting around to getting a new from for my truck so the **** ABS will work again. Finally have a nice set of Michelin's on, don't want to flat-spot these..

 

Are you sure its the PCM??? A blown crank senors can cause all your problems also and is very common. Check the crank sensor. Take it fram a guy with all the same problems and bought the PCM for 600 and then found out that Crank sensor casues the PCM to screw up. Crank sensor are cheap and easy to fix

 

A/C, speedo, tach, odometer, charging system, TC lock-up and overdrive are all out... Dad looked in our service manuals and deciphered it was the PCM and told me where to locate it over the phone (I was stuck on Camp Pendleton with no way to get back to Wisconsin). Took it out and took it apart (I have repaired electronics before and have a year in electronics classes), turns out there was an electrolytic capacitor in the upper left hand corner that had fried so badly that it came right off the board and incenerated the 1/8" wide printed copper circuit all the way back to the plug-in.

 

Dave is right, it could very well be just the crank sensor, it's wiring or even too much gap. What happens is the crank sensor tells the PCM that the engine is running. If the PCM thinks the the engine isn't running nothing electronic will work.

 

Wont hurt to check the crank senor maybe other things will come back

 

I just went through this on my truck. To check the crank pos sensor use a voltmeter to check that the sensor is getting 5 volts DC, the grey/black wire is the sensor signal, the other two are + 5vdc and ground. If you don't have 5 vdc it could be the PCM but could also be a shorted or broken wire.

If you have the 5 vdc supply then check the output of the crank sensor, you will need an oscilloscope for this. When the engine running and the scope connected between the grey/black wire and the ground wire you should see 2 pulses per engine revolution. When you rev the engine the pulse width will narrow and the frquency will increase.

If you have 5 vdc supplied to the sensor but see no pulses with the scope then your sensor could be bad or require adjustment or you could have a short to ground on the grey/black wire somewhere between the sensor and the pcm or from the pcm to the tach. This was the problem with my truck, the grey/black wire was intermittently shorting to ground.

 

dave, im having the exact same problems; no 1st gear, OD, lockup, speedo, tach or charging. all my fuses check out fine, i put new brushes in my Alt and added sheilding to my TPS signal wire and the Transmission control portion of my harness. before this, i was having OD and lockup malfunctions as described in the TSB.

i took some readings with the ignition in RUN position, heres what i got using my DVM

PCM side of harness for Crank Postion sensor.
voltage readings
Gray/black; 12v
Black/Blue; 12v
Violet/White; 11.8v

continuity readings
Gray/black to Black/Blue; 239ohms
Gray/black to Violet/white; OL
Gray/Black to Battery Pos; 312ohms
Gray/black to battery neg (or block); OL

Black/blue to Violet/white; OL
Black/blue to Battery Pos; OL
Black/blue to Battery Neg (or block); OL

Violet/white to Battery Pos; OL
Violet/white to Battery neg (or block): OL

so, something is wrong obviously. my voltage from the to the CPS is too high. i have voltage in all 3 wires checking from the engine harness unplugged from the CPS. some wires must be shorting together inside the harness. I do recall seeing the Violet/white wire splicing together and creating a common circuit. is this supposed to be a common ground?

im just guessing here, but i think my CPS signal wire to the PCM (Gray/black) has shorted with a 12v power supply wire inside the main harness. i think my 5vdc power supply wire (black/blue) has shorted against the a 12v supply wire too, which would explain the continuity between both and the same voltage signal.

thoughts, ideas?

im going to remove my engine harness again and inspect all the wiring.

 

If you have a 12v supply side short to the 5v signal supply wire it should vary with alternator output and run at about 14v. If it's staying at 12v you have a PCM problem.

 

There are places out there that will rebuild your PCM and even repair damaged traces on the board. That being said, $218 for one from RockAuto is probably about the same price as having yours repaired. Early 90's Mitsubishi's had a lot of problems with the capacitors inside the PCM leaking and damaging the board. I'd be willing to bet that one of the guys that repairs them can repair the Dodge ones just the same.

John

 

bill... what if the 12v short to the CPS circuit is preventing the PCM from seeing a crank signal? then the PCM never knows when the engine is running and in turn, never closes the contacts on the voltage regulator to ask the alternator to begin charging?

 

Good thought. I guess then you would be sort of screwed.

One thing I do know- I've replaced two PCMs on different rigs and it didn't cure the problems. Both times it ended up being connection problems. One was from the owner enlarging the connectors at the PCM with voltmeter probes. Since then I've always swapped with a known good PCM from another truck, problems still persisted. Have yet to come across a bad PCM.

An outfit in Texas I contacted about rebuilding PCMs told me the Dodge PCMs rarely fail because they have internal safeguards and as result they had no cores to rebuild so turn around was quite long as they would have to rebuild your old one.

 

where are you seeing this PCM on rockauto.com??? im on there now and it says there are no parts found... unless you are not talking about a PCM for a 12valve w/ 47RE.