I have a 1996 Dodge Ram 3500 w/ 5.9L Cummins,5spd manual transmission. The other day I started it up and found that the tach, speedometer, alternator, A/C, and CheckEngine light had stopped working. I checked the alternator ouput to verify that it was not working, it wasn't. I removed the instrument cluster to verify that the CheckEngine light bulb was good. It was. Changed it anyway. Since the CheckEngine light wasn't working and I couldn't do the key off/on sequence, I took the truck to a local mechanics shop to have them scan the computer codes. They were unable to get any codes due to lack of communication. The tech told me that the PCM was probably not getting any power, ground or something similar. He gave me an electrical schematic for my truck and I went to work checking the wiring. I checked the full time power (red/wht), Ignition power (lt.grn/blk) and got a good 12v. I used a testlight and a multimeter to load test the circuit and found no voltage drop. I checked both ground wires (blk/tan) in the same manner using the red/wht as a source of power. These tests were done at the PCM connector with it disconnected from the PCM. It seems that there is no problem with the power/grnd as far as I can tell. I also checked the two 5v outputs (org and ppl/wht) for the various sensors and instrument cluster. This is the only measurable problem I could find. Instead of 5v ouput I was only getting about 1.5v.
Well I took the chance a got another PCM and installed it. Guess what? Same Problems!!! :'( All the way down to the 1.5v outputs where the 5v is supposed to be. I even disconnected the transmission solenoid and rechecked the 5v outs and still only got 1.5v. If anyone has any ideas on this I would greatly appreciate hearing them. I don't want to blow any more money on this. (anybody need a good pcm?? :-)

 

Ok then, sorry you are having the problem and I am here to do what I can. I have a couple of questions first. Please bare with me on some of these questions.#1 What voltage are you getting off the batteries? #2 You said the alternator wasn't working. What do you mean not working. I believe the battery charging circuit depends on the alternator or were you mening the instrument cluster light wasn't working? #3 You siad you did a load test and there was no voltage drop. Please describe how you did that and the results you got. Now with the questions out of the way. We can get down to buisness. Need to know battery voltages and the alternator output. I have some prints but they are vague at best. While your getting those readings take a look at your power distribution center. Check for signs of overheating or corroded/dirty contacts. Check fuses and look at the relays for swelling, bulges and overheating. I see the the 5 volt power supply comes from the PCM and the common cable is C135. The signals on this plug are the park and neutral switch and the engine speed sensor. The 5 volt goes out to the engine speed sensor on a white lead A17 @ PCM and pin 1 at the engine speed sensor. If the 5 volts is being shunted to ground here, it may be tasking the volt power supply enough to kill the general lamp driver circuit. I is also tied to the starter motor relay. Relays can stick/short and also shunt your voltages to below acceptable levels. I have also read a post on here about the engine speed control being under the battery box on the drivers side or the battery temp sensor is under ther. One of them is under the drivers side battery and it had problems which caused all kinds of weird stuff. Be it that the speed control comes in on the same cable as 5 volts, I'd check there. I will get back to you with more as I get it. I think this is probably enough as it stands for now. Get back to me with anything you find and the answers to those questions. I will continue to look into the prints. You may aslo get a buddy to look at that 1.5 volts while you disconnect fuses and relays or connections to sensors. When the 5 comes back, you have the culprit. Good luck and I hope I have helped in some way.

 

Alittle more information to add. The PCM controls the alternator and it needs a signal from the ASD(automatic shutdown device) for charging and the crankshaft position sensor. OK I am making some head way here but it gets to be all inclusive. The PCM and the ECM are subject and so are a lot of the sensors. Hated like heck to write that. The PCM is looking at the IAT,ECT,CMP,CKP,VSS and a few others so it can tell the alternator how much to put out. It also needs a signal to tell it the engine is moving, which I don't think you have a problem with cause you said you took it in to be looked at. If the computers don't see the engine RPM's, you wont start(ECM I believe). The book keeps telling me to have the ECM checked on a bunch of these signals but I believe if it were the ECM, you wouldn't run. The PCM dosn't manage fuel but does work with the ECM(5 volts all over the place). I feel like this is getting into a great big barrel of monkeys and that I aint helpin any. So, lets check some things off the list and keep in touch on this. Check for low voltage at your sensors(0 to 1 volt) and then bar over the engine to see if it goes up high and then back down(for CMP and CKP). This will happen quickly and I don't expect you to see the entire 5 volts but you may. You may even see as high as 8 volts. The older trucks used 8 volts instead of 5(either 95 or 96 was the shift). Then start loking into those items inmy earlier posts. We need to find where we are dropping(shunting) the voltage to ground or why the signal isn't being generated. You wouldn't live anywhere close to Corpus Christi texas or Lufkin Nacodoches area. I will be in east Texas below Henderson for a couple of days and would be morethan happy to bring my tools with. If you are in south Texas, Thats even better. I make house calls for coors light and of course no beer brands will be turned down at the end of the day. I don't work and drink, too many mistakes to be made while buzzed. Get back to me please, I want to help you if I can.

 

Any luck or updates! ???

 

Thanks SlighFoxs3500 for your reply. What you said about something being shorted to ground was correct. I realized yesterday afternoon that a ground could cause the supply voltage to drop to less than 5v. Yesterday I had disconnected everything that was powered by these 5v supplies except the CrankPositionSensor. (duh) This morning I disconnected the CPS and whammo! my CheckEngine light came back on! I was pretty sure I had found the problem. Just to be on the safe side I checked the wiring to make sure it wasn't shorted to ground only when the CPS was connected, it wasn't. I went to Houston and got a new CrankPostionSensor ($88) and installed it. It fixed all the problems! It's hard to believe that a sensor is able to shut down the whole PCM to the point you can't even get codes from it :-
To do the load test on the circuit I disconnected connectorA from the PCM and attached a 12v light bulb and my voltmeter in parallel to the full time power and switched ignition power (separately) to the ground wires in the connector. If the connection was bad enough the voltage would have dropped well below 12v. It didn't so I was reasonably sure the power and grounds going to the PCM were both good. It seems some of the wire colors you mentioned are not the same as what's on mine.

1. alternator not charging
2. a/c clutch not engaging
3. check engine light would not come on while starting or running
4. gen light would not come on while starting or running
5. tachometer not working
6. speedometer not working

Now I know this all means bad Crankshaft sensor

 

Thanks for that update.
Definately going to "file" that one away for future reference.

 

Awesome and glad to have been of some help. Sounds like you would have gotten to it. Always the snake that gets you. I will also remember this one. Your load test was similar to what I had you doing but wanted to make sure.