2nd Gen Non-Engine/Transmission Electrical Woes...Maybe Bad PCM?

[Dave M]

There should be 2 12v w/ign off and a third with key on. If you don't have a manual, or access to one close by, I would consider alldatadiy.com. The $24 is worth the cost and it has good electrical drawings and troubleshooting guides. You might not be getting the 12v from ign switch or a bad ground. There are 2 ground posts at the radiator support area, 1 on each side. I had 1 so rusted the bolt had to be replaced and the area scrapped clean.



What is needed is to loosen and inspect/repair ground connections. Disconnect wires to Alternator and see what that does, short to ground will kill PCM

Thanks for replying. I do have tha factory service manual but it does not include the PCM diagnosis... thats another manual. I did look at the pinout of the PCM and had two 12 volt pins (one ign on and 1 ign off)and two grounds. I did not see a thrid on the connectors. I'll recheck the 12v and gnds at the connector. I was figuring on disconnecting the the alternator but looking for a procedure so I don't mess it up. There are 2 field winding posts (12v and gnd... both from PCM) and a big 12v cable and another GND. How do I disconnect them?



I grounded the GEN and MIL lamp driver outputs at the PCM connector and those lights came on (with key on) at the dash. So, I know the wiring is good to the dash.



Thanks,

Dave

 

[Dave M]

WHaaaaHOOOOO... ..... I found one problem. I disconnected the crank sensor and all the gage lights now come on with the bulb check with the old PCM. Now the generator is not charging.



I know the 12V and GND come from the PCM so I'll check that out. I don't know if the crank sensor is needed for the alternator to charge.



Dave

 

[DSiemens]

The crank sensor is most likely the problem. When the engine turns over and starts, the crank sensor sends a signal to the PCM that says the engine is running. The PCM turns on and it controls the alternator charging, speedo, odometer, tach, a/c. If the signal from the crank sensor is not sent... the pcm will not turn on. Been there and done that. If you have a multimeter that reads hertz (frequency), you can verify the crank sensor signal at the pcm. The FSM which you have will give you the pin # and spec.

Dan

 

[Dave M]

The crank sensor is most likely the problem. When the engine turns over and starts, the crank sensor sends a signal to the PCM that says the engine is running. The PCM turns on and it controls the alternator charging, speedo, odometer, tach, a/c. If the signal from the crank sensor is not sent... the pcm will not turn on. Been there and done that. If you have a multimeter that reads hertz (frequency), you can verify the crank sensor signal at the pcm. The FSM which you have will give you the pin # and spec.

Dan

If the crank sensor controls all that, especially the alternator charging, then that probably is the problem. I tested at crank sensor connector (the PCM side of course) 5 volts on the 2 outer crank sensor terminals and gnd in the middle with the connector disconnected from the crank sensor. I don't have a freq meter.



EDIT:I tested at both field terminals of the alternator with the engine running and got GNd on both. These two terminals are have signals directly from the PCM for control of the alternator field windings.



Thanks for the great info.



Dave

 

[DPapworth]

Hmmm -- the crank sensor must be shorting the 12v supplied to it by the PCM back to ground. This "takes out" enough of the PCM functionality to prevent the PCM from running the MIL light. Usually (in other vehicles) the various sensors are supplied with isolated power to prevent this sort of thing from happening, even in the event of a defect in a sensor.

This appears to be a less than optimal design from Dodge. The vehicles are too new to me to have noticed this; I should have checked the wiring diagrams before my earlier reply. Sorry.

Once the sensor stops shorting out, you'll have a PCM that works, but still no alternator, tach, etc. For those functions you need the sensor to actually work and send engine RPM info back to the PCM.

 

[Dave M]

Hmmm -- the crank sensor must be shorting the 12v supplied to it by the PCM back to ground. This "takes out" enough of the PCM functionality to prevent the PCM from running the MIL light. Usually (in other vehicles) the various sensors are supplied with isolated power to prevent this sort of thing from happening, even in the event of a defect in a sensor.



This appears to be a less than optimal design from Dodge. The vehicles are too new to me to have noticed this; I should have checked the wiring diagrams before my earlier reply. Sorry.



Once the sensor stops shorting out, you'll have a PCM that works, but still no alternator, tach, etc. For those functions you need the sensor to actually work and send engine RPM info back to the PCM.

This appears to be correct... except the PCM supplies 5 volts not 12v to the crank sensor but it sends 12V and GND to the field windings of the Alternator. The 5 volts is generated from the PCM which I suppose is isolated and it also has sensor grounds.



Don't worry about the mistake on the crank sensor controlling all this. I was helped by your comment on the spark at the battery terminals. That saved me a ton of "chashing my tail".



I thoroughly read the factory manual and looked at all the schematics and nothing mentioned or pointed out the crank sensor controlling the Alternator. It keep refering to a "PCM Diagnostics manual" which is not in the factory manual. If I had that I believe it would have enlighnened me on all this and would have saved me $500.



Oh well... I have a spare PCM now.



I just want to get this truck fixed cause my wife needs to do her monthly grocery shopping (I have 8 kids) and Saturday (tomorrow) my 13 yr old boy wants to go to Livermore to a Jr Rodeo to try his first time at riding bucking bulls.



At least now I think I know what the problem is. I do appreciate you guys spending time to help me out on this. I was lost on what to try next. I'll keep you posted as I go. It's not fixed jet.



Thanks all.

Dave

 

[DPapworth]

The PCM wants to sense engine RPM in order to send field current to the alternator. It does this to conserve battery charge if the key is left on while the engine is not running. It has the option of doing other fancy things like killing the alternator load at WOT for a few extra ponies at the flywheel, I dunno how they actually programmed it.

The way that is senses engine RPM is through the crank sensor. That's why she won't charge without the crank sensor.

As for 5 volts to the crank sensor -- that's the normal scheme in other vehicles. I'm surprised the MIL light won't work with that 5V shorted out, but this depends on the PCM design.

 

[Dave M]

The PCM wants to sense engine RPM in order to send field current to the alternator. It does this to conserve battery charge if the key is left on while the engine is not running. It has the option of doing other fancy things like killing the alternator load at WOT for a few extra ponies at the flywheel, I dunno how they actually programmed it.



The way that is senses engine RPM is through the crank sensor. That's why she won't charge without the crank sensor.



As for 5 volts to the crank sensor -- that's the normal scheme in other vehicles. I'm surprised the MIL light won't work with that 5V shorted out, but this depends on the PCM design.

Thanks for that explanation. I pulled the crank sensor and noticed the red wire is bare a little at the entrance to the sensor.



I cleaned it and all the grease off it and the damper and applied some liquid tape to cover the bare wire. Maybe it was shorted with all the oil on it. If it still doesn't work... it's time for a new crank sensor.



It a pain getting to the sensor to adjust it so I'm pulling the fan and fan cover to get in there easier.



Dave

 

[Dave M]

I did not remove the fan and shroud but was able to adjust it from above. The crank sensor is still bad. Gonna call Cummins to see if I can get one today.



Called Cummins and its $76 but they can't get it till Monday or extra bucks would ship to me tomorrow. They call it an "Engine Speed Sensor".



Napa will have it by 8AM tomorrow for $101 so I ordered it from them.



I hope this fixes it.



Thanks,

Dave

 

[DPapworth]

I finally looked at the schematics in the service manual.

The CPS (and VSS) get an 8 (that's eight) volt supply from pin 7 of the PCM. This is not the same as the 5 volt supply that the TPS, IAT, etc. get off pin 6. It isn't the main 12 volts from the battery either.

It is likely that the 8 volt supply comes directly off the main power bus of the PCM. It would not be as well isolated as the 5 volt sensor reference voltage, which can usually be shorted out with impunity.

If the 8 volt supply gets shorted out, it may be that the microprocessor in the PCM gets no power, in this circumstance the MIL light etc. will not work.

 

[Dave M]

WHAAHOO..It was a bad Engine Speed Sensor (ESS or CPS)

I finally looked at the schematics in the service manual.



The CPS (and VSS) get an 8 (that's eight) volt supply from pin 7 of the PCM. This is not the same as the 5 volt supply that the TPS, IAT, etc. get off pin 6. It isn't the main 12 volts from the battery either.



It is likely that the 8 volt supply comes directly off the main power bus of the PCM. It would not be as well isolated as the 5 volt sensor reference voltage, which can usually be shorted out with impunity.



If the 8 volt supply gets shorted out, it may be that the microprocessor in the PCM gets no power, in this circumstance the MIL light etc. will not work.

Well... . Those voltages don't appear on my factory manual schematics for my 97. According to my manual, the vehicle speed sensor (VSS) gets 5vdc from PCM C2-pin B31. I looked at all the pins on the PCM and don't see anything but 5 v and 12v. The PCM pin 8 on my PCM Connector PIN-OUT diagram are as follows:



PCM Black C1-A7 = Air Intake Heater Relay #1 Control

PCM White C2-B7 = Not Used

PCM Gray C3-C7 = Transmission Temperature Driver



MY Engine Speed Sensor (according to my schematic and connector pin-outs and verified by testing) gets 5vdc from PCM C1-pin A17 and the ESS gets GND from the PCM C1-pin A4).



The alternater (called generator in my manual) gets a "Gen Field Driver" signal from PCM C2-pin B10 and a "Gen Output" on PCM C3 -pin C25. The PCM has an internal voltage regulator that controls the charge output of the Alternator (or Generator in my case). Thats what I learned from the schematics of my truck.



EDIT: If you used a manual for another year truck other than a 97, then the exact details must vary from year to year.



By the way, you were dead right about the ESS (or what you refer to as VSS) being necessary for Tach, speedometer, Odometer and Generator to function. MT TRUCK IS FIXED!... thanks to your input and DSIEMENS about the ESS signal being needed for them to work. The manual was no help on providing this information. I needed the PCM diagnostics manual to determine that I guess... I have one coming in the mail.





WHAAAA... . HOOOOO... . I have a running truck again. The old PCM is good... it was the ESS (CPS) all the time.

Thanks alll for your help.



Dave



NOTE: The way I see it (correct me if I'm wrong), the ESS (Engine Speed Sensor) many people mistakenly call CPS (Crank Position Sensor). The Crank Position Sensor is the equivelant Engine Speed Sensor part on the gas engines. When I asked for a CPS at Cummins they could not find it. I closely looked at the schematics and determined it was called an ESS. When I called Cummins back asking for an ESS (Engine Speed Sensor) they found it. However NAPA listed my ESS as a CPS (Crank Position Sensor). I know... its confusing.

 

[DSiemens]

I am glad you have it fixed.

Dan

 

[Dave M]

I am glad you have it fixed.

Dan

Thanks again for the info you provided. I now have a Mopar Remanufactured PCM that I can't return so I guess I'll sell it on Ebay.



Thanks,

Dave

 

[KCloward]

My truck is doing almost the same thing on me, and having just bought it, it is very frustrating. I get maybe 13 volts on the charge instead of the regular 14 and my tach doesn't work but that is the only thing that has stopped working.

I've checked the ESS and even replaced it with another one and still no fix. I've yet to check the PCM but planning on picking some dielectric grease tomorrow after work and going through and cleaning the connectors. If this doesn't fix it I will find myself at a road block.

Also, my transmission has weird shifting problems, could this be due to the tach not working?



Thanks a bunch for the help that this post has already provided.



Bryce

 

[DSiemens]

I see that this is your first post. Welcome to the TDR. What year truck is having the problem? To get more exposure, I'd start a new thread.

Dan