Archived Bad PCM?

[vastan]

I am the original owner of a 1997 model year (manufactured in 08/1996) Dodge RAM 2500 4x4 with the 12 valve Cummins Diesel engine (111,000 miles on odometer). Two months ago I was driving and the tachometer stopped working and the voltmeter indicated that the batteries were not charging (everything else seams to work fine). Therefore, I replaced both batteries and the alternator. Nothing changed, so I checked the charge on both batteries and each was around 12. 9 volts. I returned the alternator to the auto parts store and the alternator checked OK. I washed the engine and checked all the wiring leading from the PCM to the relay box and all wires appeared OK. I even replaced the ASD relay even though the truck started fine and the old one seemed to work. I sent the PCM in to have it looked at. After receiving the PCM back and reinstalling it, both the tachometer and voltmeter (battery charging) worked for approximately 2-3 miles of driving before they went out again. I sent the PCM back to the company that originally fixed it and they told me nothing was wrong but they did update the PCM bios with a newer version. Before replacing it I checked the PCM connectors and everything looked good (no signs of corrosion or broken prongs). I replaced the PCM a second time and the truck worked fine for about 5 miles and then the tachometer and voltmeter (battery charging) stopped working again. Finally, I purchased an Innova 3160 scan tool and hooked it to the DLC to see if the PCM was at least working. The scan tool tells me it is communicating with the PCM for generic codes but when it comes to Chrysler specific codes it says “it is unable to establish a link with the vehicle”. Does anyone have any suggestions? Have I overlooked anything? Any help is more than appreciated.



Thanks,

 

[GAmes]

Welcome to the TDR. What took you so long to join?:-laf I hate to tell you this, but you spent a lot of time and effort looking in the wrong direction. If you look at the upper right quadrant of the engine crankshaft damper you will see the engine speed sensor with 3 wires coming out the top. It sends a message to the PCM that engine is turning which activates the charge circuit and the tach, among other things. Take a look at these threads https://www.turbodieselregister.com/forums/search.php?searchid=1105046

 

[Dieselnut59]

That is what I was going to say. Replace the crankshaft sensor. Easy task too!

 

[vastan]

Thanks so much for the advise. I will replace it first thing tomorrow and let you know how it goes.

 

[Dieselnut59]

Dis you get it put on? I have had to replace this on two of our service trucks. If you have not done it already, my trick when reinstalling it was to do a trick similar to what I was taught about the magneto/coil on old Briggs gas engines. Take a file folder and cut off a rectangle shaped piece to place between the pulley and the sensor when installing it. It is magnetic and will "pull" itself down against the pulley. The file folder will give it the necessary air gap. tighten the 13 mm nuts and remove the file folder shim.

I also noticed when replacing mine, the studs tried to come out instead of the 13 mm nut. The studs take a 10 mm to hold. They also retain the front gearcase cover.

 

[vastan]

To all,



I received the engine speed sensor yesterday and replaced it. I have put about 30 miles on the truck and everything seems to be working fine now. Thanks for the great advise! This was a great Christmas present.



Vastan,