No communication

Adrean Diagnostician Bakersfield, California Posted   Latest  
Question
Electrical
1999 Dodge Ram 2500 5.9L (Z) 4-spd (46RE)

so I have this truck here that starts and runs fine . Problem is it’s due for emissions testing and it doesn’t communicate with any scan tool , through global or vin specific side . I did some research and being This truck has two ecus for the engine .this is a manual transmission by the way . I’m a bit lost .. my can bus is fine I can communicate with each module in that network .. problem is I just cannot communicate with the powertran module on the right firewall , and the Ecm on the left side of the block . I looked into the sci line and on the receive line it’s 4.7 volts flat line and with key on engine off the transmit is .4 ...

the way I read this system works and by testing on a known good . Key on engine off my scan tool provides 12v on the transmits line and the module grounds it .. im thinking this one is shorted . I disconnect the ecm on the left side of the block and I get 12v now . So I replace it with a known good and same issue . Even replaced the pcm on the right side firewall and same thing .. any one ever run into this kind of problem?

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Rusty Owner/Technician
Oakham, Massachusetts
Rusty
 

Shot in the dark here but try unplugging the injection pump to see if that s get the network back.

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Adrean Diagnostician
Bakersfield, California
Adrean
 

I will try .. Ima try disconnecting all sensors see what happens .

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Albin Diagnostician
Leavenworth, Washington
Albin
 

Can you post the VIN number for this vehicle please? I have worked on a lot of Dodge pickups in my carrier and have never seen a diesel engine in a half ton pickup.

When it comes to working through a no comm problem on a Chrysler vehicle, I like to have my DRB3 scan tool at the ready, and a labscope to do the pinpoint testing with.

Since the engine will run, I would first start with the scan tool and verify it isn't a scan tool problem, then move on from there.

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Adrean Diagnostician
Bakersfield, California
Adrean
 

Sorry I was in a hurry it’s a 2500 I don’t have the vin with me ..

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Tom Engineer
Claremont, California
Tom
 

FYI we updated the vehicle for you. Soon, members will be able to edit their own messages. 

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Adrean Diagnostician
Bakersfield, California
Adrean
 

I have also tried multiple scan tools and aame problem .. also I have used a break out box and checked my sci line and it’s flat line at 0v... on a known good truck its at 12v and it’s pulled down when talking .. also this article published by auto Nerds explains it working in that similiar fashion . . When I disconnect the ecm I get the 12v on sci transmit line . But plugged in it’s at 0v 

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Albin Diagnostician
Leavenworth, Washington
Albin
 

With the ECM unplugged, can you communicate to the other modules on the vehicle with an aftermarket scan tool? I remember a few years ago being on a mobile job about like yours, where the ECM was defective, and there was a poor pin fit problem at the PCM. With my Snap On scan tool, I could comm with the PCM and other modules, but not with DRB3.

You mentioned you tried a different ECM with no success. Have you tested the powers and grounds to the ECM? Something is fishy here, since the engine will run, but the scan tool will not communicate.

Have you tried unplugging the injection pump? Those can do some funny things at times. Have you tested the 5 volt reference to the engine sensors?

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Michael Diagnostician
Holt, Michigan
Michael
 

So likely then you have a input to the pcm bringing the network down as you are correct should go from 12v to 0v as it talks. Does communication come back to the rest of the truck when its unhooked. I would venture to say its going to be a reference circuit that is shorted to ground. 

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