I'm having issues with my Ram V10 and seeing if anyone here can help. The problem I'm having is truck has NoBus crank, no start UNTIL I plug in a cheap OBD2 scanner. Then NoBus goes away, gauges work and truck will start. Thru troubleshooting I checked all sensors and am getting 10.2v at every location instead of the necessary 5v. I isolated (cut) the orange 5v reference wire on plug #2 on PCM approx 8 inches from the plug. With OBD2 scanner unplugged wire reads 10.2v and with OBD2 scanner plugged in it reads 5.2v leading me to believe that PCM is not stepping down 12v supply to 5v. I ordered a rebuilt PCM and it is doing the exact same thing only NoBus does not go away on dash and truck will crank but not start. Are both PCMs bad or is something else going on?

 

Check the PCM ground, pin 31 and 32 on connector 1, black/tan wires. They connect to the front of the engine, so also check the ground straps from the engine to the frame.

 

I did check those while trouble shooting. All grounds were good both to the PCM and all grounds on engine and body. Also checked both 2.5v leads/pins i believe they were 28 and 30 on connector 1 or 3 (one closest to fender). Got a good 2.5v on those. 12v power going to pcm were all at 12.4.

 

Ok. 28 and 30 are the CCD bus, it carries digital signals on it so voltage is not fully conclusive but is shows there is no short or similar affecting the bus. The wire you cut goes to the transmission, the sensors on the engine get 5V on C1 pin 17 violet/white. You might want to do a similar test on this wire and see if the 10V come into the PCM there.

 

I admit I did not check that one, however, I was getting 10v at Cam sensor, TPS, IAC, Crank sensor etc. and all would drop to 5v when scanner was plugged in. Also, all sensors were disconnected at time of testing. I will do a similar test on that wire in the morning and report back.

 

Identical test on C1 Pin 17 Vio/WHT 5v wire produced identical results as C2 orange 5v wire. 10.6v on wire with scanner unplugged. 5.2v with scanner plugged in. Readings were same once wire was isolated (cut). Performed test with both original and "new" pcm.

 

That's what's puzzling. I've never heard of a failure where the PCM itself puts out too much voltage on the sensor supply and both PCMs show the same behavior. I'm still thinking the PCM has the wrong ground reference. You could try to put a wire to the negative battery post and touch the engine block and/or pin 5 at the OBD2 connector (black/tan wire) and see if the voltage drops down to 5V.

Edit: Don't try to start the engine if grounding pin 5 seems to fix it. The wire going there is too thin to carry the current the PCM needs when running.

 

Its puzzling me as well especially with it dropping to normal range once scanner is hooked up. Understood. I will try this when I get home later and report findings. Thank you.

 

Ran a 6ga. Ground wire from battery to engine block. No change in findings.

 

I really don't know what would be different with the scanner plugged in, as opposed to not. I could only see it being a grounding issue..... as with the scanner plugged in, the PCM gets an alternate path to ground.

Easy test.... Stick a wire from pin 4 to pin 5, in the diag connector under the dash, and see if sensor voltage straightens out.