[fn42]

Hey all
I picked up this 01 ram to plow with and its turning into a **** show
I bought the truck with a dead trans (PO tried to replace it and broke the pump on install). I got a used pump and got the trans in, the truck drives now but has no boost

the issue I think is that the MAP voltage glitches in and out. All the sensors do it, except APPS

I've tried unplugging all the sensors on the engine as well as the VP44 and I still get the voltage drops so I dont think any are shorting out.

I tried hot-wiring the 2 grounds on the ECM and the 2 power wires and still got the same result, tried jumping the sensor ground pin of the ECT sensor to the block and it made no difference.

I noticed if I unplug connector C3 on the PCM (at the firewall) all the issues go away. ECM voltage stays rock solid. The truck will start and run fine just has no trans control.

please help! I think I got all the grounds & have no idea where to look next

 

[fn42]

ok I think this is due to a fried PCM
I reviewed all the connections between the PCM and ECM
the wire "SCI TRANSMIT" on pcm connector C3 occasionally shorts to ground

I just cut it. Problem is solved. Only issue is now the check engine light is on, but the truck runs and shifts fine so I dont really care

does anyone know what the effects of running with this SCI transmit wire cut are? seems like it might just be diagnostics?

 

[bigfish95971]

No codes with a scan tool?

 

[fn42]

nothing relevant. p1693 on the dash and I forget exactly what was in the ecm. lift pump circ code (it has a fass), and intake heater codes (found the relays were missing)

I could never get the scan tool to communicate with the "TCM" maybe thats what this link is for

 

[bigfish95971]

ECM supplies most sensors with power and receives their signals.
Does wait to start light come on immediately every time you turn the key to ON?

 

[fn42]

yeah the WTS light comes right on

this SCI transmit line ("PNK/DK BLU") connects between the PCM, ECM, and obd port. It appears that if it gets shorted to ground the internal voltage in the ECM will drop by 4-5V

I think all it really does is allow the PCM to communicate with the OBD port. So by cutting it at the PCM all I did was remove the ability to talk to it (but this didnt work before anyways since the signal gets cut off when it grounds out)