I picked up my truck yesterday from the shop. They did a motor swap for me (took 3 months) they got the motor in 3 days n running but the tach doesn't work n it being an automatic that is a bit of a problem. So it pretty much sat there for three months while the "tried to figure it out".
Ok now to the problem. I have a 2001 dodge 2500 24 valve with automatic trans, the motor was bad when I bought it in 06 it had a tic n eventually the number six cylinder went bad. I found a 99 24 valve out of a 3500 standard trans truck. The motor starts and sounds great all the gauges work except the tach. When I start the truck the tach works for 6 seconds while the computer does the check then it quits. The truck will still limp around but it won't shift gears without the tach. I'm not sure what the issue is the shop thought it was a issue with the wiring harness but idk about that since the tach does work briefly when you start it. I read one thread where it sounded like mb the cam sensor isn't reading because the hole in the timing gear housing may be .125" different. Before I go down that rd I would like some more input. Thanks so much.

It does show a check engine light but the readout in the gauge cluster was broken when I got the truck so until I get a plug in code reader I can't see what it is.
Another piece of possibly helpful info I just thought of. I believe the engine wiring harness and sensors that are in the truck now are out of the 99 truck but I am using my 01 computer because the 99 was not an automatic

 

Yeah I think its gonna be a ligistics issue ( like , input voltage Vs signal voltage, or ohms )...

Gonna have to get down to figuring out what the ECM is or should be looking for as far as the OEM signal ...

Keep posted ..