I have a 1992 W250 w/5.9 Cummins. First the check engine light came on. A week later, the check engine light went out. Then the check engine light and the wait to start light would flash at the same time about every 2 minutes, and when they flashed the aftermarket tach would quit but start working again when the lights went out. 2 days later the check engine light stayed out and the wait to start light stays on while running but doesn't work and the tach quit also. I hooked up a scanner and I have no communication with pcm. Any ideas what could be going on?

 

The fact that you've got no communication with the PCM leads me to think you might have a bad unit, or something has chewed on wires. Where does the tach get its signal? Straight from a crank position sensor, or from a line off the PCM?

 

a line off the pcm

 

Then all signs are pointing to a bad PCM, but perhaps it's getting bad info from a sensor/wire somewhere. Gotta start peeling it back, check for hacked wiring, go from there.

 

I checked and all the sensors have a 5 volt reference signal, checked all visible wiring, and disconnected each sensor / 1 at a time / in case one was causing a short. no luck

 

Maybe someone with more experience with first gens will have better advice, but it seems that all signs are pointing towards there being an issue with the output side of the PCM.