So its been happening for awhile now and Ive read quite a few threads on the o/d and lockup doing the same thing but in all those threads I never seen one that said about the tach dropping off at the same time the issue is happening. Any know where I could look? Im a first gen guy so I don't know anything about theses 2nd gen trannies. Its a 96 3500 with an auto obviously tranny rebuilt less then a year ago

 

Most likely its the crank sensor, the PCM uses that to determine wether the truck is running or not. Its either not gapped properly or its going bad.

 

any way to tell for sure and or test it? sometimes the truck will work fine, other it will do it off and on and just yesterday it dropped off and didn't come back on until i shut off the truck and restarted it. today its fine

 

don't know for sure, when my engine speed sensor/crank sensor died my alt wasn't charging

 

My alt still works fine when it happens, just tach falls off.

 

Not unless you have rigged an external Voltage Regulator, Alt function is based upon PCM knowing the engine is running, no Tach signal, no way for Computer to know engine is running.

 

What I meant to say... was the gauge still works lol. It doesn't happen for long enough periods of time for the batterys to die. It also doesn't happen unless moving so I wouldn't be able to test the actual alternator while it is doing it

 

Check that the engine speed sensor (no crank position sensor on a 12 valve) is gaped properly, the wiring and connector are intact and clean.
If that doesn't fix it replace the ESS, available at any autoparts store.