Ok so heres a tough one. I got my 96 12 valve, 5 speed, and it is doin weird stuff. Once its warmed up, tach will barely read, most the time it will only read 0. And along with this, brand new alternator will intermittenly charge, dash warning lights flash intermittenly (check engine, gen, wait to start, water in fuel) and all this happens at the same time. Ive talked to about 3 diesel mechanics, 1 specifying in cummins came out to the truck, and have turned up nothing but possibly PCM. This is very frusterating. Any input on your part is greatly appreciated!

 

My guess would be a bad ground somewhere especially if its erratic like you say it is. That's my best guess. Does it look like anyone has rigged the wiring funny? i know my 95' was a nightmare because the owner before me was a complete retard

 

That is textbook for the crank position sensor going bad or being adjusted a little too far from the harmonic balancer. The pcm will not let the alternator charge if there's no signal from the cps. It also won't let the a/c work. It's fairly easy to get to-it's mounted on the front of the timing cover, held on by 2 10mm nuts. You can loosen those nuts and wiggle the sensor around. The way I set the gap on those is I get a plastic card out of my wallet (driver's license will work, don't use the strip side of a credit card as the magnet in the sensor might erase the card) and I put it between the bottom of the sensor and the balancer. I then tighten the nuts up, locking the sensor in place. If you try that and it doesn't work it's probably either a bad sensor or a bad connection in the sensor wires. Speaking of which, there's a place on the front of the engine where the sensor wires plug into the rest of the harness. It's usually filthy dirty with grease. I'd try unplugging it and cleaning it before re-plugging it.

As a side note, the pcm on these trucks very rarely go bad from what I've seen and read. That's good, as they're considerably more expensive than a cps.