Last week while driving my 98 1/2, 24 valve Cummins, the engine stumbled several times, the "check engine" lamp came on and the tach. kept falling to zero although the engine never stalled. I'm assuming this is some sort of electrical malfunction but I don't want to damage the engine before I can get it into a shop. Any ideas?

 

Crankshaft position sensor. Located behind the starter.

 

J body is spot on! Get a replacement from a Cummins shop (or from a dealer, but only as a last resort!)

 

Thanks guys, sounds like it's unanimous, I'll check my sources for a crankshaft position sensor and let you know how I make out. You're right about not buying parts from the dealer, last time I checked the thieves wanted about $10 for the fiber washer under the oil pan drain bolt.
Dennis