might be the PMD - Pump Mounted Driver, you can see it on the left hand side "drivers side of the injection pump" on the front small black box with a harness coming out of it. I havent had one personally go bad, but I have upgraded to the FSD cooler and driver from SS Diesel Supply. Theres not really a way to test it, and its glued / epoxied to the pump. Goto
http://www.ssdieselupply.com and goto contact or whatever and get with Walt. If you got engine codes he can tell ya whats goin on. If its a 95 or older and dont know how to get codes I can tell ya how to. But thats probably what it is. How many miles are on it.