Pcm voltage regulator went out in my truck so I put in an older dodge external v. regulator. Works great at 14.6v until you drive about 5 miles then the check gauges light comes on and voltage reads pegged past 18v. I take readings at fuse block where alternator charging wire is and it is reading 14.6v. what the hell is going on? Voltage regulator is hooked up blue wire 12v switched ignition to cigarette lighter and green is to the plug on alternator. Even put on another ground from vr to ground on battery. Anyone have any suggestions?

PCM is most likely feeding the intrument cluster bad data... Being that the PCM has a problem with the regulator I'm pretty sure the PCM is the cause.

Well I dont know one day I could drive over an hour without it happening then all of a sudden 18v and check guages light comes on. Could a wire be bad.

All I can say is hook up another voltmeter and watch. As for all the communication its done over the CCD Network. So 1 bad wire would be completely system failure (NO BUS and no gauges). http://mopar.mopar1973man.com/cummins/general/ccd/ccd.htm