repeated voltage regualtor failure

[KB-Cummins]

Hey guys, I'm having some interesting charging system problems that I'm having a difficult time figuring out. I have a 90 d350 that I've had for a few months and since I got it I have replaced the external voltage regulator 5 times. The truck has a new batt and alt and I have checked the voltage drop of both the alt ground and power wires and they are both OK. With the latest voltage reg, the alt is currently charging at 15.3v. I have checked the voltage at the output terminal to the reg and I only have 9.8v. That would lead me to believe there Is high resistance somewhere in the circuit, however, that terminal is a direct output of the Alt so I'm a bit confused. The low regulator reference voltage explains the the over charging but I don't understand why I only have 9.8V at that terminal. If anyone has experienced this or has any suggestions as to where I should go with this, I would appreciate the help. Thanks

[Tacoclaw]

Voltage regulator fixes

^The good stuff is on the 2nd page, but there's some info on batteries on the 1st. My battery had a bad cell and it ate a couple VRs before it got bad enough for me to notice.

[KB-Cummins]

Thanks for the link. I was thinking the same thing originally since the battery that was in it was the cheapest battery the previous owner could get. I put in a new 850cca battery to try and eliminate that as an issue.

[cougar]

A high charge voltage will most likely be caused by a low regulator reference input voltage. If the voltage into the regulator is low, the regulator will turn up the voltage. Check the field input for voltage, it should be the same as your alternator output voltage. If is lower, replace you ignition switch. Commonplace for a voltage drop. If the field voltage is equal to the output, then check the wire to the regulator. I have encountered bad splices that caused this kind of fault.

[KB-Cummins]

That is exactly what ended up being the case. I was only getting a 10v ref to the regulator so I went under the dash to check out the ignition switch and discovered that an aftermarket alarm system had been installed at some point in the early 90's. The alarm system was Scotch-lock spliced in to about 20 different circuits in the OEM harness. Since the alarm didn't work I ripped the whole system out and conveniently the charging voltage dropped to 14.6 V. Thanks for the help.