Vehicle is a 1988 245. Following a bunch of routine work this weekend (coolant change, oil & filter, speedo gear replacement) and some floorpan repair, I buttoned the car up and took it for a test drive. At first, all was ok. About 1/4 mile in, suddenly the Charging, Brake Failure, and Parking Brake Lights are lit. Hmm.

Got home, confirmed with voltmeter at battery that alt was not charging, and assumed the D+ wire was probably shorting to ground somewhere. Ohm-meter appeared to confirm that: with car off, showed some degree of continuity b/w ground and the D+ wire (can't remember if I was checking the wire while it was still attached to the alternator. Confirmed alternator is properly grounded. Alternator is a used "Bosch remanufactured" unit with new brushes/regulator installed last summer.

So I went ahead and installed a replacement D+ wire and oil pressure sender wire, routing them from the alternator over to the fender alongside the harness for the ignition module, then fishing it back around the fender and firewall to the big grey connector and tied it in there.

No change. (dammit)

With car running, I show almost no voltage b/w D+ and ground, 12 volts b/w B+ and ground. At the battery, voltage is generally right around 12.5 but sometimes fluctuates upwards to as much as 15.

Smells like an internal alternator failure, correct? Probably a diode or similar non-DIY friendly fault? Any obvious tests I am overlooking?

thanks,
john
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1989 245 233K / 1993 945 127K