I have a low voltage problem on my 1980 265 w/ rebuilt 1976 B21F engine. The battery reads 12.5 v in the car, and the alternator has no trouble charging the battery.

The cabin voltage is always low, though (as read through cig lighter port.) At best, it might reach 12 v when driving down the road with no electrical devices active. But with headlights and wipers on, it drops to below 11 v. The windshield wipers move much slower than in my 1985 245, and the headlights are dim. Always.

The car has a "new" rebuilt alternator, and a "new" rebuilt windshield wiper motor. The behavior has not changed regardless of these component replacements. (Wipers aren't any faster or slower with the new motor.)

When the engine is off and the key is turned to activate the accessories, the voltage reads back at around 10 v at the cig lighter port. At the same time, reading across the battery itself shows 12.5 v.

Anyone seen this one before?