: 1970 Gen light always on when engine running, charging system is fine

03-22-11, 02:14 AM
Here's a good one, hopefully this will help somebody someday. This applies to 1970 Cadillac, possibly other years. If your gen lamp is always on when the car is running, and you've checked your charging system and it's working fine, check the gauge/trans fuse. The way the gen lamp works, it gets power from both sides of the circuit, and if there is an imbalance in power between sides, it will illuminate. If the gauge/trans fuse is blown, it's not getting power from that side, so the lamp will illuminate even though everything else is working fine. Be aware that SOMETHING caused that fuse to blow, and this root cause should be tracked down. In my case, I believe it was due to a failing battery connector on the back of the alternator causing high resistance and possibly overcharging.

03-22-11, 02:55 AM
1970, that can't be right, it had a candle that lit up, they didn't have electricity back then! lol

11-15-11, 10:16 PM
thanks my problem solved

11-22-11, 11:24 AM
It can also be lit dimly and still charge. That is most likely a failed diode trio within the Alternator.