Alternator problem

[Robert G]

I have a 12 year old Fiesta Ghia whose battery warning indicator lamp lights up sometimes but not at other times, seemingly illogically. This has happened since I renewed the battery after the old one died.

Basically what happens is this: Once the engine is running, the battery warning light sometimes (but not always) comes on and stays on, no matter how high I rev the engine or how much load I put on the battery by switching on lights, fan, wipers, etc. But when I drive the car the battery warning light goes out and stays out, leading me to think that all is well after all!

I've tried replacing the alternator with another old one and it makes no difference. The wiring looks OK and the connections seem good.

I'm wondering if dampness might be causing the problem, but any suggestions would be much appreciated.

[oldhenry]

Could it be resistance through a bad earth?, that is the cause of many faults and as it was ok before your new battery , did u disturb connections, loose wire etc.
had you a meter to check voltage across battery , should be 14 v ish when revved.

[Robert G]

Thanks, oldhenry. Since I posed my question the car broke down this evening and the battery was found to be flat. My wife called the breakdown service, and the guy said he suspected the IND circuit. He connected the alternator's negative terminal to earth and the battery warning light did not come on as it should (apparently). Also, when the battery was OK the battery warning light didn't come on when I turned on the ignition. The guy also said that there might be a fault with the warning light bulb.

[JimmyK]

The warning light is in the control circuit that regulates the charging, and if the warning light or any part of the circuit to it is open circuit (not connected) you will not get any charging at all. The warning lamp is actually passing current all the time when the ignition is on, and brightness of illumination indcates the currrent required in the field coils to achieve a high enough output to produce charging.
So I agree that, if the charge warning light did not come on at all when the ignition was first switched on (engine not running) there could be a problem there and you should investigate that.