Ignition Warning Light

Too low, if at all. The resistance of the bulb is used to excite the alternator so it starts to develop charge. Without a bulb the alternator is unlikely to give any charge at all. Most have an internal resistor, or a resistor in parallel with the bulb to prevent this anyway.

Does it have two or three connections on the back? Early battery-sensed alternators have a third wire which leads back to the battery to sense the voltage for regulation. A bad connection on this wire will result in poor charging characteristics. Don't get this wire confused with the small field terminal on ACR's which is used to give signal to rev-counters on diesel engines. Poor connection between the alternator and the battery, or poor earth to either can also cause oddities. If all this checks out, suspect the regulator/diode pack in the alternator itself.

Alex

Normally, with the engine running, the voltmeter sits just above 13V, and when I turn the engine off, the ignition warning light glows until the engine stops. But I have an intermittent fault. Sometimes the voltmeter sits at just under 12V (unless I put headlights on, when it goes down to 11V). If I switch off the ignition then, the ignition warning light doesn't glow as the engine runs down to a stop. You have confirmed what I suspected, and the two symptoms are connected.

Output terminal of the alternator has two thick brown wires attached. The only other terminal is the warning light circuit (if I turn the ignition on and the warning light comes on, disconnecting this wire turns it off)

It is an ACR17 on a petrol engine.

I either get a normal steady charge or no charge, normally for the whole time that the engine is running, but nothing erratic happens so I think the battery connection is working OK. So thanks to your "too low" prediction, I am suspecting either a worn slip ring brush, or a dirty connection in the warning light circuit, or the bulb itself. Fun with multimeter coming up, I think.

Jim

If the warning light circuit is OC - no charge at all with some.

On an ACR then you'll get no charge at all (for an open circuit bulb). You _might_ get some charge if the diodes are leaky too, if you blip the revs first.

If the bulb rating is wrong, then the voltage will be wrong (charge "rate" isn't really appplicable to an alternator as it is for a dynamo). Bulbs often (some makers) sit in a distinctive red holder, rather than the black ones used for the rest of the dashboard lamps. Too high a bulb rating (lower resistance) gives too low an output voltage.

Sorted - eventually. There was nothing wrong with the bulb or the bulb wiring outboard of the alternator, but there was no earth return through the alternator. So I took the brushes out - and they were well worn. So I eventually found an auto electrician that had a new set of brushes (they seem to be as common as hen's teeth!), and I fitted them. And it made no difference at all.

It was the regulator playing up, and it works fine now I have changed it. I have never known worn brushes to damage a regulator before, but it looks as though that is what happened.

Jim