Alternator Wiring

Afternoon Guys,

I wonder if anyone can help please - probably something simple but im stuck!

I've wired my basic three cable alternator using the method attached.

Both of the small gauge wires are going to 12v Switched by the ignition.

One through a lamp (Incandescent) and one direct to the alternator.

I wanted to test the alternator works and that the lamp was wired properly before starting the car, so my plan was spin the alternator on a drill and check the lamp goes out and that the battery charges - I connected the charging terminal to the battery, power up the ignition but the lamp doesnt illuminate (alternator not spinning)

However, when the charging cable is not connected to the battery, the lamp is illuminated.

Ive checked the alternator is earthed properly by running a seperate cable to the alternator to earth it, but still, as soon as i connect the charge cable the lamp goes out.

Any idea please - am i correct in assuming that both the small gauge cables should be connected to 12v from the ignition?

Thanks in Advance

Might be a silly question, but you DID ground the alternator housing to the negative terminal of the battery?

yep, as the engine hasn't been running or anything yet i was concerned that there may be an issue with the engine earth - it tests up perfectly, But yes to eliminate any doubt i ran a seperate ground probe to the alternator housing..

To Confirm - i am correct to run both spade terminals to +12v ? One direct and one through the lamp?

Hi Guys,

Finally managed to spend some more time on this issue - I removed the alternator from the car, and tested both terminals - I imagine one terminal should show continuity to ground (Alternator body) when the alternator isnt running as this would allow the lamp to illuminate (Power from one side and ground on the alternator side) then when its spinning it should break the ground thus turning the lamp off.

However.. neither terminal is showing continuity to the alternator body... anyone got one they can test which definately works and tell me if i'm right or wrong please?

Seems like something so silly to be stuck on!

Can't recall exactly, but it's possible there's an internal diode on one or the other - did you try reversing the polarity of the leads?

The ground for the warning light isn't "broken", rather the internal connection's voltage rises and the reduction in the voltage difference will dim and then extinguish the light, as it has ~12V on both sides. Sometimes a glow may be seen at night, this is because sometimes the voltages don't quite cancel out under load, but that is more likely to happen on an older vehicle.

There are test proceedures for alternators, but the best option may be to check for a local auto-electrician and have it spun up on his/her test bench - that will check it's operation, output, and should show if a winding/diode or voltage regulator is faulty. If it does fail, you should be able to replace the faulty part for a few dollars, rather than the hundreds a new or "rebuilt" alternator would cost.