Thread: help diagnosing alternator 1981 RRC

Hey HangOver,

I had sort of similar issues with alternators a couple of months ago

My alternator died, so ordered a brand new one.
Vehicle off, battery would read in the area of 12.3V
On idle, i would get a reading of 12.8V at the battery, and 13V at the alternator.
Alternator was no good.
Got a second one in, tested again, same readings with motor off, but on idle, 14.2V at the battery, 14.3V at the alternator.
Success.
For testing, have the voltmeter earthed off the alternator casing.
As for wires, colours shouldnt differ too much, but my vehicle had the following colors:
Thick brown wire (battery lead)
Brown with yellow stripe (dash light)
White with slate stripe (tach lead)
I also found that with the faulty alternator, it was putting out an excessive amount of heat (too hot to even touch). Maybe check that also.
Have you turned on your lights when you tested on idle?
If the alternator is charging, just not to full potential, might be diode or winding issue internally. Easier to check when lights are on/adding a load on the circuit.
Dash light might be a bad connection from your brown wire, or brown and white may be wrong way around?
Also, check the belt tension.
Too tight can put strain on bearings, too lose and it might be slipping on the pulley.

Hope this helps some.

Cheers,

Evo

Yep, try swapping the brown/yellow wire and the white/grey wire around, a customer came in and his wiring was like that. No tacho and red light on.

Up early and tested the alternator again.

I removed all cables from the back of the alternator, with engine running I put the multimeter + on both big terminals (in turn) and the multimeter - on the alternator casing then chassis then battery negative, all show zero voltage.

Does this suggest it's buggered or does it need a cable connected to produce voltage?

Not 100% sure on your last comment, but did you only check voltage previously or did you also check amperage?
Personally, I would just get a new alternator

I think you need an amp meter to check amps? I only have a cheap multimeter.
There is a amp/charge dial on the dash, I will look tonight but I can;t remember if that shows Volts or Amps, plus its 31 years old so it may not be very reliable.

I'll probably end up getting a replacment but wanted to be sure I was testing it correctly to show it is stuffed.

From what you have said, seems like you are doing nothing wrong.
Maybe hook up all the wires again and test it and see if the dash light is still on. If so, change the brown/yellow and white/slate wires around and see if that makes a difference.
Also, double check to make sure the mount bracket is tight and is making a good earth with the engine block.

For an alternator to work, it must be connected to a battery (fat brown wire) and the dash light wire (brown/yellow stripe) must be connected to the correct terminal, and the ignition must be on. The dash globe provides a trickle current to start the charging process, it goes out when the alternator is producing its own power. The reason you need the globe is because residual magnetism in the iron parts is insufficient to overcome the forward voltage of the diode packs. An old fashioned genny will often self excite as it has no diodes.

thanks for that!
The alternator has two fat brown wires going to two large spades, and one small wire to one small spade, there aren't any other spades to connect to and no other wires :/