Ignition light on/no output from dynamo

[rsawatson]

On my way home today, the ignition light came on whilst driving. Prior to this happening, a whirring noise in sync with engine speed could be heard - which I suspected was just the dynamo bearing on its way out. I then stopped and measured the voltage across the battery; this was 12V regardless of engine speed.

I followed an online guide and bridged the two terminals on the dynamo before measuring the voltage between the large terminal and earth. A very small increase (approx 0.4V) was observed when I carried out this test. After that, I removed the dynamo to inspect the brushes. These seemed OK and perfectly serviceable.

Do you agree that it's probably safe to say that the dynamo is at fault here? Bearing in mind the whirrring, can anyone suggest what the specific fault could be?

Thanks in advance.

[bmcecosse]

As explained many times - join the two terminals with cable and connect to the non-earth side of the battery - then earth the dynamo casing to the earth side (if it is off the car) If it's ok it will run as a motor. If not, it won't.