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MG MGB Technical - Testing Alternator

Is there a way to test the output of an alternator either on or off of the car?
I have a couple of used ones but they don't seem to make any juice.
If I pull the battery lead with the car running the engine will die.
Red light on the tachometer stays on dimly.
Thanks.
PJ Wilson

Measure the terminal voltage of the battery*.
It should go upto around 14V if the alternator is charging.

If it stays around 12V or goes above 15V the alternator doesn't work.
Usually a regulator fault, but just possibly may be a brush problem.
They rarely burn out their coils.

*or check between ground and the main feed at the solenoid.

You can use a clamp ammeter around the alternator lead if you know someone with one, but it must be a DC version, not the current transformer type.
Martin Layton

Disconnecting the battery when an alternator equipped car is running is almost certian to ruin the alternator.
John H

Disconnecting the battery with the alternator running can cause an over-voltage situation, which can damage any components powered at the time, particularly bulbs. However I've absent-mindedly turned off the battery cut-off switch before turning off the ignition and haven't experienced any problems, although I wouldn't advise it. I've also inadvertantly run a car for about 30 miles with the alternator unplugged, until the battery started to die, it didn't blow the alternator, although again it isn't advised.

Measure the voltage on the brown wire at he alternator with minimal electrical load and the engine revving at about 2000 rpm. You should see about 14.5v. If you only see about 12v it isn't charging. If that is OK turn on all the *factory* electrical loads like headlights, heater fan, brake lights etc. and the voltage will drop but should still be above 12.8v. If it is lower than this then measure again on the brown at the alternator. If that is higher than the fusebox then you have bad connections, possibly at the solenoid where the browns join the battery cable. But is that is also low the alternator isn't delivering enough current.

A dim glow from the ignition warning light indicates a disparity between the alternator voltage and the system voltage. This could be due to faults in the alternator as well as bad connections between it and the white (unfused ignition) circuit. If you have about 14.5v at the alternator brown and brown/yellow, but 2 or more volts lower than this at the white at the fusebox, then the alternator itself is probably OK and the problem lies in the brown - ignition switch - white circuit.

But if the engine *did* die with the battery disconnected, then it does look like the alternator isn't charging.
Paul Hunt 2

The procedure for testing the alternator is in the Bentley manual.
Many chain auto parts stores will test the alternator for free.
Kimberly

This thread was discussed between 30/08/2007 and 31/08/2007

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