Triumph Repair: TR4A Generator, voltage drop test, output decline

Question
Hi Howard,

I have a 1966 TR4A.  Just curious, when the generator starts to fail, will the output decline over time or will it experience a sudden total failure?

On the same subject, what is a good test to verify generator output is performing to spec?  Is it an more an issue of amps or volts?

Thanks again,

Bob

Answer
Hi Bob,
Amperage is 100
dependentt on load so you must test voltage which is the "Push".
First put a voltmeter across the battery posts (not the cable ends) and read battery voltage (Aprox 12.5v) Then start the engine and set it at a fast idle (Aprox 1500 RPM) and read battery voltage again. (13.8v to 14.5v) If it is less than that or more than that, you have a problem. It does not tell you where the problem is but just that it is not correct. Check all connections and belt tension and you should do a voltage drop test on all the power cables and ground cables and if nothing is found you can do a "Full Field Generator" test. This is done by making up a temp test cable using a 12 ga. wire that will reach from the generator to the battery. Add a short pig tail to one end of the wire end (this can be a smaller ga wire like a 14 ga.)
Remove the two wires off of the back of the generator and place them so as not to get in contact with anything and connect your 12 ga wire to the large post and the pig tail to the small post.
Put your volt meter across the battery posts again and start the engine and set the idle at about 1500 RPM and touch the test lead to the power side of the battery (Negative post, unless the system has been changed to negative ground) and watch the volt meter. You should see about 15v. If there is less than 15v you have a generator problem. You can not test a regulator with a bad generator The only test of a regulator is the (13.8v to 14.5v test if everything else is ok)
Howard