Another alternator question

[Awayonmyboat]

I have an Isuzu 35 engine fitted with two 70A Lucas A127 alternators. A couple of days ago I noticed the warning light from the domestic alternator did not always come on when starting the engine, butappeared when I switched off the engine - and batteries charged during running.

 

Today the light stays off all the time and the alternator has stopped charging (confirmed with a clamp meter). There is 13 v at the field excitation circuit when I check. My suspicion is brushes have worn out - is this a reasonable first diagnosis? The engine and alternators have done about 1300 hours work since new in 2005.

[Tracy D'arth]

2 minutes ago, Awayonmyboat said:

I have an Isuzu 35 engine fitted with two 70A Lucas A127 alternators. A couple of days ago I noticed the warning light from the domestic alternator did not always come on when starting the engine, butappeared when I switched off the engine - and batteries charged during running.

 

Today the light stays off all the time and the alternator has stopped charging (confirmed with a clamp meter). There is 13 v at the field excitation circuit when I check. My suspicion is brushes have worn out - is this a reasonable first diagnosis? The engine and alternators have done about 1300 hours work since new in 2005.

It is a definite probability, easily checked, isolate and remove the regulator from the rear, the brushes are on the back.

[Tony Brooks]

If we ignore the "but appeared when I switched off the engine" then yes, especially if you got 13V between D+ (the warning light connection and earth/negative with the ignition ON and the engine off).

 

As the ignition as normally off when you stop the engine the lamp being on would indicate blown diode(s).

Edited September 14, 2020 by Tony Brooks

[Awayonmyboat]

Tracy, Tony. Thankyou both for your quick replies. I'll check them in the next few days (currently I'm away from base. At the moment I have simply rewired so the other alternator is now charging both domestic and starter batteries). All the best.