Re: Checking alternator output (archive)

: I just had my dad's '87 635 out last nite and it died on me. It would crank over and start and then when I put the lights on it died. Earlier that day when starting, the indicators lights stayed on for a few moments and then went off after I got underway. I thought then I may have some kind of problem
: rearing it's head.
: I went back to the car today and changed the battery and it fired right up and I drove back home. I swapped out the alternator with one I had in the shop from our '86 735 that we parted out last year and checked the belt and all the connections.
: Question is: How to I verify the output of the alternator and /or voltage regulator??
: I drove the car tonite with the lights on and all seemed well with all dash lights staying out during the drive.
: TIA,
: Nathan

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The easiest way is with a digital voltmeter or with one of the LED Charge Testers from Radio Shack (about $8).

In any case, with the engine running with no high draw accessories on (and a reasonably charged battery) the battery should read 13.5-13.75 volts minimum.

Anything below that means the alternator is not working or has low output due to a bad internal regulator, brushes or diode/s.

The DME computer will quit at 10.5 volts, BTW.

Bob

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