Alternating Current (AC) Ripple Test
If the engine electronics seem to be performing erratically, it is possible that you might have
one or more marginal diodes in the alternator that are leaking. Leaky diodes will
generate AC ripples along your electrical system. The AC ripple will cause havoc on the
electronic brain. Take a digital meter and set it to AC mode. Select a voltage range to
read approximately 2 volts. Start engine and let idle. Negative lead to ground and
positive lead to BATT+ lead on the back of the alternator. You should have less than 0.1
volt AC. The lower this value the better. Ideally, an oscilloscope would be a better
instrument to use but not everyone has one. YMMV. Good luck!
In regard to Vince's response, if no multimeter is available to test the alternator, just disconnect the alternator all together and let it (try to) run on the battery. if the car runs fine, its an Alternator problem as vince suggested. if not....back to the drawing board. sounds like the dreaded brain (or brains) to me. just my two cents. good luck.
'85 928 S 5spd
Regarding #2 above:
It is absolutely fatal to the alternator to disconnect it while the engine is running. To eliminate any possible misunderstanding, perhaps the instruction might read:
"Disconnect the battery ground strap, disconnect the alternator all
together, reconnect the battery, then try to run the car on just the
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