Alternator Problems in a '79
>> Last week my alternator would not charge unless I was running
>> above 3200 rpm. As soon as I hit 3200, the gauge would jump up
>> to 14volts. Later in the week it would not charge period.
>> Car died.
>> Replaced the alternator -- nothing doing... still does not
>> appear to be charging. Needle just off the peg... -- the warning
>> light is not on (low battery indication)
>> one thing: I have been charging the battery for the last
>> three days... Is it possible that there is no problem, and the
>> reason the charge guage is reading so low is that the battery is
>> fully charged?
>> '79 928
The "won't charge 'til 3200' symptom" indicates that the field is not being excited adequately at lower speeds. Normally, there's a source for a low current excitation to the field through the regulator. This current may be fed through a light bulb that does double duty as the dash indicator bulb, or may just be fed through a resistor. I don't have a '79, so I can't say about the bulb. If you have a dash indicator bulb, look for it to be lit when you turn the key on but before the engine is running. No light would keep the alternator from charging. Once the alternator turns and is excited throgh the bulb, the excitation is supplied internally provided it is actually charging at the correct voltage. With the internal excitation available, the potential across the bulb disappears, allowing it to go out.
In a separate but possibly related issue, the newest parts and technical reference manual shows a modification that needs to be done when fitting a later service alternator to the early cars. The text discusses installing a different resistor in the dash cluster, which appears to provide more excitation necessary for the newer alternator. This manual is less than $10 from the regular soruces, and is a wealth of interesting info to supplement your expensive service manual set. Anyway, fitting the correct field current resistor may answer your situation. Worth a look anyway.
I had the same problem with my '79 a while back. It took a while, but I finally got it solved. As Dr. Bob states it sounds like you have a problem in your exciter circuit. On our '79s the indicator bulb and the resistor in the circuit are in parallel, so the bulb is independent of the circuit that goes to the alternator. I had to trace the circuit with the help of the wiring diagram and finally located the problem. There is a loop in the circuit that goes out of the main circuit board and back in without apparently doing anything and that's where I had the break. I bypassed this loop and the alternator works correctly most of the time. I'm going to update the resistor to match the new alternator, so hopefully that will fix it completely. Hint: Start tracing the circuit at the alternator end and work backwards.
'79 928 5spd Hellblaumetallic (WIP)
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