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'83 SC Voltage regulator - test/replace
I have the red idiot battery light on with the engine running. The voltage to the battery is ~12.6 V same as with the car not running. Sounds like the voltage reg or alternator, right? Another data point, I just replaced the battery. My helper put the battery in backwards, (I didn't catch it, I take full responsibility... ugh) hooking up the positive lead to the battery negative post, when we went to connect what we thought was the ground, actually touching the battery positive led there was a spark. It was then I realized the battery orientation was reversed. We put the battery in properly. Car started right up, but now i have the battery red idiot light glowing. I'm thinking I blew the diodes in the voltage regulator? To get to the regulator on an 83, i have to remove the alternator from the fan assemble. Factory manuals and Haynes all show the regulator on a bench test. A PITA. Is there a way to test the regulator without pulling the alternator?
No comments please on the bone head move with the battery install. Sharing it as a data point to help with the trouble shooting. TIA |
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07-10-2022, 04:10 PM
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ROW '78 911 Targa
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No choice but to pull the Alternator.
Could be the VR, more likely a diode on the alternator itself. Or both. Always good to take a look in there at the ground cable and main to starter cable after a reversal anyways.
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Dennis Euro 1978 SC Targa, SSI's, Dansk 2/1, PMO ITBs, Electric A/C Need a New Wiring Harness? PM or e-mail me. Search for "harnesses" in the classifieds. |
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07-10-2022, 04:51 PM
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I thought the same. Thinking I take the combination, VR and alternator and have it tested at a rebuilder.
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07-10-2022, 04:54 PM
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Heck, I’m only 5 not 71!
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Just a note on alternator diodes. There are two types of diodes:
1. High power diodes that rectify the signal to dc output. 2. Three small exciter diodes that provide the feedback signal to the voltage regulator. The exciter diodes will overtime gradually fail due to the heat environment they are mounted in. This causes an erroneous feedback signal to the voltage regulator causing alternator light to glow at idle or glow low all the time. Alternator receives erroneous signal from voltage regulator so alternator output is reduced. In the below picture I was testing the three exciter diodes. The first one I put the meter on disintegrated with a light touch of the meter probe. 
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Pat Henry Targa80 1980SC Targa (Mocha Brown) |
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07-11-2022, 05:05 AM
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Pat,
Very helpful. Thanks. I'm going to go ahead and order the VR, while waiting for delivery pull the alternator. Can then test the diodes. Best, Rob |
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07-11-2022, 07:50 AM
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I had same symptoms on my 83 but it was my stator that had a break in it. Found a local rebuilder and new stator, bearings & brushes was less than $150. He insisted that I bring the cage in so that he torque the thru bolts properly because the rotor may rub on the stator if you dont.
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07-11-2022, 09:24 AM
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I replaced the VR and next checked continuity with the diode function of my DVM between the positive batter terminal on the alternator and alt casing. I have continuity in both directions. Aren't the diodes internal the alternator? Am I correct in assuming I should have continuity in one direction, and the swapping the positive and negative DVM leads not have continuity in the other direction? Are the diodes serviceable?
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07-16-2022, 09:56 AM
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Heck, I’m only 5 not 71!
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Yes, the three exciter diodes are internal. The previous photo shows the three small exciter diodes on the alternator without the internal voltage regulator.
The below diagram shows how the three exciter diodes are connected in a typical alternator circuit.  The below diagram show the current flow through a diode. The Resistance through the diode from anode to cathode would be very low. The resistance through the diode from cathode to anode would be very high or infinity. 
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Pat Henry Targa80 1980SC Targa (Mocha Brown) |
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07-16-2022, 11:15 AM
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Location: Lomita, CA
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Quote:
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Dave |
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07-17-2022, 07:20 AM
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Targa80/MySoCal911
Thanks for the reply. Can I test the Diodes without opening the alternator checking the resistance from the + batter connection on the alternator to the alternator case? Here are the readings I got: Positive DVM lead to alt battery post / negative DVM lead on alt case - 0 ohms Negative DVM lead to alt battery post / positive DVM lead on alt case - 0 ohms Or do I need to open the alternator to test the diodes? If that is the case I'm assuming the diodes are soldered in place? The alternator is the original from Porsche P/N 911 603 120 04 14V 75A Paris -Rhone TIA Last edited by Das86TurboII; 07-17-2022 at 09:54 AM.. |
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07-17-2022, 09:24 AM
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Heck, I’m only 5 not 71!
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Dave,
You are right the two wires from stator look like they are both tied to the three exciter diodes by the black dot connection. One side of the stat or connects to the three exciter diodes and the other side connects directly to the f term on regulator. I just grabbed the diagram from the internet and did not catch that error. 
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Pat Henry Targa80 1980SC Targa (Mocha Brown) |
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07-17-2022, 10:34 AM
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Heinz
