How alternator light works, a more detailed description |
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How alternator light works, a more detailed description |
Tom |
May 3 2014, 01:30 PM
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#1
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Advanced Member Group: Members Posts: 2,139 Joined: 21-August 05 From: Port Orchard, WA 98367 Member No.: 4,626 Region Association: None |
After reading many accounts of how this circuit works, I felt compelled to investigate further as I did not understand how two positives would cause a light to operate. They won't. One must be somewhat negative to complete the circuit. Internet searches turned up the same basic explanation, still was not buying it. I think it was being oversimplified.
This is how I think the alt light works: When the key is on and engine not running, there is 12 volts + at the alt light power side coming from the fused side of fuse #9. The other side goes to a junction on the relay board with D+. With the key to off and a meter connected between D+ and ground at the relay board, the reading is 12 ohms. As soon as the key is turned to on, the reading jumps to 12.5 meg ohms and the light comes on. If the wire for D+ to the alt is removed, the reading stays the same and the light stays on. Removing the VR caused the reading to jump to infinity and the light goes out. For the light to work, there has to be power to one side of the light and some resistance reading to ground for the other. Looking at the wiring diagram, one can follow the blue wire to the junction at the relay board at D+, then up thru the VR to a set of relay contacts, then down thru a ( resistor ?, not sure) and then down to the DF connection and on to the rotor where the current will produce a magnetic field. After the rotor, it goes to ground. When the alt spins enough RPM's, a voltage is produced and fed back to the VR, causing the relay to open and removes the ground path for the alt light. I could be entirely wrong here, but this is what I see and my readings more or less confirm it. If you see an error in my thinking, please post and let me know. Thanks, Tom Attached thumbnail(s) |
930cabman |
Jul 10 2024, 04:52 PM
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#2
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Advanced Member Group: Members Posts: 3,523 Joined: 12-November 20 From: Buffalo Member No.: 24,877 Region Association: North East States |
Another old, old thread
Generator light will not light engine off or engine on. When started alternator (with built in regulator) power at the battery reads about 12V. After running for a short time, 3 -4 minutes, power at the battery reads 13.8 or so. This is a /6 conversion so accessing the alternator is a PITA |
Spoke |
Jul 11 2024, 11:15 PM
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#3
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Jerry Group: Members Posts: 7,051 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None |
Another old, old thread Generator light will not light engine off or engine on. When started alternator (with built in regulator) power at the battery reads about 12V. After running for a short time, 3 -4 minutes, power at the battery reads 13.8 or so. This is a /6 conversion so accessing the alternator is a PITA So when you start the engine GEN light is not lit and the battery reads 12V. Then after running for a short time, you have 13.8V. Sounds like the VR is not making a connection to the armature so the GEN light is off. But after driving with higher RPMs, there's enough residual magnetism in the armature that the alternator comes alive. The voltage at the battery switches immediately from 12V to 13.8V. The 911 charging system is similar to the 914 shown here for reference. The VR powers the armature through the GEN light causing it to light up and bootstrapping the alternator. The culprit is likely either wiring or the VR. Hopefully it is just wiring. |
930cabman |
Jul 12 2024, 04:59 AM
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#4
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Advanced Member Group: Members Posts: 3,523 Joined: 12-November 20 From: Buffalo Member No.: 24,877 Region Association: North East States |
Another old, old thread Generator light will not light engine off or engine on. When started alternator (with built in regulator) power at the battery reads about 12V. After running for a short time, 3 -4 minutes, power at the battery reads 13.8 or so. This is a /6 conversion so accessing the alternator is a PITA So when you start the engine GEN light is not lit and the battery reads 12V. Then after running for a short time, you have 13.8V. Sounds like the VR is not making a connection to the armature so the GEN light is off. But after driving with higher RPMs, there's enough residual magnetism in the armature that the alternator comes alive. The voltage at the battery switches immediately from 12V to 13.8V. The 911 charging system is similar to the 914 shown here for reference. The VR powers the armature through the GEN light causing it to light up and bootstrapping the alternator. The culprit is likely either wiring or the VR. Hopefully it is just wiring. Thank you Spoke, this issue is with a recent /6 conversion. Alternator (bench tested by local rebuilder) has a built in regulator. I was unable to get anything from it so the carbs, shroud, ... came off to access the alternator. Took into Phil and it tested ok on the bench. Now it's all back together and acting weird. After running a short (2-3 minutes) with a few blips, I will get 13 or 14 volts and it seems to remain until I shut the engine off. A restart will return the voltage to 11.8 V |
Spoke |
Jul 12 2024, 07:58 AM
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#5
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Jerry Group: Members Posts: 7,051 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None |
Thank you Spoke, this issue is with a recent /6 conversion. Alternator (bench tested by local rebuilder) has a built in regulator. I was unable to get anything from it so the carbs, shroud, ... came off to access the alternator. Took into Phil and it tested ok on the bench. Now it's all back together and acting weird. After running a short (2-3 minutes) with a few blips, I will get 13 or 14 volts and it seems to remain until I shut the engine off. A restart will return the voltage to 11.8 V Sounds like you can drive the car just make sure to blip the throttle to get the alternator to bootstrap. The GEN light off is the issue. Either the VR is not connecting the bulb to the armature or the wires don't connect. Try testing the system by measuring the voltage of the blue wire from the alternator and both voltages at the bulb. Just turn the key to RUN w/o engine running. The GEN light should light. |
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