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) |
Spoke |
Sep 5 2014, 04:02 PM
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#2
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Jerry Group: Members Posts: 7,051 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None |
Remove the gauge bucket with the GEN light. Start the car. Measure the voltage on both sides of the GEN light to ground.
One side should be the battery voltage about 13-14V. The other side should be the same but will likely be lower. If the one side is lower, likely there is an issue with the alternator (or wiring). I threw in the (or wiring) because a wire issue could cause just about any odd behavior. When did the GEN light start lighting up? Did you do any work to the car before? BTW, what is the battery voltage when the car is idling? |
type47 |
Sep 9 2014, 04:57 PM
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#3
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Viermeister Group: Members Posts: 4,254 Joined: 7-August 03 From: Vienna, VA Member No.: 994 Region Association: MidAtlantic Region |
...what is the battery voltage when the car is idling? 11.6V Remove the gauge bucket with the GEN light. Start the car. Measure the voltage on both sides of the GEN light to ground. On the double red/wht terminal, 11.6 V (assume battery voltage); on the blue wire side, 7.6V. At the 3 terminal plug, at the relay board connection for the red wire terminal (so this would be coming from the alternator) also 7.6 V |
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