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, 10:27 AM
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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 |
Alternator testing good may not completely test all the functions of the alternator. Not sure the test would find a damaged diode on the charging or VR sides.
Here's a little description of how the diodes work in the alternator. The alternator is a 3-phase alternator with 3 separate windings spaced 120 degrees from each other. With full diode bridge rectification, the correct alternator output across the VR would look something like this. The black line is actual voltage since the VR has some capacitance to filter the ripple voltage from the alternator. If you lose one phase of the alternator because of a open circuited diode, the voltage would droop between the operating phases and the GEN light would light a bit. If 2 phases are lost, the droop in the voltage is more pronounced and the GEN light is brighter. So with burnt diodes, the GEN light is on at idle, rev the engine and the GEN light dims. This is because there is less time between active phases of the black trace, the voltage across the VR and one side of the GEN light is almost 14V like across the battery on the other side of the GEN light. |
type47 |
Sep 5 2014, 03:21 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 |
Alternator testing good may not completely test all the functions of the alternator. Not sure the test would find a damaged diode on the charging or VR sides. Tested at Advance Auto parts. I really don't want to R & R the alt again. When I started and revved the engine, the light did not dim. I need to get out and measure voltages on either side of the alt light. I think I can get the DMM probe in the plugs. First try did not get 12 V but about 3 V. Had to stop for other things ... So on either side of the alt light there should be 12 V? That's what I read from the above info. So the alt terminal D+ found in track 76 in the schematic should be 12 V. The terminals T 3a, T 3b are the plug for the alt harness on the relay board and T 14/2 connects there too; I should be able to measure at the plug... |
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