 Easy Alternator test? |
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| rhodyguy |
 Jan 10 2015, 10:35 AM
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#21
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Chimp Sanctuary NW. Check it out.  Group: Members Posts: 22,071 Joined: 2-March 03 From: Orion's Bell. The BELL! Member No.: 378 Region Association: Galt's Gulch  |
Happy motoring!
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| mikesmith |
Jan 10 2015, 12:54 PM
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#22
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Member Group: Members Posts: 202 Joined: 5-September 13 From: SF Member No.: 16,354 Region Association: Northern California |
QUOTE(Dave_Darling @ Jan 9 2015, 12:35 PM)  I believe the alt light runs off of the alternator voltage wire that goes to the VR. I'm not sure exactly when the power for the light has to go through the VR, but it probably has something to do with the way the light is used to "bootstrap" the alternator to get it to start charging. There are two rectifiers inside the alternator. One supplies what we think of as the "output" current, the other one is used to supply current for the field coil. The alternator "works" by moving the field coil and its magnetic field around relative to the stator coils. You have to have current flowing in the field coil for it to create a field, and that current has to come from somewhere. When the alternator is running, the second rectifier supplies the current. But to begin with, there is no current, and that's where the charge lamp comes in. One end of the lamp is connected to ignition power, the other end is connected to the output of the field coil supply rectifier. To the earlier points about the charge lamp burning out; in some designs there is a resistor in parallel with the lamp so that current can flow even if the lamp fails. This connection is also why some vehicles need a diode in series with the charge lamp; if your ignition current draw is low you can actually back-power the ignition circuit from the field coil rectifier. Much more detail here: http://brianesser.com/technical-informatio...nd-information/ |
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| jdlmodelt |
Jan 3 2020, 04:14 PM
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#23
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James D. Lane Group: Members Posts: 345 Joined: 30-November 12 From: Colorado Member No.: 15,210 Region Association: Southwest Region |
Okay, alternator wasn't charging. I put 12.5Vdc on D+ and DF. I had already verified good grounding of the engine, alternator, and chassis. With both D+ and DF excited, no charge from the alternator, it just matched the battery voltage. With everything connected correctly and no jumpers, the gen light on the dash is on all the time. from D+ to ground I get 5.5 volts, and from DF to ground I get 5.5 volts. from D+ to DF I get zero volts. All said and done. I bench tested a spare alternator with the same methods and it charged great. I replaced the alternator and all is well. So, just some more trouble shooting data for others to use. Voltage Regulator is fine.
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