Alternator Circuit Weirdness ... |
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Alternator Circuit Weirdness ... |
SirAndy |
Dec 22 2007, 05:41 PM
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#1
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Resident German Group: Admin Posts: 41,656 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
This one has me stumped (sp?) ...
3.6L '95 993 Engine with stock 993 alternator. stock 993 gauge with stock 993 alternator warning light. when i start the engine, the alternator light stays on until i spin the engine above 2500 rpm. (IMG:style_emoticons/default/WTF.gif) if i just let it idle, the alternator light will never turn off. i measured the current on the battery and confirmed that the alternator is *not* charging when the light in the gauge is on. now, here is where it get's really strange. once i rev the engine past 2500 rpm, the light goes off and *stays* off. (IMG:style_emoticons/default/screwy.gif) the alternator charges just fine after that, even when the engine dropps back to idle. it seems like the alternator needs a initial "jolt" to start working properly. anybody ever seen anything like this? (IMG:style_emoticons/default/idea.gif) Andy |
Allan |
Dec 22 2007, 06:01 PM
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#2
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Teenerless Weenie Group: Members Posts: 8,373 Joined: 5-July 04 From: Western Mesopotamia Member No.: 2,304 Region Association: Southern California |
Mines kinda similar. The light stays on under 3k no matter what, but I have confirmed that I'm getting good output to the battery even at idle.
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clarkcou |
Dec 22 2007, 06:04 PM
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#3
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Member Group: Members Posts: 115 Joined: 17-October 03 From: Peoria, Arizona Member No.: 1,257 Region Association: Southwest Region |
Had this problem in an older BMW (1975). Corrosion at the pivot bushing was not allowing a good ground, added a ground wire from the case to the block and the problem went away. May be a similar issue (bad ground) ?
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SGB |
Dec 22 2007, 06:21 PM
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#4
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just visiting Group: Members Posts: 4,086 Joined: 8-March 03 From: Huntsville, AL Member No.: 404 Region Association: South East States |
Could this be connected to the oil pressure weirdness? Did that get resolved?
I'm thinking the alternator is waving "buh-bye"... |
Bruce Allert |
Dec 22 2007, 06:38 PM
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#5
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Hellions asleep Group: Members Posts: 3,289 Joined: 19-March 03 From: Eagle Creek, Orygun Member No.: 441 Region Association: Pacific Northwest |
I've had several American cars that did exactly that. Rev it & the light goes out. (IMG:style_emoticons/default/confused24.gif) Nothing ever happened for the length of time I had the car.
........b |
r_towle |
Dec 22 2007, 08:30 PM
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#6
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Custom Member Group: Members Posts: 24,584 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States |
rtfm,
Do not run this engine under 2500 rpms or vee vill leave zee dumkopf light on. |
draperjojo |
Dec 22 2007, 09:22 PM
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#7
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sixstrings Group: Members Posts: 411 Joined: 26-December 05 From: Salt Lake City Utah Member No.: 5,317 |
Sounds like a problem in the voltage regulator circuit to me.
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Spoke |
Dec 23 2007, 07:27 AM
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#8
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Jerry Group: Members Posts: 6,986 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None |
First question: is this new behaviour or has it been like this all the time?
The alternator circuit for the 914 is a bootstrap circuit. In other words, the alternator powers itself. The alternator provides the voltage to the armature (spinning part of the alternator) to provide magnetize the armature so when it spins, the alternator produces voltage. No voltage to the armature means no voltage output of the alternator. Without an outside helper to get some voltage to the armature, the alternator will not turn on and produce voltage. That helper is the idiot light. Not sure about the 911 setup so this example pertains to at least the 914. There are 2 sets of diodes in the alternator. One set powers the car and one set powers just the voltage regulator and the armature. If the armature is not powered then it is not magnetized and no voltage is developed by the alternator. When first started, the armature has no power and the only power to it and the VR is through the dashboard idiot light. The current through the idiot light provides just enough current to get some magnetization going in the armature thus it produces some voltage. Not quite 12V but some voltage and the idiot light stays on. Rev the engine and the alternator spins faster and the little magnetization in the armature produces more voltage until the output is 12V or higher. At this point, the second set of diodes provides enough voltage for the VR and the armature to fully turn on and the idiot light goes out and stays out. This bootstrapping should happen at a real low RPM so you don't have to rev the engine to get the circuit going. Maybe one of your diodes is bad? I've seen in my 71 914 that if the car is started and the engine runs real slow (like 200 RPM), the idiot light stays on. Blip the throttle and the light goes out and stays out. Spoke |
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