Alternator fails to excite?

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JHZR2

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Hi,

What causes an alternator to fail to excite/generate?

At the first start after sitting for a long time, my battery/alternator light will stay on along with my abs light. The engine will run and I'll get 11.8-12.2v. As soon as I give a tiny bit of throttle, both lights go out, dc bus voltage is 13.6-14.1 (I have a 14.1v regulated alt), and turn on/off doesn't create any issue - until next cold soaked start.

Belts are within a year and tight, connections are clean and sound.

Thanks!
 
Just pulling the fuzzy's out of my behind ..but it sounds like you aren't charging until you breach the threshold voltage of something like a Zener diode. You appear to be floating on the battery until that threshold is exceeded.

Since we don't know if you even have that type of voltage regulation (no year/make/model) ..it's hard to say. My alt regulation is via pulsed PCM grounding.
 
My Cabriolet does that after it has been sitting a while or during cold-damp start after the rain. Once I give it a shot of gas the light goes out and it doesn't come back on.

Does this with the new alternator I bought and with the old one. I researched it and apparently it is normal for this model to do this.

I don't worry about it.

Not saying yours is the same problem, but do some research it might be...
 
Brings back memories of our old GM SI series alternators. On our race cars we would use "self exciting" single wire regulators that acted the same way. Before they would charge you would have to blip the throttle to get them to charge.
 
Th excite wire into your alternator should get +12 (battery voltage) all the time the key is on. The voltage regulator controls how much of this +12V gets to excite the coils.

Some people in boats have reversed the excite and sense wires, but unlikely in a car with a harness.
 
Car is a 91 BMW, I think it has a separate sense wire, will look for corrosion, etc.
 
Just had to replace the factory alternator in my 2002 F-150 with only around 43,000 miles on it. It would not charge once the engine reached around 2,500 rpm's. Weird
21.gif
. This was even after replacing the serpentine belt.

Whimsey
 
Old BMW?
At least clean all cables and grounds. I recommend adding larger wires, or supplementary cables.
They are cheap with wire gauges from the factory - the bare minimum.
Add age and corrosion, and you have problems. Also clean your fuses.
 
There is nothing wrong, or at least, nothing critically wrong. All my BMWs of earlier vintage did the same. Blip the throttle to 1100RPM and drive. I have an original alternator in my 88 that I retired at 350K miles. If the thing stops charging, do not give up on the alternator until you replace the regulator/ brush pack. The last one I bought cost me 10 $
 
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