- GT Style -

<i>mcorrow </i> says big red

<a href="https://forums.audiworld.com/v8/msgs/93503.phtml">the positive is the big red</a>



<i>boxerfan </i> says little blue

<a href="https://forums.audiworld.com/v8/msgs/93409.phtml">ensure that you see battery voltage on the field wire when the ignition is switched on</a>




Which is correct?
I wasfiguring big red to + of volt meter &amp; grounding to chassis.

mcorrow

I hooked the voltmeter up to the only two wires attached to the alternator. The positive to the big wire, the negative to the little wire. IIRC, before I started the car, with the ignition on, I saw battery voltage(almost) there, and after starting the car, I saw 13.9-14.1 volts there.

If you connect them backwards, your VOM should show negative....

It just made sense to me that the LARGE wire would be battery positive... (I seem to remember also that I accidentally touched that wire to a ground and got a large spark).... Yes, I know I'm supposed to unhook the battery first.,.....

- GT Style -

that's what I had been thinking also

RussB

The large red wire is the OUTPUT from the alternator. Within the alternator case it's connected to the cathodes of the six diodes, which are in turn fed by the alternator's stator windings.

The small blue wire is the "exciter" wire, and is an INPUT lead. (It's sometimes called the "field" or "trigger" wire.) The alternator draws a small current from it when it's not turning to create a magnetic field.

Mcorrow's measurements between the big red and small blue wire were essentially meaningless.

To measure the alternator's output voltage, you are correct: measure between the big red wire at the alternator and ground.<ul><li><a href="http://en.wikipedia.org/wiki/Alternator#Automotive_alternators">http://en.wikipedia.org/wiki/Alternator#Automotive_alternators</a</li></ul>

RussB

To generate juice, the alternator must have a magnetic field (which is rotated by the pulley). Initially, at standstill, the current to create this field is supplied by the battery through the small blue exciter wire. So you must see voltage here when the alternator isn't spinnnig.

Once it's spinning, measure the alternator's output voltage at the big red wire.

mcorrow

Hmmm.. wonder why I was seeing 14.0 volts there... No ground wire?

boxerfan

Are you having trouble staying charged? The voltage between the big red and small blue should be 0V. Measure each one against ground and the voltage should be the same from one to the other with the car running. When the voltage is equal the light on the dash goes off. Too high or too low and the idiot light comes on.

RussB

<center><img src="http://www.humanspeakers.com/audi/images/charging2.gif"></center><p>
A traditional volt-ohm-meter draws a lot more current than a modern DVM (digital voltmeter) or DMM (digital multimeter). When measuring voltage with a VOM, you're simply placing a meter movement and a series multiplier resistor between the two test terminals.

By placing the negative lead of your VOM on the blue wire and the positive lead of your VOM on the red wire, you may have been sucking current (the VOM's current -- anything from a few microamps to milliamps, depending upon the quality of the meter: the better the meter, the less current it will draw.) through the voltage regulator and maybe the rotor winding. The alternator warning light (which may have glowed dimly while you had your VOM connected) would need to be a fairly high resistance -- which it prolly is.

Refer to the schematic diagram above. (Note that the blue wire is called the "trigger input". (It's synonymous with "field" and "exciter".) The voltage regulator has at least one junction drop (0.6 Volts for a silicon semiconductor), so that would account for the slight discrepancy that you saw, and who knows what voltage may have dropped across the rotor winding. You were temporarily drawing the voltage of the alternator's blue wire terminal down to about +0.6 Volts.

Edit: I just had a read of the above, and I'm not entirely happy with this idea. I don't know what the oil pressure warning unit does and I don't have a full car schematic nearby.

mcorrow

Anyway, you should definitely see 14.0 volts from the large wire to ground when running.... if not, then your alternator has failed...

RussB

Are you certain that you measured about 14 Volts between the red wire and blue wire? When? With alternator stopped? With it spinning?