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Fri Sep 14, 2018 10:57 am

Vanagonauts of the world,

My Alternator stopped working all of a sudden, resulting in a no battery recharge situation and the alternator warning light remaining lit.

I changed the brushes / voltage regulator, checked the grounds and even changed out the alternator with a spare that was working when pulled - no dice.

Searching through the archives, I only see reference to the workings of the D+, i.e. that it supplies a tickler voltage to boot the alternator windings - but nowhere does it state what the voltage should be.

I measured the voltage of the unplugged D+ wire and got +/- 5 volts (positive lead of voltmeter to D+ and negative lead to a good ground). does this sound right?

Thanks in advance!

W

Fri Sep 14, 2018 11:09 am

I'd expect battery voltage. On the blue wire, with the wire unhooked from the alternator and the key switched on.

Mark

Fri Sep 14, 2018 12:22 pm

I too would have expected at least closer to battery voltage.
Any chance someone could measure it for the community?

W

Fri Sep 14, 2018 1:00 pm

There could be and probably is a voltage drop on the blue wire because..the ALT charging warning lamp is in the circuit. Might only get 5v..not surprising.

If you ground the blue wire the warning lamp should illuminate with the key on, engine not running. If you un-ground the blue wire..the warning lamp should go out.

If the blue wire is connected to the alternator key on, not engine running..the warning lamp is illuminated. Battery voltage flows through the bulb to the alternator which is not generating any voltage.

When the engine is running if the warning lamp is illuminated..it is because voltage is out of balance between the alternator and the battery. Both sides should be the same voltage (or pretty close) and make the warning lamp go out.

Voltage is either going into the alternator because it is not charging..not enough 12v electrical pressure from the alternator from the battery though the warning bulb so it lights up. Or..the alternator is over-charging and 12v electrical pressure at the battery is now overwhelmed by the overcharge through the warning lamp and it is illuminated.

Bosch thoughtfully put a diesel tachometer output connection on many alternators and the position looks like a blade connector. D+ should not plug in there and should be a loop connector..but not always.

I don't think you have an exciter wire problem. I think you have two alternators with bad diodes..it happens. If everything is grounded correctly including the battery ground both at the chassis and battery clamp and that big ground strap under the car between the chassis and engine transmission (that one will keep the starter from engaging) then charge up your battery an drive over to your local FLAPS and most will offer a charging/electrical system check. No charge :)

See what the test readout says?

Fri Sep 14, 2018 1:07 pm

It is a diode, not a lamp, also know as an LED.

Most of the excitation current goes around the led, via a parallel bypass resistor of a lower value than the led's limiting resistor.

Mark

Fri Sep 14, 2018 1:41 pm

Crazy Van Man is correct..alt warning lamp is a LED with a series 150 ohm resistor.

Still should have a voltage drop though.

Fri Sep 14, 2018 1:44 pm

Thank you all for thoughtful replies.

I suppose it is possible that I have 2 alternator with bad diodes, as unlikely as that seems. I think I will recheck and reclean and reset all my grounds, and try again.

In the meantime, if anyone could measure the voltage on a properly working system, that would be great and would add to the community's knowledge base.

I'll let you know what the end of the story is.

W

Fri Sep 14, 2018 2:06 pm

Not as clear cut as you might think. Various models had various other things connected to the blue wire that could affect the reading, while some vans had nothing else sharing it. Some people have added aux battery relays that pull some current from the blue wire. You didn't tell us anything about the van in question.

Mark

WiboBusMan wrote: ....
In the meantime, if anyone could measure the voltage on a properly working system, that would be great and would add to the community's knowledge base.

I'll let you know what the end of the story is.

W

Fri Sep 14, 2018 2:56 pm

1984 Canadian Westy. No ox counter. AFAIK nothing else connected, as I'm the third owner, my father being the second and not prone to playing with the wiring. First owner, a Canadian fly boy bought it while stationed in then West Germany during the cold war and during Germany's status of total capitulation post ww2. He handed it to my father in pristine condition.

I removed the foil from the instrument panel and put 12.39v to terminal 9 and measured 12.00v at terminal 13, the blue wire's source. One would expect mild line resistance the length of the blue wire, but not to account for such a drastic voltage drop as I'm reading (+/-5v). I may well have found my problem. Could be the wire or the connector at the junction box or perhaps at the D+ alt connector...

Stay tuned and if someone could measure off their working system...

W

Fri Sep 14, 2018 3:09 pm

In an 84 Westy with stock 3 way fridge there is a relay under the driver seat that connects to the blue wire D+ circuit. Maybe something else put inside that compartment is shorting out the blue wire circuit. At the least look closely there and try unplugging the blue wire from the relay there to see if that helps.

Mark

Fri Sep 14, 2018 3:39 pm

I will check that out! Thanks.

W

Fri Sep 14, 2018 5:41 pm

Or the exciter wire may have decomposed internally due to corrosion and/or vibration.

Sun Jan 24, 2021 7:44 pm

I’m also seeing about 2 volts on my D+ wire. Did you end up figuring out what your issue was?

Mon Jan 25, 2021 12:26 am

The most common problems are corroded connectors or broken wires in the engine bay. Sometimes bad ignition switch, corroded, melted wires under the dash. Or cracked blue foil on the back of the speedo.