Alternator light

[Willin]

Hello
I am having an issue with the alt.light in my 73.
Rebuilt Bosch alt. New VR from laps. Pretty sure grounds all good.
In warm weather, at startup , the light will glow till I blip the throttle. So, right away.
Now that it is cold out, I notice that I have to rev. the engine more the colder it gets, to get the light to go out. I just fired up this morning at 15*F. Light will not go out. Took for a short drive and rev-ed it. Light staying on.
What is going on with cold temps. causing this?
Thanks in advance!

[busdaddy]

Drive belt slipping?
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[Amskeptic]

[mikedjames]

Its also worth checking the alternator brushes are not worn down, and the brushes can move freely in the brush holders.
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[SGKent]

Some electronic parts can be defective but only show up as temperature sensitive. Go thru the alternator and VR checks as in Bentley or what Telford wrote below when it is not working to confirm the defect.

[Willin]

I will certainly check the belt.
I have done those tests before, I will try them again. Or take it the three blocks to laps to have them test. They found that my first vr was bad, and the one they sold me went bad. Maybe now the third one is bad.
I will keep an eye on things. No heat out in the big sky garage, so will have to wait for a warm day. And I don't drive it as much in winter.
Thanks again.

[Starbucket]

Is the trans to body ground strap clean and shinney? that's how the volts get back to the battery from a rubber mounted assembly.

[Willin]

There was some solar gain out there today, so I did the Telford tests.
Seems my alt. is not working.
This unit has only @4000 miles on it. PO put it in, says "rebuilt" on it, it's a Bosch. Its , maybe, 5 to 10 years old.
What happens to these? Hope this isn't the life of the rebuilds these days.
I'll need to wait for the new year for time to dig in this deeper.
Thanks!

[SGKent]

[timvw7476]

You don't mention if your VR is the classic Bosch contact points type or a modern solid state unit. There are decent VRs in the classifieds here. Those are the ones that work, and well. (not solid-state)
Also double check the plug into the VR itself, the issue(s) I've had amount to that harness, or one of it's wires failing to seat or work loose, & the Too Long Stud Syndrome you find on rebuilt alternators when replacing one.

[Willin]

What I meant to ask was,
If the diodes went bad, what causes them to fail?
If it's the brushes, what causes them to fail? (Thinking out loud)
Alternator is not that old, I have gone through all wiring. Unit has worked fine for the 5 years I've had bus.
VR is solid state type from O'reillys. I had one of them fail (dash light staying on), so I did the Telford Test. It showed the Alt. was bad. Went to O'reillys, and their test machine showed the VR was bad. The store replaced it with the same style.All good to go till now. That was this past summer. Easy enough for me to have them test again , before taking thing too much apart.
This could be a repeat of that scenario. Hoping for easier solution than replacing alt.
Thanks all for your time! I have received lots of good info here getting this bus back on the road!
Jim

[telford dorr]

[Willin]

Thank you Sir! Sirs!
Have not disconnected the battery in months, and I have made sure the cooling boot is intact.
How about this? Like I mentioned at first the alt light would glow till I increased the revs. Warm weather, light goes(went) out right away. Cold temps, have to rev a little more. Would turning on the booster fan when the light was on mess the diodes up?

[timvw7476]

[telford dorr]

[Willin]

It was warm today so I pulled the Alt.
Checked wiring to starter and to vr. All good.
I have the old alt the po gave me. The Haynes manual says the diodes must flow in only one direction. Check with test light at no more than 24 volts. It also mentions un-soldering the diodes first. I did not. I used my 20v drill battery test light.
Tested old alt. first, flow in both directions.
"new" alt same , both directions.
Obvious question is what happened?
I went ahead and bought the only alternator I could find. "Quality Reproduction " from Cip1, and they only had one.
Is there some place folks send cores in for rebuild these days?
Or are the diodes available, and straight forward to replace?

[Wildthings]

I would hope that a town as big as Bozeman would have a mom and pop automotive electric shop.

I believe that the diodes are going to be common enough as they are likely the same as found in other Bosch alternator of the era. In the past I have scrounged wrecking yards for alternators (not necessarily Bosch ones) that I could take diodes and other parts from,

[telford dorr]

Diodes are available, but hard to find. You'll need a good medium sized soldering iron, 60/40 rosin core solder, and a solder sucker to disassemble them from the diode trio board. Work carefully, as things in there are somewhat fragile. Always unsolder the diode wire from the diode trio board, not the wire from the diode. NEVER put any force on the solid wire coming out of a diode - they cannot take a joke! If you need to bend this wire, use two pairs of pliers! Also, a cheap arbor press is handy to remove and install the diodes from the heatsink plate and alternator case.

Diode trio board above negative diodes:

Positive diodes, showing new diode installed:

Positive diodes:

A test stand is handy to test the finished alternator before reinstallation!
Type 4 alternator:

Type 1/2 alternator:

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More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)

[Willin]

I will check locally, there is one shop, maybe.
I would like to have a spare around.

In the pic of the t-4 stand, looks like you were building a bike wheel? Spokes?

Thanks all for your replys !
Jim

[Willin]

One more questions, Please.
I looked at my wiring. Big red wire says 10g/ 5.25mm. That is smaller than the 6(mm) , listed on the wiring diagram.
Bigger wire be better? What size?