[T3] Alternators for T-3's...

Jim Adney jadney at vwtype3.org
Sat Apr 28 08:25:11 PDT 2012

I'm always amazed by the amount of interest this topic brings up. 

Having driven Type 3s with OE generators for over 40 years, I've 
never had any need for something "better." Yes, VRs, brushes, and 
bearings wear out and need occasional replacement. But alternators 
also have VRs, brushes, and bearings. Early alternators, up thru 
about 1974 used external mechanical VRs, but by the mid '70s everyone 
had started building solid state ones into the alternators. Both 
styles occasionally need to be replaced. I've replaced quite a few of 
the later style, built-in, solid state regulators. For Bosch 
alternators it's easy to do, and the VR assembly includes a new set 
of brushes. I think it's a lot harder, or impossible, on some of the 
Japanese alternators (meaning, you get to replace the whole 
alternator, instead of just the VR.)

My impression is that the "problem" that ends up frustrating many 
DIYers is a simple lack of understanding of how to troubleshoot 
electrical problems in general and charging systems in particular. 
Thus, the charging system seems like a black box that needs to be 
replaced, rather than a collection of discrete components, each with 
its own function, and each separately testable and repairable at much 
less cost.

I should note here that if you are minimally competent with a 
voltmeter, I have a writeup that I'll gladly send to anyone who 
requests it. It will walk you thru testing the OE generator system 
and allow you to pin down and fix most of the common problems that 
arise with it. 

Also, if you're having trouble with your OE generator itself, that 
you can't fix there, I can usually fix it here, cheaply. They are 
heavy, but Priority Mail flat rate boxes make this reasonable. If I 
can't fix it, I have lots of good ones here that I can sell.

In all cases, the voltage stability is controlled by the VR, down to 
the lower rpm point where the alternator/generator can no longer keep 
up. Both systems have these lower thresholds.

Both systems have their weak points. The generator system has a low 
rpm threshold that is above the idle speed, simply because generators 
have an upper rpm threshold that limits the pulley ratio. Alternators 
have no upper rpm threshold, so their pulley ratio can be high enough 
to put their lower threshold below idle, but they have 9 (sometimes 
6) solid state diodes which can be zapped instantly by an accidental 
short or overload that would not bother a generator.  

I realize that the 30 Amp generator that comes with 12 V Type 3 
systems may seem small to many people, but it's more than enough to 
keep up any possible load you could put on the car while driving. If 
you like to run all 4 headlight filiments, the windshield wipers, and 
the  4-way flashers, all while idling and running enough sound system 
power to entertain a block party, then, yes, you're going to run your 
battery down.

What most people don't understand is that you'll probably run your 
battery down doing this with a 50 A alternator system, too. Because 
your 50 A alternator can't come anywhere close to 50 A at idle. 
You'll probably be lucky to get 10% of the full rating out at idle.

I've got a DC clamp-on ammeter that I'll try to remember to bring to 
Co Springs. If there's someone there with an alternator upgrade. It 
would be interesting to check it out.

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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