6V Generator - Lost polarity randomly

[earlywesty]

I have a bone stock 6V '58 36hp bus. The other day I was coming home from a cruise and the charging light came on during the last half mile. Belt was fine, all wires intact etc. Still started up fine, battery still good.

Was only getting 6.45V at high idle at the battery. Went through the tests and it was seeming like generator was ready for rebuild. As a last ditch, I re-polarized the generator and voila, it's charging fine now a little over 7.2V.

Why would it lose polarity out of the blue? I have been driving this bus on this powertrain for 5 years regularly and it's never had an issue. I have a larger trip coming up and I want to make sure there is't something else/a source problem I should be investigating.

any thoughts.

[BulliBill]

Hmmmmm... I've never heard of that happening to an occasionally to frequently used generator. VW warns in the Bentley Manuals to polarize a generator after it has been in storage for a long time, or brand new/never installed. But I've never (until now) heard of a generator losing polarity while in regular use. I'll be watching for other expert answers...

Bill
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Coeur D'Alene - Lake Shore Volkswagen
Mission VW - San Fernando
Thornton VW - Stockton

Thanks for any help!

[&Dan]

Had a Ford generator and regulator switch polarity several times over the course of a day. Neighbor dropped by to help unfuck the problem, he's an auto electric pro, said he'd never seen anything like what was happening- baffling.
I ended up replacing the harness between the gen and reg despite it all checking out fine w/ VOM. That cleared up the problem though checking the replaced wires after the fact still yielded no clues.
In the end, I suspect elves.

[BarryL]

Brushes free and good length with spring tension good? Armature clean?

[telford dorr]

I'd suspect the regulator. The only way for a generator to lose polarization is for current to flow backwards through the field windings. This can occur if the regulator keeps power applied to the field while the engine is stopping, as the generator output voltage falls below the battery voltage. Speculating: if the battery disconnect relay doesn't open fast enough, the generator field could remain powered backwards long enough to depolarize.
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