Generator Light Issue on 72 Bug

[Mike Morehouse]

Good morning.

I need a little advice with a generator light issue on my stock 72 Bug with generator. I drive my Bug mostly short trips in town - errands, church, work, etc...both day and night driving. Car is stock with OEM generator. No added accessories. Battery was replaced in 2012 and starts car fine.

I never have any generator light issues on normal trips in town.  Last night I took it up the interstate on about a 22 mile trip at about 65 mph and at the 20 mile mark the gen light came on. Car contined to run fine. Got to my destination and picked up a play script and got back in the car to make the 22 mile trip back home. Gen light stayed on the whole time. FYI, I did notice left and right turn signals were flashing very rapidly when I put them on on way home.

As I got close to home it did appear that headlights were getting dimmer. Made it home OK. Pulled in garage and left engine running to look at headlights. They did look dim. Shut engine off. Checked fan belt. It was intact and seemed to have plenty of tension. I connected my Harbor Freight trickle charger to battery and left it on over night.  It is still charging this am. If I do start car this am, headlights seem fine and there is no gen light on. 

IIRC, the last time I made his same length trip, the same thing happened.

Why would I have gen light issues on a 20 mile trip at 65 mph when I never see that around town?

Would a 30 or 35 minute trip cause a gen problem that a 10 minute trip around town might not?

I guess what I am saying is no gen light occurrences on numerous local trips but 2 consecutive light issues on 30 min trip up interstate.

I'd welcome your thoughts as this makes me wary of taking an road trips in the Bug.

If I didn't give enough info on symptoms please let me know as I do need some help. 

Thanks Mike Morehouse in Alabama

[Ray]

Perhaps a bearing is getting tight and the heat makes it worse causing belt slippage?

Sent from my Verizon Wireless 4G LTE smartphone

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[stokester]

The first thing I would do is use a meter on it to see the actual output voltage of the generator. 

Nick

[Mike Morehouse]

After trickle charging the battery over the last few days till I got a green light indicating it was charged,  the battery output with the engine off is 12.65. I started the bug and let it idle.  No gen light.  Revved up the engine and checked the voltage at the battery and it was reading about 14.3 vdc.

Wouldn't this indicate that the generator is ok?

What to check next?

Thanks Mike

Mike Morehouse

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[John Sroka]

Hi Mike.   The charging system could work fine when cool and then fail when warmed up.  A Google search shows  that the gen light coming on at highway speeds is not an uncommon problem, these same articles have good information on diagnosis and correction.  Maybe plan a 20 mile round trip on the interstate in daylight (headlights on to provide a load) and see if you can reproduce the symptom.  You can then isolate the problem to regulator, generator or wiring.   

[Mike Morehouse]

John, thanks. Do you have alink to the article about gen light on at highway speeds?

Mike

[stokester]

Yup.

A 100% charged battery should read 12.6 volts, 12.4 = 75%, 12.2= 50% and so on.

Over 14 volts from the generator should be enough to keep it charged and provide enough juice to run the vehicle.

So your battery shows that it is fully charged and the generator is providing a charging voltage but there may be other factors.  The battery may read fully charged but until you do a load test its capacity is not assured.  The generator has brushes with springs and a commutator which may be worn causing them not to make full contact.

As John mentioned, heat may be a factor.  As the armature and other components heat up there may be other things going on.

Nick

[John Sroka]

From your description it does sound like a circuit is opening up (failing) in the charging system when everything is warmed up and under load (lights on).   The trick is going to be getting the problem to recur, then testing your charging system before things cool back down.  Rob and Dave’s VW page has this write-up on testing:

 

Rob and Dave’s VW page http://www.vw-resource.com/generator.html 

 

Be real careful of the spinning pulleys/belts while measuring voltages.    If you do further web searches you will find tips on testing the generator brushes by sticking your finger through the vent holes, I’d suggest using a stick or another expendable item instead…

 

Let us know what you find.

[Mike Morehouse]

Thanks John. It would be easier if it was RPM related not time.

I'll see what i can diagnose.

Mike

[Glen Hadley]

Actually, over 14V at the generator is probably too high.  A max of 13.8V is generally considered enough.  Sounds like it is overcharging the battery, much over 12.5V resting is a bit high.  My guess is the regulator - it could be shutting down completely on a long trip after the voltage hits some threshold instead of keeping the voltage up.  If it's holding 14+V, the generator is OK - that's what you should see if you ground the control terminal of the generator, which is a sign that the regulator is the problem.

glenh :<)>

[Mike Morehouse]

Glen and others who have replied.  I have connected my DVM to the battery so I can see what voltage the gen is delivering to the battery why I drive around town. Yesterday  was seeing about 14.2 -  14.3

In order to check he regulator, whch wire do I disconnect at the reg and ground?

Mike 

[Glen Hadley]

The 14.2-14.3 VDC is a good voltage for bulk charge of the battery, but can cause it to boil (gas) if held after the charge is complete.  This may shorten the life of the battery, but it should still charge.  A much lower charging voltage will cause the battery to sulfate, decreasing the capacity over time.  It's sort of a compromise with the older 'not smart' regulators.  That's why most 'modern' alternators are under the control of the engine computer, which can implement a '3-stage' charging algorithm that maximizes the battery life.  My recollection of the regulator on the VW is that it has a 'current' coil that will shut charging down for a while when the charging current gets low.  A failure of this to reset could be your problem.  I have taken regulators apart and filed the contacts inside to restore them to usefulness (temporarily, they will burn again....).

The 'definitive' check to distinguish between failure of the generator and the regulator is to ground the "DF" terminal of the generator, at the generator or at the regulator.  The generator should go to something like 15V, more than the regulated voltage when it is under control.   To make this work, you would need to do it after the light comes on and your battery voltage is lowering, you seem to have sufficient voltage 'around town'.  Next time it fails, ground that DF terminal (the little one on the generator - don't try to ground the hot lead!) with a jumper and read the output voltage of the generator.  (don't leave the jumper on very long, it can damage the generator and/or battery). If it goes up to >14.5 V, it is the regulator causing the trouble, if it doesn't (no change in voltage, basically) then the generator is the problem.  Good luck!

glenh :<)>