New Alternator Time or Just A Loose V-belt?

[Jim Ed]

I’ll be driving along in the day time and the needle on the dash voltmeter will drop to just under 12 volts.

I turn on the headlight switch and the dash voltmeter goes back up to over 14 volts.

I turn off the headlight switch and it stays over 14 volts.

Then everything will be just fine and once I think everything is alright it happens again.

Is this from a loose v-belt or is my alternator talking to me saying its time for a new one?

1973 Beetle
Any help is appreciated.

TIA!

[Marc]

First question I have is, where's the voltmeter connected?
It's possible that you have a loose/dirty connection in the B+ circuit. Its resistance could climb (resulting in a drop in voltage at the point where the meter's hooked in) and the increased draw cause by switching on the headlights causes it to heat up and "self-clean" a bit - that'd explain the symptoms you report. Otherwise I'd say the alternator field brushes/slip rings could be making poor contact. One way to tell which would be to run a dedicated wire for the voltmeter from the battery positive post. Keep it small so if it shorts out it'll fry rather than make the battery explode; even better, add a small inline fuse at the battery end.

Assuming you have stock steel (not chromed) pulleys, the belt should be tight enough that you can turn the motor over by hand by turning the generator pulley (or nearly so) - that equates to about ½" deflection when you push in on one side of the belt midway between the pulleys. You should be running an 11.3 x 912La belt with an alternator rather than the 9.5 x 905La that's adequate with a generator...it's not an issue in normal driving when the electrical load is nominal, so the 9.5 is good enough to carry for a spare, but if you need to buy a new one be sure to get the 11.3.

[Jim Ed]

First question I have is, where's the voltmeter connected?.

To the fuse box. I am not of which fuse.
I also have an alternator tester that plugs into an aftermarket 12 volt power outlet.
The 12 volt power outlet is also connected to the fuse box.
It has three lights that light up.

When the alternator is working the green light comes on.
When it is not working the green light does not come on but the middle yellow light comes on.

So, when the voltage on the voltmeter drops below 12 volts on the dash voltmeter the alternator tester green light goes off and the yellow light comes on signifying that the alternator is not charging.

After I turn the headlights on low beam and then turn it off again, the dash voltmeter goes back up to over 14 volts and the lights on the alternator tester the yellow light goes out and the green light comes on signifying that the alternator is charging.

What I want to do next, when I get time, is to connect a digital VOM directly to the battery and observe it and the dash voltmeter.
If the dash voltmeter shows a voltage drop then the VOM connected to the battery should also drop.
If this happens then to me it means the alternator is due for replacement.

It's possible that you have a loose/dirty connection in the B+ circuit. Its resistance could climb (resulting in a drop in voltage at the point where the meter's hooked in) and the increased draw cause by switching on the headlights causes it to heat up and "self-clean" a bit - that'd explain the symptoms you report..

I replaced the main wiring harness in 2005.

Assuming you have stock steel (not chromed) pulleys, the belt should be tight enough that you can turn the motor over by hand by turning the generator pulley (or nearly so) - that equates to about ½" deflection when you push in on one side of the belt midway between the pulleys. You should be running an 11.3 x 912La belt with an alternator rather than the 9.5 x 905La that's adequate with a generator...it's not an issue in normal driving when the electrical load is nominal, so the 9.5 is good enough to carry for a spare, but if you need to buy a new one be sure to get the 11.3.

Yes, I am using a Continental 11.3 X 912 v-belt. I do not think it says La after the 11.3 X 912.
It slips a little when I try and turn it over at the small pulley.

TIA!

First question I have is, where's the voltmeter connected?
It's possible that you have a loose/dirty connection in the B+ circuit. Its resistance could climb (resulting in a drop in voltage at the point where the meter's hooked in) and the increased draw cause by switching on the headlights causes it to heat up and "self-clean" a bit - that'd explain the symptoms you report. Otherwise I'd say the alternator field brushes/slip rings could be making poor contact. One way to tell which would be to run a dedicated wire for the voltmeter from the battery positive post. Keep it small so if it shorts out it'll fry rather than make the battery explode; even better, add a small inline fuse at the battery end.

Assuming you have stock steel (not chromed) pulleys, the belt should be tight enough that you can turn the motor over by hand by turning the generator pulley (or nearly so) - that equates to about ½" deflection when you push in on one side of the belt midway between the pulleys. You should be running an 11.3 x 912La belt with an alternator rather than the 9.5 x 905La that's adequate with a generator...it's not an issue in normal driving when the electrical load is nominal, so the 9.5 is good enough to carry for a spare, but if you need to buy a new one be sure to get the 11.3.

[Marc]

Before replacing the alternator I'd pull the brushholder and inspect the slip rings. Sometimes the brushes are worn enough that they aren't making good contact with the slip rings and they get scorched. The brushes don't always need to be replaced, sometimes it's as simple as rerouting their pigtails to give a bit more slack. There are several different brushholder assemblies depending upon the vintage and country of origin of the alternator so most VW parts places have given up trying to keep them in stock, but any shop that specializes in rebuilding electrical components will have some loose brushes that you can fit (I've even got them from vacuum cleaner repair shops). Shine up the slip rings with some emery paper and the alternator's good for a few more years - or until the bearings go bad.

[Jim Ed]

I went ahead and bought a new or new rebuilt Bosch AL-82 and it's on the way.
Whew! It was pricey.
I will try your tip on shine up the slip rings with some extra fine emery cloth and try and use the shop vac to get the filings out the day or so before I replace the alternator. Then I might have a good spare.

Before replacing the alternator I'd pull the brushholder and inspect the slip rings. Sometimes the brushes are worn enough that they aren't making good contact with the slip rings and they get scorched. The brushes don't always need to be replaced, sometimes it's as simple as rerouting their pigtails to give a bit more slack. There are several different brushholder assemblies depending upon the vintage and country of origin of the alternator so most VW parts places have given up trying to keep them in stock, but any shop that specializes in rebuilding electrical components will have some loose brushes that you can fit (I've even got them from vacuum cleaner repair shops). Shine up the slip rings with some emery paper and the alternator's good for a few more years - or until the bearings go bad.

[Jim Ed]

Before replacing the alternator I'd pull the brushholder and inspect the slip rings. Sometimes the brushes are worn enough that they aren't making good contact with the slip rings and they get scorched. The brushes don't always need to be replaced, sometimes it's as simple as rerouting their pigtails to give a bit more slack. There are several different brushholder assemblies depending upon the vintage and country of origin of the alternator so most VW parts places have given up trying to keep them in stock, but any shop that specializes in rebuilding electrical components will have some loose brushes that you can fit (I've even got them from vacuum cleaner repair shops). Shine up the slip rings with some emery paper and the alternator's good for a few more years - or until the bearings go bad.

I will try this. The old alternator would then be good for a spare.

[Jim Ed]

I finished replacing the alternator yesterday. It took about 1 week which included a couple of bad weather days.
I installed a Bosch 51 Amp AL82N. I got it from aircooled.net.
The Bosch 51 Amp AL82 I replaced lasted almost 21 years. I got it from CB Performance.
The voltmeter gives a really good reading now.
I got the fan off by installing an old alternator pulley and gripping it in a vice on the work bench.
I did not need a big air impact wrench and was able to do it by myself.

[Marc]

...I got the fan off by installing an old alternator pulley and gripping it in a vice on the work bench.
I did not need a big air impact wrench and was able to do it by myself.

One of many techniques which usually work - just note that they all rely upon the integrity of a tiny Woodruff key and its slot. The fan nut should only be torqued to ~45 lb-ft; it's 36mm size (same as axle & gland nuts) tempts some to blast it on much tighter - when that happens, the only non-destructive technique is an impact wrench.