Alternator/generator light proper voltage measurement 66 bug 12v

[mikesbeetle]

Hello:

I’ve researched numerous posts on the topic since it seems popular but can’t find my answer. I have a 66 bug for about 10 years now. Recently I noticed a very dim alternator light when driving around. No big deal, battery seemed ok. Just recently, during a drive the light came on and now stays fully lit (when car is on). Fan belt is fine. I measured voltage on the cable to this light on the speedometer and only get 1.5 volts or so. Everything else to the speedometer seems a little under 12.

The bug was a 12v conversion when I got it and uses an alternator with I believe an internal voltage regulator since I can’t locate one. Including a picture for reference. I'm assuming I should be seeing near 12 on that cable? Where does the other end connect to? Wiring diagrams I find seem to point to a voltage regulator.

Thanks for any help.

[VW_Jimbo]

Normal battery voltage is 12.66 volts direct current of vdc.

Alternator should put out between 12.8 to 14.6 vdc. Really depends on several variables, but I like to see 13.1 - 14.2vdc. If it will maintain this through a load test, then all is good.
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Jimbo

There is never enough time to do it right the first time, but all the time necessary the second time!

[ashman40]

[Cusser]

Test voltage across the battery terminals with engine running. Should read about 13.5 to 14.5 volts.

Do NOT disconnect the battery while engine is running !! Can damage any alternator.
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1970 VW (owned since 1972) and 1971 VW Convertible (owned since 1976), second owner of each. The '71 now has the 1835 engine, swapped from the '70. Second owner of each. 1988 Mazda B2200 truck, 1998 Frontier, 2014 Yukon, 2004 Frontier King Cab. All manual transmission except for the Yukon. http://www.thesamba.com/vw/forum/album_page.php?pic_id=335294 http://www.thesamba.com/vw/forum/album_page.php?pic_id=335297

[mikesbeetle]

Thanks for the guidance everyone!

It looks like based on the results the alternator is bad but here are my results:

Notes:
I had the battery on a trickle charger like usual and it was at full charge before starting my testing. Charger was disconnected before any testing started.
The GEN and OIL light both come on when I turn the key on.
As I sit in the driver seat, the OIL light on the right goes out when I turn the engine on. GEN light on the left stays on, even with higher RPM.
I’m not sure where the 2 wires go from B+. They both go behind the firewall, my guess is the right one goes to the battery and the left to the front of the car. There is no junction under my rear seat like you said is on some cars.

Readings on wires that went in to the speedometer are in the image below for reference to make sure I’m using the correct reference for the wires.
The GEN light wire whether the engine was on or not read about 1.9v
The bottom terminal to the speedometer (black cable) read about 11.3v - ignition on and/or engine on. It was interesting to me the blue/green cable on the left only read voltage with the engine on.

Suggested tests performed:
Disconnect D+ from alternator and test loose wire: 10.65v
Test D+ terminal with wire disconnected: -.82v

-Reconnected cable-
Test voltage at B+ terminal (ignition on/engine off): 11.48v
Test voltage at D+ terminal (ignition on/engine off): .5v

-Start Engine- (GEN light on/OIL off)
Test voltage at B+ terminal (engine on): 11.6v
Test voltage at D+ terminal (engine on): 1.26v

-Rev Engine to 2100 rpm and hold- (GEN light on/OIL off)
Test voltage at B+ terminal (engine on): 11.8v
Test voltage at D+ terminal (engine on): 1.2v

Battery reading - Engine on:
12.18v at Idle across the posts

Reference photo.

[ashman40]

From those readings, it sounds like there is a short to ground inside the alternator D+ terminal.

Time to remove you alternator and take it to your FLAPS or local automotive electrical shop to test it. If it is bad, then you should be looking for a replacement new or rebuilt. Just be sure to test the replacement before you go thru the effort of reinstalling it. I would hate for you to find you got a dud and have to go thru the effort of removing it and reinstalling a replacement all over again.
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AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[toxicavenger70]

If it was me I would check all the wiring to see if all the connections are cleaned and wired correctly.

Here is a thread showing were the internal regulator is located. https://www.thesamba.com/vw/forum/viewtopic.php?p=8836146

If there is a short on the D terminal like Ashman said you can remove the black box on top (holds the brushes) and see if there are any issues with the connections on the back of it.

[mikesbeetle]

I appreciate the input. Is there any risk to taking off that component on the alternator and inspecting it with the battery still hooked up or should I completely disconnect the battery first?

[ashman40]

When doing ANY electrical work... disconnect the negative terminal from the battery.
Testing is a slightly different story.
_________________
AshMan40
---------------------------
'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[glutamodo]

I skipped down to the reply box the instant I heard the equivalent to: "glowing charging lamp" without reading the other replies yet. I just wanted to interject VW's own statement on this phenomena in the Generator Era:

I lived with this for years on my 62 Beetle, not knowing which link in the chain was the culprit until it came to a head. I was driving in I-70 in the mountains of Colorado, where I lived at the time. Fortunately this did not happen in the very long Eisenhower Tunnel, but one of the other shorter tunnels, the charging lamp started to glow really bright and then... nothing. The ignition switch gave out. That was the culprit. I was able to coast to a stop on the shoulder outside the tunnel and hot-wire the car. Later, I dismantled another compatible ignition switch (not exactly easy as I don't think they were assembled with the intent to ever take them apart again, kind of like the mechanical fuel gauges of the era) and marry my tumbler section to that switch, so I fixed it retaining my keyway.
_________________
Andy T.

"What is Glutamodo, Horror of Toyko, besides total, fiendish, quality-a-you brain!"

[herbie1200]

Do not waste time with all those measurements.

The light check is very simple.

Turn ignition on, engine stopped. Then:

1) The gen light should go on.

2) go to the alternator, disconnect the D+ wire, the gen light should go off.

3) Finally short the wire to ground, the gen light should go on.

If these 3 point are OK, the gen light and its wiring are OK.

Please report and we go forward in the analisys.

[HENDO]

[scrivyscriv]

The light comes on based on differential voltage, or differential potential.. it doesn’t care which side is ground or not. It just needs the voltage one on leg to be higher than the other, and it will light up.

When your alternator fails and is overcharging, it will light up.
When your alternator fails and doesn’t charge, it will light up. Current is flowing different ways in those examples but it doesn’t matter.

So yes if you’re revving up and the light gets brighter, your alternator is overcharging, usually a diode is shorted in that case.
17 volts is not okay. It will make the battery explode if you drive too long like that

It’s repairable. Most would recommend taking it to a starter repair shop versus buying a Chinese aftermarket new pRt
_________________
Robert in Memphis
Dünkelgrügen 1967 Java Green bug thread
Engine rebuild thread
If you're ever in the Memphis area, you are welcome to stop by for advice and help.

[HENDO]

[HENDO]

Ill just pull the alternator out of my 66 and use it. I have that all torn apart to restore anyway.
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84 Westy AKA "Sludge"

[ashman40]

[HENDO]