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Wed Jul 29, 2015 9:05 pm

Greetings everybody. I'm starting to loose sleep on this. I just installed a fresh engine in my beetle with an alternator conversion. Also pulled in a new wiring harness following the instructions on the Bug-Me video so wiring is all new, engine new and tested. I've double checked all my connections. When I connected the battery, the alternator light immediately came on. I turned the key to the run position and the light went out. When I started the car, the light stayed out. I checked output voltage of the alternator at B+ while running and it was 14V but I only did this once so i need to verify this. With the key in the off position, I checked voltage at D+ and at the alternator light, it was 11V. Voltage on the ignition side of the alternator light was 2V, again with the key off. Checked the brushes and they are in great shape. I'm kinda lost at this point. The readings I've taken so far are really puzzling and I've spent literally hours combing the Samba for help interpreting them. Maybe someone here could point me in the right direction in figuring this out before I start replacing parts.

Thu Jul 30, 2015 4:51 am

You don't mention what year Bug it is or what type ALT you installed, which may or may not make any difference. There is no one accepted or standard way that everyone wires them up either, like fingerprints no two are exactly alike :wink:

So, hard to say without detailed pics of your wiring at the back of the speedo, wiring and connections at the ALT, and a brief wiring diagram of your particular installation.

My guess is that either the light circuit is wired wrong, or the ALT D+ has an internal short to power.

To help narrow it down, disconnect the D+ wire right at the ALT that goes to your light.
Connect the clip of a hand held test light to B+ power, and hold the tip to the D+ terminal.
With then engine off, D+ should supply a ground and the light should light.
With the engine started and running, the light should go out.

Once you determine this, post back for more.

Thu Jul 30, 2015 2:38 pm

Alternator model is bosch al82n. My bug is a '65. I'll get a pic up later, until then i'll describe what i've done on the wiring. I ran the large red wire from the battery to B+ and the red wire going to the dash also to B+. The small blue wire is going to D+. That's all on the alt. I'll post up my results on the test measurements along with a pic as soon as I can. Thanks.

Thu Jul 30, 2015 6:52 pm

I Googled bosch al82n, and found out it has a built in regulator, so that narrows it down some, thank you :wink:

So then, the test I've explained above is a very simple way to test that sort of ALT on it's own and takes the car's light and wiring completely out of the picture for now.

To recap, first disconnect the D+ wire from the ALT.

Then, with the battery connected to B+ on the ALT, connect a test light between B+ and D+.

With the key on but engine off and ALT static, the light should light because the ALT is not charging and it supplies the ground.

With the engine running and the ALT charging, D+ should produce more voltage than the battery does, so the light will go off.

Hard to explain, but super easy in practice, so hope you're following me so far :arrow:

If your ALT passes this test, then the problem is in the wiring to and/or from the light in the speedo.

Thu Jul 30, 2015 9:33 pm

OK Bruce, pretty simple tests to run. Here are the results:

key on engine off and ALT static - no test light

engine running and the ALT charging - no test light

Also, I double checked the output voltage from B+ with the engine running. At idle it's 13.0V, when idled up to about 2000rpm, it peaked at 14.5V.

Thu Jul 30, 2015 9:57 pm

Here are a couple of pics from the install and wiring. Blue wire on D+ goes to alt light, red wire on left B+ goes to headlight switch at 57b, red wire on right B+ goes to starter and battery.

Fri Jul 31, 2015 4:55 am

Hmmm, seemingly mixed results...
The stock dash light is essentially a hard wired test light with exactly the same circuit as we have just hooked up as a substitute.

As your post title says, if the dash light stays on with key off, then you should get the exact same result with the test light, but you say the test light doesn't light when connected to D+ with either KOEO or when the engine is running :?

Like I said, D+ should be a ground when the engine is off. Are you certain the test light works?

The ALT does charge though, which even with the light inop or disconnected some will self energize after the engine is running, but that still doesn't explain these symptoms you describe.

Try a simple test of the dash light by disconnecting the D+ at the ALT.
With key on/eng off (KOEO) the dash light should be out because the circuit is open.

Now touch the blue wire terminal to ground on the ALT casing.
With KOEO, the dash light should light because the circuit is closed.

Fri Jul 31, 2015 11:14 am

I suspect that the voltage regulator on your alternator has gone bad or was bad to begin with.

I had a 05' Golf that had an alternator that was less then a year old and the battery would keep going dead. Check battery - fine. Check alternator, both in and out of the car - good. disconnected the battery lead from the alternator and checked it in the morning - battery still had charge. Plugged it back in and check a few hours later - dead battery.

What sucks is it probably means a new alternator. Unless you can get the place where you bought it to accept what you are telling them since they will test it and it will test out just fine. I guess you could check that also by just putting a test lamp on the D+(?) wire from the alternator and see what you get. I may be wrong but you shouldn't be getting anything there so if you are it would mean I am right. Also you kind of know that already since the light indicates a low charge from the alternator which would be consistent with my hypothesis.

*Edit* I re-read the previous responses and it looks like the alternator light does not function the way I thought. So my comment may not be applicable.

Fri Jul 31, 2015 7:29 pm

Unfortunately AZbuckeye, I did pick the alt up 2nd hand. I couldn't pass it up for the price knowing i was going to need one. So you may be right that it was bad to start.

Fri Jul 31, 2015 8:00 pm

A quick and dirty toggle switch or a battery disconnect could be an easy fix if I am right.

Fri Jul 31, 2015 8:01 pm

Bruce, I have the results from the latest test. Touching the blue wire terminal to ground on the ALT casing with key on and engine off - dash light on.

So to summarize,

Trouble - key off engine off, dash light is on.
Turn key to on, dash light goes out and stays out when running.

Test results:

D+ wire disconnected, test light between B+ and D+.....
key on engine off and ALT static - no test light
engine running and the ALT charging - no test light
Alt output voltage from B+ to Alt casing ground with engine running - 13.0V - 14.5V

blue wire terminal to ALT casing ground with key on engine off - dash light on.

Looks like the wiring and dash light are good.

Fri Jul 31, 2015 10:26 pm

I think I may have found the trouble. I read the following from troubleshooting tips at Rockauto.com:

"an alternator can charge fine when the engine is running but drain the battery when the engine is off. It probably has something to do with the rectifier diodes in the alternator not properly blocking current from draining away from the battery"

I then searched The Samba for info on replacing the alt diodes and came across the following test procedure from Samba expert Ashman40:

"With the ignition OFF, disconnect the blue wire from the Gen light in the speedo. Test this wire for voltage. My guess is you will find voltage here. It may be less than 12v but it will be well over 1v. This is usually a sign that the internal diodes in the alternator have failed. Part of their function is to prevent current from the battery from flowing backwards into the B+ terminal. If you remove the B+ wire from the alternator stud and the voltage goes away you will know for sure. "

I found I had 11V at the disconnected alt light wire at the speedo. When I removed the B+ wire from the alt, keeping the one from the battery and the one from the dash clipped together, the voltage went away just like Ashman said. I also found that the alt and oil lights operated normally coming on and off when turning the key. Looks like I'm getting a new alternator. Not great news but at least the problem seems solved. Thanks for the help guys.

Mon Aug 03, 2015 9:49 am

Wiring in a relay could save you the alternator until it actually goes bad.

Fri Jul 16, 2021 2:15 pm

let me post something in this old thread.... extra check.

If:
light between D+ (disconnected the dash-light) and BAT turns off when you start the engine but the dash light is off when ignition key is ON but engine off (with D+ reconnected) and the OIL is off too, then the problem can be a fuse.

Check the "interior lights" fuse.
I spent a whole weekend bickering and then i cleaned all the fuse contacts, and now it works.

Stefano