The 1975 is not recharging the battery, so I obviously have either a fault with the alternator or the wiring/charging circuit.

In the interest of being just slightly less of a dope than I currently am, what would cause the charging circuit to fail if it is not an internal alternator issue. In other words, if we assume that the alternator is healthy and there’s no issues with the brushes etc. internal to the alternator wear with the wiring I have gone sideways to result in it not charging?

This seems like a useful topic to get into the forum here, thanks in advance for the shared knowledge.

Voltage regulator not regulating?

Do a full field test on the alternator.

With the car running and the voltage regulator unplugged, hook a DVOM to the battery to measure battery voltage.

Send positive 12v to the field connection for the regulator.

You should see something around 14 to 16 volts at the battery, and hear the alternator whine as it loads up.


If that gets the alternator charging, you have a bad voltage regulator.

CAUTION: Don't leave it hooked up more than a few seconds to test. The alternator will overcharge the battery and boil out the acid. And that is not a good thing.



Clay

Grounds. Check them. Clean them.
Even a small ground fault will cause you alternator not to charge your battery.

Read my thread from a few years back, which REALLY got into a weird issue that involved powder coat getting where it was not supposed to be, and causing juuuuust enough resistance to leave me stranded on longer drives.

http://www.914world.com/bbs2/index.php?sho...d++powder++coat

Also, I had an issue where while replacing the VR, I somehow pushed one of the terminals on the relay board through the potting. Alternator was not charging. Drove me NUTS. I found that one totally by accident while checking the chassis ground behind the battery and the terminal that had pushed through brushed my arm and startled the hell out of me.

Zach

Also, if your bulb for the alternator doesn't light up when you turn the key (or you have replaced with an LED) it will not get the small charge necessary to excite the field and allow it to charge.

Yes! One of the old Mercedes that I had mysteriously stopped charging. It was a 1981 280CE grey market car. I called the shop that rebuilt all of my starters and alternators and their first question was if the alternator light came on when the ignition was first turned. Guess what? The bulb was dead. I replaced it and the alternator began charging.

Charlie

Hopefully this is it.

Thanks for all of the responses - the bulb works (glows with key on and when running).

And it's not an LED bulb?

Not to hijack the thread but wanted to state that I'm having a similar issue. The alternator light is ON when in accessory mode or with the car started. I'm going to remove the voltage regulator (if anyone can give me a quick location of that it would be great - waiting for my 914 shop manual to arrive - total noob here). Hoping it maybe the voltage regulator.

The light wasn't on 2 weeks ago (it got shipped from BC over 3000+ Km to Ontario) as per the previous owner. Not sure if something came loose in transit. I've attached my charger that tests the alternator which says its fine. So I'm hoping its the voltage regulator that's bad.

engine bay driver side at the relay board


Click to view attachment

Highjack away if it helps, no worries.

My dash light is not an LED btw, so problem elsewhere.

Possibly a bad relay board. Clays test may rule this out too.

I'm an hour east but if you need parts like the VR I do have some known good parts, just return or replace.

You can do a rudimentary test on the alternator that Clay mentioned by removing the VR and shorting the D+ and DF pins on the VR connector on the relay board. Then start the car and measure the battery voltage. It should be around 16V for a good alternator.

All great advise.
Do these basic checks as well.
Alternator belt tight ?
Battery terminals clean ? and tight ?

I was going to say grounds... also.
How about a loose belt?
Bad connections on battery?
As I have leaned its always the simple things that fail.
Matt

Hi folks – just wanted to close this out. Turns out I had a good alternator but a bad voltage regulator. Thank you for the help and knowledge, very much appreciated.