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Posted by: stoneman30hotmail Sep 21 2023, 05:58 PM

I read some threads and "how to test alternator regulator". But they're pretty rudamentary. I don't get 13.5 V at the battery, it just stays at almost 12V. So I gather that usually means alternator. I do see 13.5V at the D+ pin and 3-4 at the DF pin. Does that mean the regulator is trying to work? I thought the 13.5 V meant that the alternator was putting 13.5 somewhere. I tried to short these to "full field" the alternator and fried a small wire without change at the battery. The regulator looks original and I would of course rather change that.

Posted by: Spoke Sep 21 2023, 06:26 PM

You can try removing the VR and shorting D+ to DF then starting the car. This will cause the alternator to put out maximum voltage. See what you get at the battery. The battery voltage during this test should be north of 15V.

BTW, is the GEN light on during normal operation of your 914?

Posted by: stoneman30hotmail Sep 21 2023, 06:51 PM

The GEN light was on to start with, then started to fade. The car has been in repair for a year as soon as I got it.

Posted by: L-Jet914 Sep 21 2023, 11:24 PM

If you can find someone with an old VAT40 or a MT3750-AVR that works, they can hook it up and put the amp clamp around the alternator output cable to see if it's outputting current at idle and then again around 2000rpm with loads turned on. Your other choice is a amp clamp for a DVOM that is capable of reading at least 60+amps.

Posted by: stoneman30hotmail Sep 22 2023, 11:01 AM

17V with that test. So regulator! The trick was taking off the old regulator I guess. Tryinng to short while running didn't work.

Posted by: Spoke Sep 22 2023, 11:07 AM

Seems to point to the VR. For sure it's easier to swap a VR versus the alternator....

Posted by: stoneman30hotmail Oct 9 2023, 08:45 AM

I'm not so sure now. While running I get a nice 13-14V at the battery. But the gen light is still on as there is 17 V at D+. So I'm expecting the alternator to quit.

Posted by: GregAmy Oct 9 2023, 08:53 AM

So the alternator is putting out...


Bad diode in the alternator. That 3-4V differential is enough to turn on the GEN light. Looks like an alternator swap is in your future...sigh...

On the plus side...the D+ 17V is only connected to the GEN light. So if you're willing to tolerate that light for a while (or willing to disconnect and tape off the wire) then it is not hurting anything and you can change the altie at your leisure.

I drove mine like that for some weeks while I got the part (had a core rebuilt by a crusty old dude in NH for $125, killer workmanship) and worked up the motivation for the fidgety job (a friend on top while you're working the bottom helps). - GA

Posted by: ClayPerrine Oct 9 2023, 01:39 PM

That is going to shut down the alternator completely.

All you can do is ignore the light until the diode is replaced. Ran a whole weekend at an MUSR with a bad diode. It shortened the battery life, but didn't leave us stranded.

You can always find a junk 914 alternator and swap in the diode plate if you are a CSOB.

Otherwise just buy a rebuilt alternator.

Hope that helps.

Clay

Posted by: GregAmy Oct 9 2023, 01:45 PM

Hmmm, I know what you mean. But I have excited disconnected 914 alternators by zipping them up to high RPM (not something you'd want to do right after startup).

My other solution in the race car (before I just removed the alt entirely and went "total loss") was to have a button that momentarily dropped D+ to ground. And I do mean very "momentarily", like just a quick tap of a spring-return button. Or, run the wire through an appropriate-sized resistor to fuse S9 (where the GEN light is connected).

Or, just ignore the light. As long as the voltage is being properly managed to 13-14V at the battery it shouldn't shorten batttery life.

Or - the correct answer - replace or rebuild the alternator. - GA