Printable Version of Topic

Posted by: whiterabbit Mar 30 2021, 12:59 PM

I've been browsing this site for awhile, as I restore my '75 914. I'm mostly done, just working on finishing touches and some drivability issues. You guys have been a huge help but the details of my current issue has me stumped. Below is a summary of the issue:

1-With key OUT of the ignition the charging light is "ON"
2- Key in on position, the charging light turns "OFF", oil light turns "ON"
3- Engine Running, charging light is "OFF"
4- Engine running, voltage regulator plugged in, voltage at battery reads 12.4 volts (not charging).
5- Engine running, voltage regulator unplugged & jumper between D+/DF voltage at battery reads 12.6-12.8 volts (still not really charging).
6-Occasionally, ignition key needs to be cycled several times before the starter is triggered to start the car (probably unrelated?)
7-Charging light turns "OFF" when alternator plug (specifically DF connection) is disconnected from relay board.

Background info: New alternator 100 miles ago, belt tight, newer voltage regulator, new alternator wiring harness, newer ignition switch, transmission to trunk floor ground strap clean and tight. 20 miles ago, the charging system was working as it should.

My theory: Alternator failure! But I don't want to throw parts at the situation if there are other clues in points 1-7 above.

Posted by: ClayPerrine Mar 30 2021, 01:08 PM

Before condemning the alternator, verify you have +12V at the D+ pin. If you don't, your full field alternator test is invalid. You can just run a wire from the positive battery post to the DF connection on the relay board and see if it charges.

Clay

Posted by: whiterabbit Mar 30 2021, 01:28 PM

Thanks Clay!!

Update: I verified that I have +12V at the D+ pin. Running a test wire from + battery post to DF connection and start engine, voltage at D+ changes to 12.9V.

Posted by: ClayPerrine Mar 30 2021, 02:50 PM

Now that you have verified everything else, I think you can just get another alternator.

Posted by: whiterabbit Mar 30 2021, 03:08 PM

Thanks for the confirmation. I'll report back any findings after I replace the alternator.

Posted by: Spoke Mar 31 2021, 05:09 AM

This is really weird and exactly opposite of what you would expect. In this diagram, the GEN light is powered by fuse F9. F9 is switched from the battery (not shown in the diagram).

One way the GEN light would be ON with the key off and OFF with the key on is if one of the 3 diodes connected to B+ of the alternator has died and shorted out.

You can test this by removing the cable from the alternator to the relay board and measuring the voltage on the cable D+ wire. If it's close to the battery voltage, it is likely that one of the diodes has died and the alternator needs replacing.

Did you do anything to the electrical system after installing and getting the new alternator running? Did you disconnect the battery with the engine running?

Posted by: whiterabbit Mar 31 2021, 06:32 AM

I also found items # 1 and 2 strange. At first I thought it was a failure in the ignition switch but that wouldn't explain the oil light turning "ON" as it should.

I didn't do any additional electrical work after the alternator was replaced nor did I disconnect the battery while the engine was running. I replaced the alternator last fall just before putting the car away for the winter. I drove the car about 80 miles over a couple weeks time. The day I parked it for long term winter storage, I shut off the engine, removed the key and noticed that the charge light was still on. I disconnected the battery and decided I'd deal with it in the spring...and here we are.

I'll report back on the results of Spokes suggested test. ...and here's a photo of my 914.

Posted by: GregAmy Mar 31 2021, 07:06 AM

Remember, the light comes on by a differential in voltage...regardless of the direction.

I thought it strange too, but it's certainly possible that an internally-shorted alternator could provide the hot side, and something in the keyed hot circuit could find a ground somewhere.

I'd start by disconnecting the three-prong plug from the relay plate and seeing if the GEN light stays off. If so, then you have a clue that that end is suspect.

Pulling the alternator is such a god-forsaken PITA hassle that once out, just get the damned thing rebuilt so you don't have to do it again, even if it checks good.

Posted by: whiterabbit Mar 31 2021, 09:00 AM

Agreed! I did confirm that removal of the alternator plug from the relay board turns the light "OFF", so the short must be in the alternator assembly.

+1 on alternator R&R being a PITA...on your back, working by feel only, contortionist arms/wrist/fingers...UGH!

I'm dropping off a spare alternator to a local rebuilder today. The one that failed was a re-man bosch unit from AutohausAZ.

Posted by: whiterabbit Apr 6 2021, 09:35 AM

Just an update on this charging light issue:

I replaced the alternator. Then I had a new set of symptoms.

1-With key OUT of the ignition the charging light is "OFF"
2- Key in on position, the charging light is "OFF", oil light turns "ON"
3- Engine Running, charging light is "OFF"
4- Engine running, voltage regulator plugged in, voltage at battery reads 12.4 volts (not charging).
5- Engine running, voltage regulator unplugged & jumper between D+/DF: voltage at battery reads 16.4 volts (battery is charging, VR failed??).

Replaced voltage regulator with spare and charging system is working as it should. So, it looks as though the alternator and voltage regulator both failed resulting in the symptoms of my original post.

Posted by: DRPHIL914 Apr 6 2021, 10:24 AM

what are the chances of that happening? did one cause the other? i am always curious of the "why" not just what.

Posted by: Spoke Apr 6 2021, 10:56 AM

Good to hear. What is the battery voltage now with engine at idle and 2k RPM?

It is possible that one item died and took the other with it.

Posted by: whiterabbit Apr 6 2021, 12:56 PM

Voltage at battery, at idle speed and 2000 RPM is 14.5 volts. It does not change as the engine speed increases.

Posted by: ClayPerrine Apr 6 2021, 02:13 PM

That is exactly how it is supposed to work.

Congrats!

Posted by: whiterabbit Apr 6 2021, 06:46 PM

Anyone have any suggestions on how to run a postmortem on the voltage regulator?

Posted by: ClayPerrine Apr 6 2021, 10:52 PM

Why bother? They are cheap, and disposable.

Clay