Hi...Bought a great rebuilt 914 recently. The battery was getting over charged & fried. At the battery its reading up to 18V. Same with a new battery. I initially replaced the voltage regulator on the relay board but this made no difference. The board has has been checked by an auto electrician as OK. Obvioulsy I have then swapped the alternator for a rebuilt german one. Unfortunately me & the mechanic have run out of ideas. Other than changing the relay board too. Any help appreciated!! Dave

What brand of voltage regulator? DID you replace the alternator wiring harness when you replaced the alternator?

Thanks.....A good Elring one & have tried another unbranded one too!

What exactly is the alternator wiring harness? Haven't changed any wiring so far.

Wiring harness picture. Lots of discussions about differences in voltage regulators and how they work on the relay boards. Some of the late solid state do not seem to work as well as they should.

@dangermoose
@914sixer

What brand of voltage regulator? DID you replace the alternator wiring harness when you replaced the alternator?
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Thanks.....A good Elring one & have tried another unbranded one too!

What exactly is the alternator wiring harness? Haven't changed any wiring so far.
[/quote]

It is the harness that connects the alternator to the relay board.
Sounds like the new voltage regulator is bad. Inspect the harness for the wire condition. Remove the relay board cover, pull the three wire alternator connector and check the condition of the wires and connectors. Running high voltage could have damaged it.

Bruce

When you say late ones.....do you mean new? I've tried 2 different new ones with the right original Porsche part numbers which also fit other VWs with 2L Type 4 engines.

Guess I could try an old used one?

[quote name='bdstone914' date='Sep 21 2019, 07:27 AM' post='2748958']
@dangermoose
@914sixer

What brand of voltage regulator? DID you replace the alternator wiring harness when you replaced the alternator?
[/quote]
Thanks.....A good Elring one & have tried another unbranded one too!

What exactly is the alternator wiring harness? Haven't changed any wiring so far.
[/quote]

It is the harness that connects the alternator to the relay board.
Sounds like the new voltage regulator is bad. Inspect the harness for the wire condition. Remove the relay board cover, pull the three wire alternator connector and check the condition of the wires and connectors. Running high voltage could have damaged it.

Bruce
[/quote]
Thanks...
All the associated wiring looks good, but I could try replace it if I can find replacements. I will inspect it all again. Otherwise I will replace the board too.

Does the GEN light come on when you turn the key to ON before starting the engine?

Does the GEN light go out when the engine is started?

Check out the diagrams below. It helps to make some measurements to determine what is working in your system.

Note in the first diagram the signals D+, DF, D-, and B. Measure all 4 of these signals with respect to chassis ground. There's a stud behind the relay board and one behind the battery. Get chassis ground there.

D+, DF, and D- are on the alternator cable coming from the engine. Signal B is the battery positive. Measure all 4 with engine running. Let us know what those readings are.

D+ should be 14V
DF will vary between 5V and 12V
D- should be 0V
B should be 14V

Here's a picture of the relay board on my 914. The wire exiting the board to the right is the alternator cable. D+, DF, and D- can be found here.

Nice red earth wire.....

I just realized this is not my 914. It is SirAndy's from his fuel pump connection for carb conversions.

Many thanks....I will get the mechanic to do the checks this week.....DAVE

My other thought is.....both alternators are definitely the right part.....one newly refurbished.....so should either of them be turning out 18v?

Is that the issue somehow rather than voltage regulator or other wiring?

Before you do a lot of electrial work. Make sure that the plugs for the voltage regulator has not pushed through the potting on the relay panel. You do this by just feeling around under the relay board under the regularot plug. If the D- plug is pushed through your voltage regulator will do exactly as you have described. That exact thing happened to me last summer.

The potting is old and simply changing the VR can push the plug through.

Zach

The alternator can output 17-18V if maximum field current is applied to the rotor. Looks like both alternators tried both function properly and are not the problem you are having.

No field current = no alternator output.

The VR provides the field current to the rotor. The VR monitors the D+ voltage from the alternator and adjusts the field current to the rotor via DF. Without the VR there should be no alternator voltage.

As Zach points out, there could be other physical issues affecting the operation of the charging circuit. Do the measurements suggested above and let us know what you find.

Thanks for the helpful education on how the VR regulates the alternator.

As Z suggested.... I took the board off to look at its physical shape & the mechanic hand tested it best he could....but I agree this still seems to be the hopefully likely culprit.....

A used replacement one that looks in good shape has arrived today.....so we will try that asap!

Otherwise we have to go over all the wider associated wiring again.

Fingers crossed

Hi again all...

In case anyone is still interested..... I thought I would let you know the outcome of all this!

Turns out it was the wiring harness between the alternator & board. Even this all looked good & had been continuity tested, further checks found that 2 of the individual wires were crossed in the way they'd been put into the socket! God knows why this came about!

Oh well.... at least I now have a spare alternator & board!!

I'll be posting more of the remaining issues soon!

Dave
UK