Alternator high voltage issues

[SReppel]

I've searched and read a lot of posts and still have not found an answer.
I just put the engine back into my 78 Targa and noticed that the alternator was putting out way too much voltage - 15.5 to 17.5 volts.

Two years ago my Tach was bouncing so I pulled the alternator and had it checked.
The shop said it was ok and it turned out to be a bad voltage regulator.

While I had the alternator removed I noticed that the PO had cut the wires and used crimp connectors to refasten them. I removed them and soldered all the wires and shrink sleeved them.

This time I figured the voltage regulator went bad again. So I ordered a new on and installed it. No change, still putting out the same high voltage. I went and spoke to our local Porsche Shop owner and he said it didn't make sense and gave me two more older style regulators to try. No go again, still high voltage.

I checked the transmission to body ground again and connections to the starter and even got another battery, no change again.

I also spoke to the local shop that rebuilds alternators and starters and they said if the alternator is putting out voltage then the regulator should control it.

I am about ready to pull the alternator again and have it checked.

Has anyone every experience a problem like this with their alternator?

Any help would be appreciated.

Scott

[High Life]

Originally Posted by SReppel

I also spoke to the local shop that rebuilds alternators and starters and they said if the alternator is putting out voltage then the regulator should control it.

This - Its not an alternator issue - its a regulator issue - it may be the regulator needs adjustment - not sure if the regulator can be "mismatched" (Marchal vs Bosch) but the regulator should be adjustable. Find an old school alternator guy and see if they can help you adjust it.

[darrin]

Agree, without the regulator, the alternator (by definition) is going to put out the high voltage.

On my '86, the regulator is integrated into my alternator, while some older 911s had a separate, external regulator. Given your comment about the PO and rewiring, wondering whether you're replacing an external regulator that the PO may have wired out of the circuit and that your failed regulator is internal to the alternator?

[SReppel]

Everything has been working find until I reinstalled my engine last week. I had it out to replace the clutch etc.
The alternator does not have an internal regulator, it the old type that needs and external one.
I'm about ready to just get a new alternator with an internal one if I can't find the problem. 4 regulators and the can't all be bad since one is brand new.

[High Life]

Originally Posted by SReppel

. 4 regulators and the can't all be bad since one is brand new.

I wouldn't say any of them were bad - (they prolly are all good) just need adjustment.

A regulator senses the alternator output and "switches" on/off to maintain the voltage preset in the regulator.. This can be (or should be able to be) adjusted to the volts you desire. I just am not familiar with The Porsche models - so I cant tell you how to physically do it.

[High Life]

To add to this - as I have been reading a bit more - what AMP is the alternator?

Interesting post here: (See Post #10 specifically)

Alternator problems

[SReppel]

what AMP is the alternator?

Looks to be the stock 75 amp one. I just pulled it out and will have it checked tomorrow.

[High Life]

Originally Posted by SReppel

Looks to be the stock 75 amp one. I just pulled it out and will have it checked tomorrow.

Look at #2 post here:

Do the alternator and VR have to be matched??

Originally Posted by Lorenfb

The regulator and alt. don't need to be matched. The later SC type
of Marchal alt. is 70 amps (the best ever Porsche alt. except 996).

There are two possibilities for your system charging to 15 volts:

1. The voltage reg. is bad, or
2. The field diodes (The three that feeds the D+.) are open
or at least one is.

If you have the early alt. with the red plate on the back
(one of the worst alt.), then problem is the reg. since
there are no field diodes.

Have Fun
Loren
'88 3.2

Later on he describes how to adjust regulator.

[30westrob]

I have seen this before and it turned out to be a bad diode in the Alt. At first it did not make sense but the diode that was bad was one of the three diodes that send voltage to the regulator. Since the regulator was only getting 2/3 of the alternator output it over excited the field, resulting in overvoltage. Rob

[spoke]

Originally Posted by 30westrob

I have seen this before and it turned out to be a bad diode in the Alt. At first it did not make sense but the diode that was bad was one of the three diodes that send voltage to the regulator. Since the regulator was only getting 2/3 of the alternator output it over excited the field, resulting in overvoltage. Rob

You can test to see if a diode is bad by measuring the voltage from D+ to chassis and compare that to the battery voltage. They should be the same if all diodes are good. The schematic below is from the 914 but I think the circuit is basically the same.

The diodes you refer to are the 3 on the left side of the alternator below.

[Artfrombama]

Yes
I was having high voltage issues, borrowed a Motorola service manual and did some research. Turned out the regulator had a dirty ground, a main body ground behind the fuel filter.

[SReppel]

Brought the alternator to a local repair shop and a quick inspection just looking at it revealed a burnt wire. I'll find out more on Monday after they take it all apart.

[targa80]

The three small diodes are called exciter diodes that provide the feedback signal from the alternator to the voltage regulator. The diodes will over time fail due to heat damage (they become brittle and cause an open in the signal path) each exciter diode taps off of one of the three main power rectifier diode banks. A loss of one exciter diode will cause a low voltage output from the alternator and will cause the battery to be under charged. I have had this fault twice since 1991. The first time in 1994, I had the charge light on low idle and it dimmed with increased rpm's. Replaced all three with 1N4001 general purpose diodes.(note all three diodes disintegrated while removing them). The second time in 2017, one of the 1N4001 failed and only caused the low voltage output (under 12V) the charge light did not come on at normal idle. The only way I knew I had an issue was when I could not start the car due to a dead battery. Changed the bad exciter diode and fixed the problem. Also added an old school VDO volt and ampmeter combo where the ash tray use to be just so I have a reliable charging indicator besides the idiot light.

The only way I could imagine an over charging issue is if a rectifier diode shorted instead of an open failure.

[SReppel]

Had my alternator checked by a local repair shop and they did not find any problems. It is working as it should. They said the same thing, that it is has to be a regulator problem or in the wiring going to it.

Did a continuity test between the regulator plug and the wires going to the alternator. the Blue and Black wires checked OK but got an open line on the single brown one and the two that are tied together.

Question is, should I get a reading between the plug and single brown wire attaching to the alternator?

I've tried 4 voltage regulators and two are new.
I'm at wits end trying to find the problem.



Here is the back of my alternator.

[sapote]

OP, please use the terminal names such as DF, D-, +61, D+ to describe the connections between the alternator and the regulator, not color wires.

If you have high charging voltage, then do these measurements on the regulator and report:

1. voltage from VR D- to alternator body. (If there is voltage dropped between VR and alternator D-, then the VR sees a smaller charging volt and tries to cause the alternator to generate more
2. from VR's 61 (or +61) to alternator +61. (IF less than 0V then VR sees lower voltage and commands the alt to pump out higher volt.)
3. from VR's DF to alternator body (this is a square wave oscillating from 12V to 0V so maybe you need to measure in two modes: DC and AC (the DVM should be switched to AC mode)

Best,
Brian

[30westrob]

Yes the brown terminal plug should connect to the brown at the alternator. Porsche always uses solid brown wire as ground. The regulator brown connects through the 14 pin engine harness connector by the regulator panel. I would start there for your problem. Rob

[Artfrombama]

Regulator case to chassis should be 0 ohms. If it is not clean your grounds.

[sapote]

Originally Posted by Artfrombama

Regulator case to chassis should be 0 ohms. If it is not clean your grounds.

The trouble is the DVM will never display a zero value

In this case, it's better to measure from VR's D- to Alt's D- terminal if they have the D- connecting wire, and not from case to chassis.

[timmy2]

Originally Posted by 30westrob

Yes the brown terminal plug should connect to the brown at the alternator. Porsche always uses solid brown wire as ground. The regulator brown connects through the 14 pin engine harness connector by the regulator panel. I would start there for your problem. Rob

Rob, just an FYI.

No brown D- wire in the 14 pin connector on a ‘78.
Directly run from remote VR to Alternator case in the CDI wire harness branch.

Alternator to case and case to body grounds are the ones to test if VR is proven good, (in another car) and all alternator diodes check out.

Looked at photo, the D- brown wire should be in the bundle of 3-4 browns on a ring terminal at the alternator location, for continuity
At least that is how it was built originally.

[sapote]

OP,

15.5 @ idle and 17.5 @ 3000 rpm and higher? Or the voltage varies between 15.5 to 17.5 at a fixed rpm?