Alternator voltage question

Nick, I got battery voltage on alternator red wire and blue wire with engine key *off*. could that be a problem? Thanks.

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[Casey at Pelican Parts]

If I recall correctly the red is hot at all times ground will be ground at all times and the blue should be hot with the key on. Does the battery light stay illuminated after the car is turned off? There is a diode that can fail causing that issue.

- Casey

[Franks89-911]

Battery light goes off when car turned off.

[DRACO A5OG]

Okay first you should not be working on the alternator with the battery negative side connected. VERY DANGEROUS!

What seems to be the issue? Are you replacing the alternator or just testing it?

Compliments of our Resident retired EE Dr Gerry aka 86 911 Targa

[Jdub]

How odd - my red and blue wires on the Valeo are exactly reversed of what I am seeing here.

[DRACO A5OG]

^^^ Whoa, and she still works?

But wait a minute, the blue wire connector is much small ID then the B+ terminal. So unless you swapped the terminal or connector, it is impossible to mix those up.

Blue is a 7MM nut
Ground is an 8MM nut
Positive ( B= ) is a 10MM nut

Remember this picture is upside down or you looking from rear of the engine facing down

[Jdub]

I guess the lesson here is that they change these connections over time. I have a very old Valeo I just pulled out of my '78 (internal VR) and the configuration is different.

A picture is worth a thousand words:



...and cleaned up for viewing:

[rick-l]

Originally Posted by Frank (guest)

I got battery voltage on alternator ...and blue wire with engine key *off*

Originally Posted by Franks89-911

Battery light goes off when car turned off.

These two statements are in conflict with the laws of physics but we are assuming these measurements were made with everything connected.

How were these measurements taken and what is the problem with the alternator.

[Franks89-911]

Hi All, I'm a noob to both message boards AND porsche repair, so be gentle! I'll do my best to explain better than I have heretofore. 89 911, Targa 3.2L standard stock engine (i.e., no turbo or weird mods).

Here's what's happened so far:

1. Took out alternator and fan assembly. Alternator being rebuilt (now for second time) at local shop.

2. Reinstalled alternator & fan after 1st rebuild. Belt spinning, good tension, no slipping, shims all good, etc. But no voltage whatsoever (tested both at battery w/voltmeter and cig lighter w/ plug-in digital meter). Revving to 2k and beyond did nothing. Battery light on dash stayed on the entire time when the engine was on, flickered only slightly at 3k rpm but stayed on and no increase in volts. Battery light came on as soon as ignition key was turned to on position, before I started the engine, and stayed on.

3. I have 3 wires/ groups of wires connected to my alternator. As you look down at alternator from the engine, I have 1 blue wire that goes on the 7mm bolt on left side, 2 brown wires that go on the 8mm bolt on the right side, and the 2-3 big red wires that go on the 10mm bolt on the bottom. Just as in Draco's pic above.

4. I asked Pelican if maybe wiring was bad in car and that's why alternator still didn't work after a rebuild. Pelican doubted wiring was issue, but suggested checking wiring in car while alternator out. So, I hooked up the battery to the car and started testing.

5. Battery is new, charged up, and giving me a crisp 12.6v.

6. I get 12.4v on alternator wires when I test the red (+) and brown (-) wires, at all times when the battery is connected. With key on, *and* with key off. Same thing with the blue wire - whether or not key on, I also get 12.4v at blue.

7. Now when I turn the key to on position when alternator uninstalled, battery light doesn't come on.

8. Rick - I don't know what the specific issue is with the alternator, other than it is not giving me the 13 - 14v I so desperately crave. Hopefully the second rebuild will do the trick.

**My question: Should I be getting constant voltage on the alternator wires when battery is connected, even if the key is turned off? Is that a problem - draining battery when car off, etc.?**

Thanks all for your advice.
Frank

[WyattEarp]

Originally Posted by Jdub

I guess the lesson here is that they change these connections over time. I have a very old Valeo I just pulled out of my '78 (internal VR) and the configuration is different.

A picture is worth a thousand words:

I just got done swapping the alternator in my '83 and it was wired just like this one. Alternator in it was a Valeo as was the new one. Blue wire in the top center.

[rick-l]

Originally Posted by Franks89-911

6. I get 12.4v on alternator wires when I test the red (+) and brown (-) wires, at all times when the battery is connected. With key on, *and* with key off. Same thing with the blue wire - whether or not key on, I also get 12.4v at blue With the key on.

OK the alternator is disconnected and you get 12 volts on the blue wire. That is normal. If the alternator was installed the light would be lit with the current path through the regulator and field coil to ground.


EDIT: Added assumed test condition [red]
sounds like the rebuild should have included a new regulator.

[rick-l]

The red wire is a direct connection to the battery so be careful.