924Board.org

Meneer aard · Joined: 27 Jan 2006 Posts: 73 Location: Netherlands

Yesterday ive got a problem with my alternator. While i was driving the voltage drops to 10V and the battery light comes on. Checked under the hood, nothing wrong with the v-belt (not missing, tension is right).

I think my alternator is (going) bad (3 years ago rebuild), The current (with engine @ idle) is 13.10V when the engine runs @ arround 2500 rpm the current is arround 13.40V! I checked things such as the ground, wires everything is good.

On the D+ connection @ the alternator i get 6V?? I have no idea what it could be.
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Porsche 924 Euro 1977

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

be sure the ground is good. I'm going to run a seperate ground cable from alternator to battery to be sure.

The ground is normally connected between the alternator and the small ground wire to the engine... I don't like the size of that small ground wire.

Better to be sure

Just above 13V isn't that much for a charge...
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"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

Joes924Racer · Posted: Mon May 19, 2008 2:41 am Post subject:

Called an exciter wire, grounds the alt.
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1979 porsche 924 Na
1980 porsche Turbo 931GT Replica
Have u ever driven a turbo.

Martijnus · Joined: 29 Dec 2006 Posts: 2019 Location: Netherlands

thanks, didn't know that

however, I wonder why it's called exciter wire... since it isn't exciting anything...

_________________
"Rule: Turbo's make torque, and torque makes fun." (C. Bell)

924 "50-jahre", 1981.
MSII/extra, LPG, ITB's, 5lug.
To be turbo'ed in a while.
Killed her at the Nurburgring, Porscheless at the moment

Paul · Posted: Mon May 19, 2008 4:40 am Post subject:

It excites the alternator's coil at startup by using the battery idiot light.
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White 87 924S "Ghost"
Silver 98 986 3.6l 320 HP "Frank N Stein"
White 01 986 "Christine"
Polar Silver 02 996TT. "Turbo"
Owned and repaired 924s since 1977
Porsche: It's not driving, it's therapy.

Smoothie · Posted: Mon May 19, 2008 6:43 am Post subject:

Yes actually it's either a ground or a 12v+ source depending on mode.
Before engine startup, with the ignition switched on, but not yet started, alternator post D serves as a ground for the idiot light. At that time, voltage comes from the battery through the light, and grounds at alternator post D. Then when you start the engine, the alternator needs to see that 12v+ from the battery at D to put it into charge mode (the alternator is "excited" into charge mode). Then when the alternator starts charging, it puts 12v+ out through post D - now it's no longer a ground.. Now you have 12v+ at both ends of the exciter wire which causes the idiot light at the middle of the wire to turn off. Any time there's an imbalance in voltage between both ends of the exciter wire, the idiot light will begin to light anywhere from dim to bright depending on the voltage difference.
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"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

!tom · Posted: Mon May 19, 2008 2:41 pm Post subject:

Sounds just like my problems.

Check the 3-wire connector in front of your battery, and make sure all pins are making a good contact.

Also as others have pointed out make sure you have a good ground, and make sure the alternator drive belt is good.

As you can see above, mine ended up being the regulator, and I had the exact same 6-volt on the D post issue.

It would also be a good idea to check the brushes. A replacement regulator will probably come with new brushes.
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78 924 NA
5-lug