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Registered User
Join Date: Aug 2019
Posts: 21
911 SC alternator problem

Hey guys,

ok finally thank you! My 911 is on the road!! Its so cool!!
Car: 1980 911 Targa US version.

But... got a problem (seems this year i got it all......)

My generator is not charging the battery. i measure 12,6 Volt at the battery, slowly sinking...

It is a alternator with the voltage regulator inside.. (I rebuilded the ignition and as studid as i am i connected the cable from the external voltage regulator too ...

ok i tested: Often when i turn ignition on, the battery warning light is not on!! I made a shortcut blue wire to ground --> battery warning light goes on...

once a time few minutes ago the battery warning light was on when turning the ignition.. startet the engine and measured 13,5 volts...

So what do you think. Did i destroyed the internal voltage regulator due to the addition connection with the outside voltage regulator?

Did not found weather to check if its the voltage regulator or the alternator as itself?

regards from stuttgart!

Old 09-14-2019, 09:17 AM
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Superman's Avatar
Join Date: Mar 2000
Location: Lacey, WA. USA
Posts: 21,070
I recently feared I had installed my alternator (internal voltage regulator) incorrectly. I took the unit to a local alternator/starter rebuild shop for testing. The testing procedure involved connecting the electrical circuits to test equipment and then turning the alternator to see if it makes current. It tested fine. I installed it (correctly) and the alternator is working properly as the testers predicted.
Man of Carbon Fiber (stronger than steel)
Old 09-14-2019, 09:25 AM
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Walt Fricke's Avatar
Join Date: May 2004
Location: Boulder, Colorado
Posts: 5,660
Phillip - The VR has three connections: field (DF), positive (D+), and ground (D-). External, internal, all the same three connections. The D+ is going to be at the same voltage as the thick red wire from the alternator which connects at the starter to the thicker cable from the battery +, through which the battery gets recharged. The D+ is a small wire, so it can't be used for things which need a fair amount of amps to work. It is a signal wire, if you will.

Both the internal and the external VRs connect the D+ to the warning light in the dash. The other side of the light bulb is connected to the battery +. When the alternator is producing current, there is no voltage differential between the two sides of the bulb, so it doesn't light - no current is flowing through its internal wire. When the alternator is not producing current, the battery power flows through the bulb and is grounded via the VR-alternator.

The field wire from the VR is what controls the alternator to keep the voltage where it needs to be.

The only wire which emerges from the internally regulated alternator is the D+.

So how did you connect the external regulator to the internal regulator? More to the point, how did you connect the DFs?

Though I don't know what would happen if an alternator had two devices trying to vary the current flowing through the DF wire.
Old 09-14-2019, 02:17 PM
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Registered User
Join Date: Aug 2019
Posts: 21

the car is from 1980 so it was delivered with a alternator with a external voltage regulator and the wiring harness.
The car got 1999 a alternator with internal voltage regulator... so i was so stupid and connected the plug from the external VR again... 2 regulators connected...

At the alternator the black cable is not connected. The blue cable is connected on top #61, brown at 2D- , big red at 2D+

seems to be right.. Will start remove the alternator now...

Regards Phil
Old 09-15-2019, 05:37 AM
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ROW '78 911 Targa
timmy2's Avatar
Join Date: Jul 2011
Location: Salem, OR
Posts: 8,777
Only the internal VR mounted on the alternator would have been active.
The black wire that was disconnected coming from the remote VR is the only active control from it.
Have the alternator tested and leave the remote VR disconnected.
Euro 1978 SC Targa, SSI's, Dansk 2/1, Bitz kit EFI.
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Old 09-15-2019, 09:20 AM
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