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AndrewCologne 08-17-2018 12:55 AM

odd indicator light signal behavior
Hi all,

a good friend of mine got some problems with his 911 S from 1975. with the right turn indicator signal light in the rev counter when switching the signal to the right.

When switching the indicator light to the right, the right signal in the rev count unit only blinks once correctly and then ... all following blink signals do seem odd as they let light up both (left & right) indicator signals and the relay sounds different!

Please check the video ... first: left signal appears ok, ... and second: the right signal behaves odd.

This behavior of the signals in the rev count unit only appears when switching to the right.
At the outside the blinking is totally normal.

Do you have an idea?
The relay shouldn't be the guilty one as emergency light signal an the left indicator light signal behave totally normal.
I checked the wiring and nothing seems odd to me.

Thanks a lot for any hints

Ian Comerford 08-17-2018 01:49 AM

Have you had any of the wiring apart recently? Reason for asking is that I rebuilt my car last year after some extensive bodywork/paint and I have a similar problem. In my car when you indicate left it flashes left once on the gauge and then flashes repeatedly on the right. Externally the indicators flash correctly. I have been advised it is likely to be a bad earth as the car worked correctly before stripping. Not sure if this help.


targa80 08-17-2018 11:03 AM

I am not familiar with the wiring of the tach turn indicator lights in the 75S model but more than likely it has not changed with to 80SC model. The tach turn indicator lights acquire their ground from the four post flasher relay via contact C from contact 31. Since your left tach turn indicator functions normally but your right does not then you are getting a voltage feedback on the right teach indicator ground line that's why both lights dim. There are several posts with schematics that can help you sort out the abnormally. If the 75S wiring is not the same then you will need to look at that.

steely 08-17-2018 01:26 PM

Might this help?

spoke 08-17-2018 06:13 PM

The both tach indicators flashing at the same time is a common failure mode of vintage 911 and 914 OEM flashers. The flasher has a secondary coil which closes the 4th pin on the flasher called K or C2. Over time this secondary coil weakens and loses its efficiency.

This K/C2 pin connects to the common connection of both indicators. When the heavy current of the external 21W bulbs is flowing, the secondary coil closes a contact to connect K/C2 to 12V as shown in the simplified circuit on the left.

But the secondary coil weakens over time and may flash a few times then gives up and both indicators flash at a reduced brightness.

Explanation using diagrams below. Left diagram is correct operation and the right diagram is the failed operation.

In the left diagram, the right turnsignal is flashing and K/C2 is connected to 12V. The RIGHT tach indicator current flows from 12V through K/C2 then through the LEFT exterior bulbs.

In the right diagram, the right turnsignal is flashing but the coil pulling K/C2 to 12V is weak and K/C2 remains open. Current flows through both tach indicators and they both light dimly.

In the case of the OP, the failure is observed only on the right side. My guess is the right side bulbs are pulling less current than the left side bulbs.

There are a couple of solutions:
1) Get a new OEM flasher
2) Install an EP26 LED-compatible flasher with K/C2 connected to ground.
3) Keep the current OEM flasher and make a little Y-pigtail with spade connectors like shown below. Remove the common connection at the indicators (tape up the wire) and connect the Y-pigtail to both indicators then ground on the tach.

AndrewCologne 08-18-2018 01:17 PM

Thats why I love to be here!
Thanks to everybody helping me here and ...finally Spoke's solution with the y-wire trick made it working as it should.

A nice evening to all and all the best from Cologne

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