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spoke spoke is offline
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Join Date: Dec 2005
Location: Allentown, PA
Posts: 864
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.










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Old 08-17-2018, 06:13 PM
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