Factory tachometer problem

[wstaab]

Problem is no power to the red/white wire that supplies the tachometer bulb.
The red/white wire is connected to a ground somewhere because is checks for continuity.
It appears that when the instrument lights are activated the power is back feeding through the red/white wire and using the tachometer ground to complete the circuit which causes the signal from the MSD unit to fail. Trying to find the wiring problem would mean pulling the dash which is not an option. I may try to disconnect the red/white wire and splice it to a instrument light feed on another gauge.
Appears like a wiring problem from the factory. Car number 3126 delivered in 2015.
Car has 400 miles on the odo.

[patrickt]

Originally Posted by wstaab

Problem is no power to the red/white wire that supplies the tachometer bulb.
The red/white wire is connected to a ground somewhere because is checks for continuity.

If you can't easily find the glitch in the harness itself, I would just cut the wires, tape off the sides going to the harness, and then run a clean feed from one of the gauge bulb feeds that works properly, along with a nice clean ground feed. Remember, a hot side feed to a filament bulb will show continuity to ground (through the filament) when there is no 12v+ on it.

[Blas]

Ah, so pulling the bulb would have been a good test it seems. Could the fault be in the connector plug itself? The small contact wires in the bulb plug perhaps bent an shorting? Splicing the power for the tachometer bulb to another bulb power feed is acceptable. The power for the instrument lighting all comes from the dimmer anyway.