Missing tach wire??

stubble88 · 08-04-2025, 05:26 AM

Engine bay 14 pin connector. Found taped off blk/vio with 14 pin male connector coming from car. Engine harness 14 female side does not have a blk/vio wire. Even pulled the tape back. Nothing.
I do not have a working tach. Should there be a corresponding blk/vio wire on the engine harness?

zuch · zuch's Garage

What year car? On 3.2, the tach is fed from the DME/computer, not the back of the car.

stubble88 · 08-04-2025, 06:01 AM

Quote:

Originally Posted by zuch

What year car? On 3.2, the tach is fed from the DME/computer, not the back of the car.

Harness is from an 83. Wondering what this blk/vio is?
Comes from DME but where does it get its info from? Crank sensors?

wazzz · wazzz's Garage

It is derived from the ignition pulse signal by the DME. The DME itself gets the engine speed from the crank sensors, but the signal is not used as is. It is processed by the microcontroller.

stubble88 · 08-04-2025, 07:44 AM

So I should have some tach reading while cranking correct?
Did all the required tach testing and continuity all checked out. But I have no tach movement at all.

wazzz · wazzz's Garage

No you shouldn't have tach reading. For this you need to get the tach signal from pin 21 of DME and send it to the tach, with probably some adapter circuit in between. Don't know the details to make a CDI tach work with an EFI tach signal. I know it is possible, but signal levels may need some adaptation.

The signal from the DME is a pulsed signal varying between +12V and ground (0V).

On the CDI engines the tach signal is provided by ignition on pin TD. I don't know what the TD pin signal level is.
Some skilled people will chime in.