The factory tach plug on my 92 cummins W250, has 4 wires, Gray, Orange, and two black wires, are the two black wires 12 volt, one constant and one ground?. is the gray for signal, and orange for tach light, thanks, George.

 

hold test light to wire, turn headlights on. whichever one lights up with the headlights, its the 12v switched, the other is the tach signal.

 

but to clarify, you'll have

-tach signal
-12v light switch
-12v ignition
-ground

 

Has anyone found and aftermarket tach that would utilize the factory tach signal?

 

Yes, most. Isspro and autometer.

 

All of them that I find show a need for the signal to come from the coil or the alternator.

 

I'm using a 3-3/8" isspro w/hour meter off that harness. Set the dip switches for 2 pulses per rev. Seems to be somewhat accurate although it has a significant flutter at idle. I could probably correct that with a capacitor or some other filter.... or I might just mount a dedicated pickup to read flywheel teeth.

 

An isspro EV series tach will run spot on straight from the stock CPS. Verified with mechanical tech on crank snout.

 

Ditto, but in my case I think the yellow and white wires had to be swapped?

Twostep, you in Berea Australia or something? You look upsidedown...

 

I run this tach as well. Also white and yellow wires had to be swapped.