Current Sensing/Triggered Tach

[Jay911bird]

Hey everyone, first time posting here! I'm installing a pro flo 4 into my 66 mustang which uses the rally pac tachometer. The tachometer is wired in series with the ignition switch to the positive side of the coil.

I've been searching and haven't found much info about wiring it up to work with the system other than purchasing the msd tach adapter. From what I've found the tach will not function correctly without the resistance noted in the picture. I've also seen suggestions to try attaching to the negative side of the coil but I'm worried this could also potentially damage the tach.
My question is the orange wire that connects to the positive terminal on the coil. Is it only a 12v triggered by the ecu? Could I save myself the money of purchasing the tach adapter and using my original 12v keyed source for the coil power and "tape off" the pro flo's? Or is the factory style not adequate (voltage wise)? Just trying to potentially save some money and extra wiring.

[CorkyE]

The latest Hot Rod magazine has an article relative to older Mustang tachometers. This article focuses on a company that updates old factory gauges to work with modern electrical accessories. I don't know if you'll find the answer there but you might get a direction to go in. Good Luck

[2700 Cast]

We dont have a test unit to try.
But it looks like the MSD # MSD Magnetic Pickup Tach Adapters 8920

Should be the unit to use with your Tach.

Check it out & do some research.

[Jay911bird]

Yes that is the tach adapter I was referring to. The tach I have is actually a reproduction unit but installs into the car through the factory harness. The instructions that came with it state it has been "tested and approved for use with some aftermarket electrical ignitions" but that as far in depth as it goes. I've reached out to the manufacturer as well in hopes of getting some more in depth answers but haven't heard back yet. From what I've read the reason for the resistor inline to the coil is if the owner were to leave the ignition switch on with the engine off and coincidentally the old points were closed; It wouldn't cause a high current short and fry the coil. Obviously the points system is gone and the grounding is controlled by the ecu. I'm still in the process of wiring up the pro flo and this is one of the road blocks I've encountered. Hoping someone else has used this in an older vehicle with this style tach with success without dropping the extra cash on the adapter.