Factory Tac and fuel gauge not working

7173Mustangs.com

Help Support 7173Mustangs.com:

midlife

Shorts checker
Joined
Jan 24, 2012
Messages
4,127
Reaction score
554
Location
Tucson, AZ
My Car
No Mustangs at the moment.
Tach wire goes to + side of the coil for the factory tach to work. Engine gauge feed harness may be different solely due to length of wires or terminations for non-gauge sending units (e.g. oil, water).
 
Joined
Feb 17, 2018
Messages
77
Reaction score
30
Location
Cloverdale, CA
My Car
71 429CJ
I am not familiar with your Duralast distributor, but the Ford factory tach uses current and not voltage to operate. Is the tach still connected to the factory tach wires from the coil to the tach? Typically installing aftermarket distributors and coils requires bypassing resistor wire in the factory wire harness to provide the distributor and coil with 12v. GM and most aftermarket tachs use voltage. Rocketman can help you convert your tach to use voltage or repair it.

Here is an excerpt from a magazine article on this topic.

A Ford tach's peculiar problem: The preceding are common generic issues across all manufacturers. However, there's an added wrinkle with legacy Fords. On most factory and aftermarket tachometers, the signal wire connects to the negative (-) side of the ignition coil. But most Fords with points-triggered ignition systems have a two-wire inductive tach that connects to the coil's positive (+) terminal, piggybacking onto the ignition "run" circuit from the ignition switch to coil (+). Remember, on a points system that circuit incorporates a ballast resistance wire. Upgrading to a modern ignition requiring full-time 12 volts requires getting rid of the resistance wire. Without resistance in the circuit, that old Ford tach won't work!

Built for failure:
This Ford design is still problematic even if the ignition system is still stock, particularly as originally designed back in the 1960s: The OE Ford tach wire is inline (in-series) with the ignition "run" circuit. In other words, the ignition switch's "run" wire passes through the tach and then on to coil (+). If the original tach ever shorts out, the car won't start. Total electrical failure. By 1973, Ford marginally improved this circuit, with the tach now wired in parallel (not directly in-line, there's an added "bypass" wire around the tach). At least in theory the car should at still run if the tach shorts out. Wanna bet your engine on it?
Incidentally, if you ever connect the stock Ford two-wire tach to coil (-) by mistake, that error will fry the tach for sure. Any way you slice it, updating the Ford design is a critical reliability and safety fix.

I currently run the factory current sensing tach using an MSD 8920, works great. I’ve used the 8920 for more than 30 years with dual point, Hall effect and Mag trigger distributors with the same ease on my 71 Mustang. Currently running Pertronix HP510 CD along with the Mag Trigger dizzy and my 51 year old original current sensing tach accurately displays rpm as well as displays preset Rev Limit for 3 seconds upon turning key on. Tach is now wired in parallel. I’m just bringing this up because there is another good option to converting the original current sensing tach. The 8920 is a small box easy to install and hide, three wire including ground. Another nice thing about the MSD 8920 is going back to completely original wiring is not an issue.
 
Last edited:
Top