Tachometer problem

[Kammertime]

I purchased a 73 conv. last year. The prior owner added the tach and center gauges from a wrecked 73 Mach 1.

The tach would work one day and the next day would not work. This happened on and off until the last 4-5 times I started the car it wouldn't work.

Is there something simple I should look at ? It does have a Pertronix module installed. 

I am new to the 71-73 world; most of my experience is with 69-70 Mustangs.

:thankyouyellow: 

John Kammer

 

[rvrtrash]

The tach and gauges use a different wiring harness than the "idiot lights" cars. A lot depends on whether or not he switched the harnesses when he did the swap. If he just spliced stuff in, you may have a bad connection somewhere. On a factory wiring harness car, if the tach dies, the car may not even start.

Steve

 

[red351]

Mine sticks sometime when the weather get colder. Sometimes a slap on the dash works or just wait for some heat.

 

[midlife]

Does the car still run with the tach not registering? There's one failure mode of the factory tach that does that. If the car doesn't run with the tach not registering, it is either a bad transformer in the tach or crappy connections to/from the tach.

Is this a 3 wire tach (non-Ford)? If so, there's still other possibilities, but most likely a bad connection somewhere (ground, signal, power).

 

[Kammertime]

It is a Ford factory tach. It appears that the PO replaced the complete wiring harness. The car still starts without the tach working.

 

[Stangmeister]

Does the car still run with the tach not registering?  There's one failure mode of the factory tach that does that.  If the car doesn't run with the tach not registering, it is either a bad transformer in the tach or crappy connections to/from the tach.

Is this a 3 wire tach (non-Ford)?  If so, there's still other possibilities, but most likely a bad connection somewhere (ground, signal)

 

[Stangmeister]

I have two wires on my ford tach, and both are black.  What's up with that???  How do I know which wire goes where?   Help?   Image attached...

 

[midlife]

Kammertime: it appears that the tach gauge mechanism is faulty and not the input transformer. You'll need to send it out for repair. Tachman will try and sell you a 3 wire conversion tach, but if you are insistent, he will repair your tach.

Stangmeister: I hope you have a male and female plug coming out of the tach. The gender will tell you which wire to match. If not, you have a 50/50 chance of connecting it right the first time, a 100% chance the second time!

 

[Stangmeister]

Yes, I'm sorry, I forgot to mention that it does have a male/female connector plug on the end. However, there is no matching connector under the dash to plug into. Is it just a matter of going to the coil with the positive and grounding the negative side? Or is there more to it than that?

 

[TheRktmn]

The tach is part of the ignition circuit and connects between the ignition switch and the coil + side.

Going by standard Ford wiring practices and the red/black leads on a 67-70 tach the tach male is red and input from ignition switch, tach female is output is black and goes to coil via the 'pink' resistor wire.

This drawing is from a 67/68, but the circuit is the same for all 65-73 Mustangs. Why it shows opposite values for the connectors is beyond me, I can only guess that the draftsman was doing an install only, not concerned with electrical.

 

[midlife]

OMG! Do NOT use that figure! The lead from the ignition to the tach is female (remember: female connectors are always hot when energized), and the output from the tach to the coil on the underdash harness is always male.

Thus, simply plug the male lead from the underdash harness to the female lead from the tach and vice versa. There is no ground involved here.

 

[Don C]

As it (the sketch) should have been:

 

[TheRktmn]

OMG!  Do NOT use that figure!  The lead from the ignition to the tach is female (remember: female connectors are always hot when energized), and the output from the tach to the coil on the underdash harness is always male.  

Thus, simply plug the male lead from the underdash harness to the female lead from the tach and vice versa.  There is no ground involved here.

Yeah, that why I said they were wrong.

"Why it shows opposite values for the connectors is beyond me, I can only guess that the draftsman was doing an install only, not concerned with electrical. "

The drawing was included so he could see HOW the tach is wired, not the correct wiring. I thought I had clearly explained that and the red-black male-female discrepancy in the drawing.

I generally don't make changes to factory docs, I just point out the discrepancy as I see it. Don did change it so it is 'correct' by Ford's normal wiring practices.

 

[Stangmeister]

Thank You Rktmn, that dwg will be very helpful. My Stang is in the shop this week getting a rear adjustment, but I’ll let you know how the tach hookup goes.

 

[Stangmeister]

Got that last message. Thanks!

 

[kcmash]

Mine sticks sometime when the weather get colder. Sometimes a slap on the dash works or just wait for some heat.

Tap on the glass got mine working today!!

 

[Kammertime]

How can I test the tach to see if it is the problem or if I have a wiring problem somewhere else ?

 

[Don C]

Disconnect the 2 leads to/from the tachometer and connect the power leads together, bypassing the tachometer.

 

[Kammertime]

Don C ,

I want to bench test the tach to make sure that the problem is in the tach before I send it out to be repaired.

Thanks,

John