Topic: Rally Pac tach issues

[k-code]

Have an early 65 k code I've finished restoring and added a RP to the car. It wasn't sold with one from the factory.

Obtained a RP from Fred B., installed and it worked for a few hours then the tach stopped performing.
Fred sent me another tach from his parts bin, same thing, worked for a few hours then stopped. Everything is hooked up correctly via the resistor wire. Under dash harness is original but all engine compartment wiring is fresh AlloyMetal Products correct for an early 65.

After the second tach stopped I started testing with the mm and found full voltage out to the + side of the coil, not between 7-9 volts as per the shop manual. Checked the resistor wire and it tests as fully open, no resistance.
So it appears the resistor wire has lost its resistance over the years. Also checked across the coil posts and it's 1.5 ohms. It's a repop yellow top coil.

The car still starts and runs so the internal transformer of the tach(s) must be good. The tach needle moves a bit and is receiving power, but it's basically non functional.

My question is, would a resistor wire failure damage the tach but still permit the engine to fire and run normally, no miss?
Do resistor wires fail over time?
Am I missing anything? Anything else to check?
Don't want to repair or replace this tach and have the same thing happen.

Thanks

[Bob Gaines]

Are you running stock points or Pertronix?

[k-code]

Sorry should have stated that. Stock dual points, stock distributor

[midlife]

You're misinterpreting the results.  Without the engine running, and the key in RUN, there will be full battery voltage at the + side of the coil, as there's no current.  The purported 5-9V is with the engine running, but I've found something more like 11.5V.
If the resistor wire fails, it will be an open circuit with infinite resistance; it cannot fail to zero ohms. 
I'd check all of your connections, as it sounds like one of them is loose.  If the tach fails but the engine continues to run, the problem can be isolated to the guts of the tach behind the input transformer.

[k-code]

Ok, thanks for this.

A buddy measured the resistor wire for me as I was inside the car holding the probe to the wire.
After reading your reply I figured I better measure it myself with alligator clip extensions, with two different multimeters and got 1.5 ohms on one and 2.1 ohms on my other, so my resistor appears it?s doing what it?s supposed to be doing.

I've gone over my connections so many times, not to say there isn't a bad one but I'm stumped.

Could a bad coil cause the tach to do this? I am running a repop yellow top.

[midlife]

Check the resistance of the coil. It should be 1.5 ohms. Yes, this could be the cause of the tach guts failing for some reason or another, but it is hard to fathom.  I presume the tach has not been converted to the three wire version and is in factory configuration.

[k-code]

Yes coil is 1.5 ohms, but I'll check it again just to be sure. And yes both tachs were factory configuration. Happened to two tachs so yeah it is a strange one.

[k-code]

Measured the coil again and this time I got 1.8 ohms. I used alligator clips instead of just the probe so may account for the slightly higher reading. Using a DMM not analog.
I can't see 3 ohms making enough of a difference to burn out two tachs?

[midlife]

It could be that both tachs were refurbished by the same person and were done incorrectly.  I'd send one to Phil Severence in Idaho and see if he can determine what's wrong.

[k-code]

Ok thanks! I?ll give him a call