Tach issues...

[Loa]

[Edit: see further posts]

Hello,

When I got my Challenger last year, the tach was dead. Before having it rebuilt or buying a new one, I wanted to make sure it was dead. It had been left unplugged and I just tried to plug it. I made sure the long screw was on a working 12 volts, and that the shorter bolt was connected to my coil's negative side.

I wasn't hoping for much, but to my surprise, the car won't start at all when the tach's plugged. As soon as I remove it from the coil, the engine starts normally.

Is that indicative of a bad tach, or another problem?

Thanks

Loa

[FY1Cuda]

Only guessing, but if your tach is shorted internally then the 12v you put to it would be feeding back to the negative side of the coil (preventing the coil from building up a charge).

[Loa]

Very good guess. After reading your post I did a continuity test between the two screws: they're shorted.

Is it worth it to take the tach apart and look for it?

Loa

[FY1Cuda]

Probably not.  I think your best bet is getting a new innards for it.
http://rt-eng.com/rte/index.php/RTE_67-74_Tach_kit

[Loa]

Hello,

I got the tach board from rt-eng and it still doesn't work. The soldering plan is pretty bad, so I've had to figure a few things out. But at present, the tach needle barely moves when I rev the engine. By barely I mean it goes from 0 to about 400, very slowly. The needle moves freely when I blow on it and returns to 0, I've tried adjusting the little screw, I've checked every contact point for continuity and the coil's resistance is dead on spec.

Added info: I have access to another tach that, while it's very old, was supposed to work. It doesn't on my car, but I don't have access to another classic car to test it. Is it possible to test it on modern cars?

Loa

[Chryco Psycho]

Depends how modern , a lot of nw cars have coils right on the spark plugs

[miketyler]

I also bought one of those boards and never got it to work properly. I need to pull the panel back out and fix it as well as my buzzing speedo. It would reach a certain speed and the needle would begin to oscillate so bad that the needle started coming apart.

[Azchallengerboy]

Hi Guys, my tach shorts out the coil also.  Is the printed circuit board that classic industries sells come from RT?When I order mine up I want to know that I get a board that will work.

[Adam]

I have used the RT-Eng tach kit on two different tachs with great success; however they were both functioning tachs to begin with... maybe whatever prevented the old tach from working is affecting the new guts..

[miketyler]

Could be right. Mine initially behaved like it had damped movement. Maybe back off the screw on the jewel movement? Need to pull it back out to check voltages