how do I test a tach

deac · Post by **deac** » Mon Aug 25, 2003 1:13 pm

What's the easiest way to tell if a tach is working? I have a dwell/tach meter if that'll make it any easier.

TIA,
Deac

Guest · Post by **Guest** » Mon Aug 25, 2003 1:51 pm

did a little searching, correct me if I am wrong. There are three wires on the back of this autometer monster tach, a green, red, and black one. The black is ground, the red goes to the positive side of the coil and the green goes to the negative side? Am I right? I think the red goes to a hot connection, would the positive side of the coil suffice?

Thanks
matt deacon

uberbug · Post by **uberbug** » Mon Aug 25, 2003 2:20 pm

If you can find a "function generator" that will make a square wave, that would work best. On a 4 cylinder, 500 hz should be 1000 rpm. There are 2 sparks per revolution.

You will need 12v, ground and the signal that is supposed to come from the coil hook to the function generator.

This is in theory anyway, reality might differ.

It might be easier just to hook it up to the car and see how it works.

matt deacon · Post by **matt deacon** » Tue Aug 26, 2003 6:19 am

Thanks, I'm still a little confused on how the coil can measure revolutions, but I guess you explained it to me. So the coil has a two pulses for every rev, and that's what the tach gets?

deac

uberbug · Post by **uberbug** » Tue Aug 26, 2003 11:49 am

Actually its the points that open two times per revolution. What you measure is the frequency of the opening of the points

What you are measuring is how often the voltage at the points goes from 0v or ground (when the points are closed) to 12v (when the points are open).

When the points open, the coil sparks. This happens near 0 degrees and 180 degrees (give/take the "advance").

Guest · Post by **Guest** » Wed Aug 27, 2003 7:29 am

The green wire goes to the coil Term 1 (the same terminal as the condensor)

The Red can go to any Positive connecection (does not need to be on the coil) and Black to ground. If you have a white wire that is for immumination of the guage and can be tapped into the instrument lights.