im designing a SIMPLE and CHEAP shiftlight for our cars, but ineed to know the voltage range on the tach wire to do so, and my car is in the shop right now. anyone know it?

 

It's not a variable voltage thing, it's a variable frequency thing at 12V that determines what RPM the tach reads.

 

ah ha, gotcha. how about those values?

 

267 Hz at 8000 rpm, if my math is right. That's for a four cylinder.

 

howd you go about figuring that out?

 

Well, My 1984 Civic Wagon used the same signal for the coil and tach. I'm guessing most cars with a distributor are the same. On a four stroke engine, all cylinders will fire every two revolutions of the crank. So that's two pulses per revolution with a four cylinder engine. Multiply by the engine speed to get pulses per minute, then divide by 60 to get pulses per second, or Hertz.

So:
2 pulses x 8000 rpm / 60 = 267 Hz

I don't know if it's correct, but it makes sense to me. The signal is negative, by the way (or from the negative coil terminal, anyway).

 

after some research last night, you are exactly right. thanks. im gonna have an adjustable shiftlight range from 6k - 10k, im just trying to figure it out. the damn variable resistors are expensive as ****, keep in touch though. peace.