What is the signal for the tachometer?

[jeff1234]

I wanted to see what the signal for the tach looks like. I connected my VOM between a lead coming from red plug #12 and my signal ground splice. I got a steady reading of 2.06 vdc. Revving the test engine resulted in no change. Should that signal be steady voltage like that or am I measuring with the wrong type of test gear.
I want to add a tach to the stand but wondered if an aftermarket tach would work. Also, I'm not seeing rpm on my OTC 4000E. If I understand this system the distributor should be reporting to the logic module and that should be displayed by the tach.

[cordes]

It's a PWM signal. I think it's compatible with aftermarket tachs, but I'm not sure. I recently looked at it with my oscilloscope, and it changes quickly enough that I don't think a multimeter will do it unless is has the RPM function on it. I have one from my college days with that function and it works very well.

[jeff1234]

Cordes, thank you. Inspite of having numerous tach over the years I never really understood how they worked so its time to dig deeper. The oscilloscope info from your observations confirms my suspicions. I will try to borrow a tach to test the hookup.

[Aries_Turbo]

It's a PWM signal. I think it's compatible with aftermarket tachs, but I'm not sure. I recently looked at it with my oscilloscope, and it changes quickly enough that I don't think a multimeter will do it unless is has the RPM function on it. I have one from my college days with that function and it works very well.

is it PWM or just a frequency signal? I would think it would be a changing frequency to indicate RPM but I havent logged it.

Brian

[cordes]

I think you're right that it's just the frequency. It matches the Dist. signal, which makes sense.

[tryingbe]

It's just an on/off 0-12v digital signal, my Microsquirt output to the Daytona Digital dash with no issue.

[jeff1234]

Thanks guys, I ordered a cheap tach.

[jeff1234]

Update, I installed the tach with signal wire to the negative side of the coil, thre red and white wires to positive and black to negative. It worked as expected. I tried hooking the signal wire to red plug #12 wire and it didn't work however I didn't put the black wire on the signal grounds splice. I'm guessing that would make it work.