Tachometer Function Question

[w3wilkes]

So as not to thread jack another thread.

In post #14 of this thread

It talks about current driven tach in the 240Z and voltage driven tach in the 260Z and up. Does current mean the amps increase as the revs increase and likewise voltage means voltage increases with revs?

I have a tach issue where my tach works fine from idle to about 3000 revs and then pretty much stays close to 3000 for all higher revs. I have a ZX electronic dizzy and the balast resistor has been removed (I didn't personally do this, but an old school Z guy that I trust did it for me). All was well for about 3 years (probably no more than 4000 miles). Then one day about 5 years ago the tach just slowly crept all the way up on its own unrelated to actual engine revs. I pulled over, shut the car off, started it back up and it's been in the idle to 3000 okay mode since. The car runs just great, just not the tach. It's not a big deal, it just bugs me. I've cleaned all the connections with no change in symptom. Looking for any suggestions... Thanks

Edited April 1, 2023 by w3wilkes

[cgsheen1]

Well, not exactly. You're correct about the early tach being driven by the amperage flowing to the coil. The later tach is "counting" the sparks essentially. It gets it's signal from the "-" side of the coil and it's reading the pulses that are given to the coil by the electronic ignition.

The distributor shouldn't impact the tach operation (the wire loop on the back is just reading the coil amp draw like an ammeter). Most of the guys here with 240's have switched to the "matchbox" distributor without issues. You might check the wiring - it's a bit more complicated with the early tach having to run back-and-forth - but I suspect that your tach may just need a good old fashioned ultrasonic cleaning...

Edited April 1, 2023 by cgsheen1

[Captain Obvious]

10 hours ago, cgsheen1 said:

Well, not exactly. You're correct about the early tach being driven by the amperage flowing to the coil.

 (the wire loop on the back is just reading the coil amp draw like an ammeter). 

Well not exactly. You're correct about the early tach running the coil current through it, but it's not reading it like a ammeter.

It's reading pulses. Steady state does nothing. The signal has to change.

If your points happen to be closed when you turn the key to the "ON" position, if it was an ammeter, the tach would peg high and stay there until the points opened.

[cgsheen1]

Kk, corrected my post above.