Inaccurate Tachometer?

[RETed]

I assume you're talking about stock Mazda gauge clusters?

The short answer?
Significantly.

I can only comment about stock FC tachs, since that's my experience.
Mazda has an "adjustment calibration screw" on the circuit board to compensate for such variations.
When we were messing with these things, we've seen these things regularly off by an average of 400 - 500 RPM's.
I've seen some off by almost 700 - 800 RPM's, but this was a rare case.

Readings were done at discrete 1,000 / 2,000 / 3,000 RPM increments.
Signals was generated with a custom built microcontroller.
Signals was monitored with a lab-grade o-scope.


-Ted

[infernosg]

^^^ Correct, stock Mazds S5 N/A cluster. I have read a couple things regarding recalibrating the system but I wanted to make sure that was the track to follow or if it could be something wrong with the trailing coil.

I was reading something written by Granny's Speed Shop regarding recalibrating the system with a 12V battery charger. Apparently this outputs a 120 Hz DC signal, when applied to a RX-7 tachometer and should cause it to read 3600 RPM - does that seem about right?

The other indicator that something is off is if I allow the car to perform it's cold start routine (rarely) it shows upwards of 3400 RPM. My biggest concern is the idle. Right now the tachometer is showing 750 RPM, but if mine is reading ~300 RPM high at speeds, does that mean my idle is also off and therefore actually ~450 RPM?!