Originally Posted by PeterH
I have two questions related to running my factory installed tachometer in my Geo Tracker.
1. what sort of signal does it take to drive the tachometer? I'd like to generate a signal of the right type to drive this during setup and testing. Guessing I might use an Arduino board to generate a signal.
2. How do you tell the proper number of pulses per revolution approprate for the tach? This Tracker had a 4 cylinder ICE motor before I started this project.
Ok, maybe I have three questions... is there an easy way to accurately measure the rate my motor is turning, other than using the sensor? I'm looking for some way to confirm that all my instruments are accurate. I'm picturing that ancient strobe light we used to use way back before electronic ignitions, to adjust timing on ignition.
If you had a four cylinder engine then you would have had two ignition pulses per revolution. Most likely, your tach will also be calibrated for two pulses per revolution. You should review the wiring diagram for the ICE installation; if you had a tach sense wire running from the ignition coil (-) terminal to the tach, then try driving that line with a 12V square wave of about 50% duty cycle. If the tach signal was provided by the engine management system, then still, you could try a 12V square wave, or be safe and start with 5V and go up until you get a reading... it might be prudent to put 100 ohms (or so) in series with your test signal output and the tach input on the instrument cluster, just to prevent any Real Bad Stuff from occurring.
Tachs that are designed to run off the coil negative connection, have to be tolerant of input voltages of over 100V because of the flyback pulse when the coil current is cut off and it triggers a spark across the plug. So driving 12V to the tach input of older tachs is unlikely to cause any damage; in fact you may find that the tach will not respond unless you create a substantially higher voltage.
I don't think that any current manufacturer uses the Smiths style of inductively coupled current sensing... that went out in the 60's.