Tach Tech

[mkinne]

Any way to bench test a tachometer?

TIA
Matt

[John Rogers]

If you need to see if the tach is good vs if the wiring to the tach is good, you could pull the tach out of the dash and hook a test tach to the terminals on the back and then run the engine to see if they read about the same. If you don't have a test tach, you could remove the tach and all connectors and connect it across the connectors on another 914 or similar early ignition coil. Make sure you note which wires go where. Good luck.

[jkeyzer]

It is probably not hard to make a circuit to do this - in fact there are probably some already available. You just need to simulate the points opening and closing at a certain rate and feed this signal to the tach input. This would be roughly a 12V square wave with frequency 0 to 7000*4=28000 Hz (28 kHz) which is very doable.

I think I've got that right...

[Dave at Pelican Parts]

Half that speed, Jeff. A four-stroke engine sparks each cylinder every other revolution.

--DD

[jkeyzer]

D'oh.


Maybe a 555 timer and a variable resistor would do this. You don't need much accuracy just to test out the tach, so an RC oscillator would give you something that was good enough.

[mkinne]

Was hoping for something a little simpler, like...
"Apply 12v here and the needle should move."

- Matt