spot_55

I put a 470ohm resistor in the the tach wiring from the pcm to the the tach (auto meter) wire and fed 12 volts to the other side of the resistor, My tach does work however when looking at the info through my scan tool and looking at the tach, it will show half of what the tach is reading. So driving down the road the tach will show 3000rpm and the scan tool shows 1500. while sitting at idle the tach will show 1200ish while the scan tool shows 600 rpm.

My question is if i go to a higher ohm resistor will it bring the tach back to where it should be? it is set for 4cyl operation(both wires brown and orange are cut).

FastKat

iTrader: (10)

Changing the resistor will not adjust output of the tach. It sounds like the tach should be wired in 8-cylinder mode instead of 4-cylinder mode. I know the 4-cylinder signal is pretty common, but based on what you're seeing from the tach, wiring it in 8-cylinder mode should fix the problem.

BTW, the tach signal is driven by pulses per minute. Other sensors, such as your coolant temp sensor, are driven by resistance. They are two entirely different kinds of sensors/gauges. The only reason you need a resistor and pull-up circuit in the tach wiring is to improve the strength of the signal. The actual pulses per minutes are not changed.

spot_55

Thanks for the reply. I had it hooked up 8 cyl and i had no readings,that was however before i added the resistor so i will reattach the wires for 8cyl operation and see what happens.

spot_55

FastCat, you were correct I hooked my wires back up for 8 cylinder operation on the back of my tach and it now show the proper R.P.M . Thanks again.