I’m looking for an idea for a frugal tachometer for my garage use. For when I’m doing a tune-up on my early 90’s Corolla with electronic ignition. The way I do it now: I haul my o’scope out to the garage and wrap a wire around the no 1 spark plug lead few times as an inductive pick-up, and use the time base on theo’ scope to measure the rpm. It works ok but it’s cumbersome.
I could buy one for $39.
But I’m thinking to myself there must be a more frugal alternative. After all, I don’t need all those other functions. I already have a digital multimeter, why buy another just to measure rpm? I don’t need a fancy inductive pick-up. There’s a tach diagnostic test point on the diagnostic connector of the engine. And why the need for a 9 volt battery to power the instrument, when there is a 12 volt battery right there on the car? No, the $39 method isn’t the most frugal alternative it seems to me. But it is a good back up plan.
Any other ideas? I’m open to a minor home-brew electronics project. Aren’t there on-board tach sensor/circuits on newer cars that convert the tach input to a voltage, powered by 12 volts? Maybe I can just buy one from an auto parts store, like I was buying a replacement, install it in a box with wires coming out for the tach input, +12 volts and ground, and the voltage output?