Garage Tachometer for a Cheapskate?

Maintenance/Repairs
1

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.

http://www.harborfreight.com/lcd-automotive-multimeter-with-tachometer-kit-95670.html

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?

2

If you’re looking for convenience you buy the proper tool that someone else already designed to provide that. Such as the tach you showed.

If you can find a cheaper tach go for it. The Rube Goldberg methods are more hassle than they’re worth.

Tester

3

I think your use of the o’scope is a great way to save $39 and very creative. I’ve never felt the need to set the idle speed of our vehicles and am not certain that it is possible, but if I feel the urge, I’ll use your method with the 'scope I do own rather than spend $39 for a tachometer. When I get my lawnmower out this spring, I am going to use my 'scope to check its rpm.

4

Check Ebay. I just saw a few tachs for under $20 and one or two under $10, with free shipping.

5

The salvage yard I frequent charges $2 entrance fee. Most dash gauges (tach, speedo, gas) go for less than $10, it’s a pick n pay set-up. I’m fairly certain that you could find something for a Tojo. Maay even be something you could install in your own dash. Cool! Problem is: what if it doesn’t work? I’d go with Tester or Busted here. Rocketman

6

All good ideas. My current thinking if I can’t get anything else to pan out: A home-brew pulse generator that outputs current pulses at exactly 650 rpm (the spec for the idle rpm for my Corolla). I’ll use the current pulses to trigger my timing light, and with that I can visually set the rpm to spec by turning the distributor while sync’ing it to the crank pulley rotation.

7

Just remember: If the engine is running 650 RPM the distributer will be turning 325 RPM.

8

We had an adjustable strobe in our lab. Cool tool. Used it to verify cooling fan speed to check belt slippage and elec motor performance. I bet u could find one on ebay