earlybroncoguy1 Reader
11/29/22 10:00 p.m.

I bought an old electric tach for a project car, engine in that is not running yet. I'd like to test the tach before I install it, but I realized all of the engines in my other cars are EFI and coil on plug. I considered hooking it up to my lawn tractor, but it's a magneto type ignition, plus it's a 2 cylinder, the tach is so old it's not settable for engines with less than 8 cylinders, even if I can get the right pulses from a magneto driven coil, the tach won't read the correct rpm with only 2 pulses per crank revolution. I don't think hooking it to one coil of a coil on plug ignition will work either, since they only fire once per crank revolution.

Or am I overthinking things? The wiring for old tachs is so basic (positive to +12v, negative to negative side of ignition coil, it gets a pulse every time the points open in the distributor - 8 pulses per crank revolution for a V8). I know MSD and other aftermarket CD boxes have a terminal for a tach output, but where can you tap into a tach signal on an OEM?


ShawnG MegaDork
11/29/22 10:10 p.m.

Autometer sells an adapter box for exactly this.

Keith Tanner
Keith Tanner MegaDork
11/30/22 12:04 a.m.

Wouldn't having it read at some multiple of actual RPM (hook to a coil on plug, for example) tell if you it works or not? Its a yes/no kinda question anyhow, if the needle moves you're good. 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
11/30/22 7:01 a.m.

There is a way to test a tach's calibration using a cheap battery charger.  The cheaper ones don't quite put out DC voltage, there is a residual 60hz wave in the output.  The tach will be able to read this and show whatever engine speed is 60 sparks per second for its calibration (four, six, eight, etc)


This is also why you don't use a battery charger when reprogramming a computer, you have to use a shockingly expensive true DC power supply.  The waveform in the voltage coming out of the charger can corrupt all the ones and zeros moving around.

Noddaz PowerDork
11/30/22 7:07 a.m.

Interesting concept.  When you figure it out, please share how you did it here.

MadScientistMatt UltimaDork
11/30/22 9:55 a.m.

Depending on the tach, some will trigger off the negative coil of a fuel injector or a 12 volt square wave. Some will even take a 5 volt square wave. At the other side, some Toyota tachs needed 300 volts to trigger. If the coil has no built in ignition module, it can drive a high voltage tach.

Running a V8 tach on a once every other engine rotation signal will give a 1/8 speed reading.

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
11/30/22 10:08 a.m.

Get a points distributor and spin it with your drill?

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
11/30/22 10:11 a.m.

In reply to Robbie (Forum Supporter) :

You'd also need a 12v source and a resistor (a 1000 ohm unit should be plenty, it is what I use, but a lightbulb is probably even better) so there is a square wave for the tach to read.

GameboyRMH MegaDork
11/30/22 10:34 a.m.

My AE92 has a tach driver made from a relay like this to convert the square-wave tach signal from the MS3 to a spike signal to drive the factory tach expecting ignition coil feedback:


Pete. (l33t FS)
Pete. (l33t FS) MegaDork
11/30/22 8:04 p.m.

Found the link I was thinking of.



PREFERRED SIGNAL SOURCE...an inexpensive 12v battery charger can be used as a deadly accurate signal source for the re-cal of your tach. This signal source is so steady and accurate that there is no need to compare the reading with another V-8 calibrated tach. Battery chargers typically use a rectifier bridge to convert the AC power source into DC. This turns the 60 hertz sine wave input into a 120 hertz ripple wave DC output. The ripple wave output is be read as 3600rpm by the stock rotary calibrated RX-7 tach. For a V-6 application, an RX-7 tach should be adjusted to read this signal as 2400rpm. For a V-8 application, adjust your RX-7 tach to read this ripple signal as 1800rpm. Please note that you must use a 12v battery for a power source, and the battery charger as a signal source only. The ground wires for both the battery and the battery charger should be connected to the ground terminal of the tach.

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