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automotive tachometer adaptation

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aaa123

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After installing a newer engine into an older european car (engine swap), I am trying to find a solution for driving the stock tachometer.
The old engine has a distributor/coil ignition where the tachometer is driven from the coil.
The new engine has the newer type ignition with no coil and a tach signal wire coming from the computer.
The signal from the engine computer will not drive this tach.
I do not have an oscilloscope but I may have enough information from simple DMM measurements. With the old coil ignition, the DMM set for AC Voltage shows around 28 volts and it wanders up and down a couple of volts. With the newer engine the meter shows about 18 volts on just the bare signal wire from the computer, and that wanders a bit as well.
With just those two measurements I am thinking the tachometer just needs a higher signal voltage to make it run. It's automotive so I have about 14.5 volts to play with from the alternator. I'm thinking maybe if I doubled the signal voltage supplied to the tach, that might make it go? I've been poking around for a while looking for solutions and I hit upon charge pump ICs that can take 15V and put out 30V using capacitors. Would that work, or does anybody have more insight on why the tach won't move?
 

adamey

Member
It would help greatly if we knew the engines (# of cylinders, make, model, year) for both.

The tach is designed to work off the pulses it receives from the ignition coil being switched on/off via the distributor. So if you have an 8 cylinder engine then the pulses will be at a speed that is 4 times engine rpm (8 cylinders in a 4 stroke engine means the coil fires 8 times every 2 revolutions).

If your new engine has coils for each cylinder then using one of those as the tach signal will only give 1 pulse every 2 revolutions, so your tach will read 8 times lower RPM than actual.

If you can tell me the make of the new engine, then I can determine what the tach signal actually is (probably a square wave) that the engine computer is outputting.
 

Diver300

Well-Known Member
Most Helpful Member
It's highly unlikely that the voltage needs to be increased to make the tacho read.

The voltmeter won't read the voltage on the coil at all well as it is a very odd waveform and probably has a big DC component as well.

As a first attempt, connect the tacho input to the coil. Ignore any computer output and leave that disconnected. Just because there is a signal available does not mean you have to use it.

If your coils drive a different number of cylinders, you will need to recalibrate the tacho.
 
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