Tach Question

[akwalker]

General question. A friend of mine has a boat with a 1978 70 hp 3 cylinder Evinrude. His tach quit, and he asked my help diagnosing the problem. I checked for 12 volts DC to the tach with the key on, and its there. I also checked the AC signal wire from the rectifier at the tach, and at idle, there is 5 volts DC. As the idle increases, so does the signal voltage. I'm thinking the tach is all done, but here's the kicker. He picked up a new tach from Bass Pro, and it won't work either. I'm thinking the new tach is bad as well, but wanted to make sure I didn't miss anything.

 

[jhebert]

Re: Tach Question

Speaking generally:

On a typical outboard motor the tachometer signal is derived from the permanent magnet alternator and rectifier. The usual arrangement is to have a half-wave tap on the rectifier. The signal thus consists of an alternating or pulsing DC , with the voltage swinging from close to the battery voltage at peak to a bit below zero at the minimum. The frequency is in proportion to engine speed.

If you connect a DC voltmeter to the signal, the meter will integrate the voltage over time and it will show a DC voltage. It is hard to predict exactly what voltage your meter will show because in part it depends on the meter and its circuitry. You could measure a voltage and it might not indicate the tachometer signal was correct--it could just be a steady voltage, not a pulse signal. If you use a particular meter, sometimes the service manuals will give a specific voltage that should be seen. But, again, it is dependent on the meter type.

The tachometer itself does not measure voltage, but rather it measures frequency. Since the frequency of the tachometer signal varies in proportion to engine speed, a measure of the frequency of the signal will be a measurement of engine speed. All that is left is to perform a proper calibration, and this is usually accomplished with a selector on the tachometer.

Now, as for why yours does not work, it is hard to say. I can help you with your diagnosis by eliminating the tachometer: take your tachometer and test it with another motor. If you tachometer works, then you know it is not the problem. (And I would suspect the new tachometer ought to be good, so if it did not work, I'd begin looking at something other than the instrument itself as the problem.)

Also, some fancy multi-meters have a frequency meters built into them. If you have such a meter you could feed the tachometer signal to it and see if it reads a frequency.

I am an engineer-type of guy, so when I had a tachometer problem I just dragged an oscilloscope out to the boat to see what was going on.

 

[yamamarinetech40]

Re: Tach Question

Well, I imagine you don't have a scope....so did you set the new tach's pole setting correctly...it doesn't know the application before hand so you have to tell it....one thing about the key on power, did you use the tach's ground to get the 12v or another ground...common mistake made is to get an easy ground and check power, but may not have same ground at tach. Even if no signal from stator you should see slight twitch with key turned....