Strange Tach Readings

I've got a 1981 Johnson 90 HP. I installed a tach this past spring, and it has consistently shown around 5600 RPM at WOT. My last two or three times on the water it's trying to tell me the engine's turning in excess of 7500 RPM - NO WAY. Engine doesn't seem to be running any different, and I still get the same speed at WOT as always (about 43). I put the tach on a signal generator at work, with a 600 Hz squarewave it's showing around 6150 RPM, off a little but not nearly enough to explain my huge "performance gain".

Anybody with some ideas on this? My first inclination is to blame the tach, but no matter how hard I try it won't act up on the workbench, so I just don't know.

Thanks.

Re: Strange Tach Readings

You would do well to test the rectifier. First map the wiring, then remove it and run the test according to Mr Reeves' instructions here: http://forums.iboats.com/showthread.php?t=190638

Since a bad rectifier can damage other components, if the unit tests bad it should be replaced or left disconnected.

Re: Strange Tach Readings

Well, the rectifier is only a few months old. I replaced it this spring, but it won't hurt to check it.

Are there any failure modes where the rectifier would cause a higher than actual tach reading? That's puzzling.

Re: Strange Tach Readings

What is the condition of your battery terminals? Loose and/or corroded battery connections usually take out single diodes, but can do that more than once eventually causing a complete bridge rectifier failure. Cross-polarizing takes out both sides of the bridge rectifier instantly.

As Ezeke stated, do not run the outboard with a fried rectifier.

Re: Strange Tach Readings

Battery terminals are clean and tight. Battery connections at the engine are clean and tight. Just checked the rectifier, all diodes in it are ok. When the engine is running the charging voltage is the same as always (about 14.6 volts per my panel mounted digital voltmeter).

There's really not much else it could be on the engine is there?

Re: Strange Tach Readings

A simple way to determine if the tach is bad is to use another tach. Some shops will have a tach ready with alligator clips just for this, ask them and see if they will let you borrow it, or borrow one off a buddy.

In the event you don?t have access to a shop tach, you?ll have to refer to the manual to check the signal source. The signal is a pulse-positive output that?s provided directly from the stator assembly through the rectifier. A peak-reading voltmeter set to ?positive? and the lowest volt scale will enable you to read any voltage output (on this circuit) from the stator.

Re: Strange Tach Readings

I recently noticed my tach had started reading high. I found the +12V connection at the back of the tach was loose. (The nut had vibrated loose but the terminal connector was still on the post.) I don't understand why this would cause a high reading and not a low one, but tightening it fixed the problem. I have a Faria tach.

I would check the voltage supply connections at the back of your tach, the plug at the harness and all the way back to the battery.

Hope this helps.

GY

Re: Strange Tach Readings

gyoung said:

I recently noticed my tach had started reading high. I found the +12V connection at the back of the tach was loose. (The nut had vibrated loose but the terminal connector was still on the post.) I don't understand why this would cause a high reading and not a low one, but tightening it fixed the problem. I have a Faria tach.

Click to expand...

That gives me something to check. My tach is a Faria as well. It is strange that you got a high reading with a bad power connection - on my bench I can make it read low by dropping the supply voltage but so far nothing I've done has made it read high.

I don't have access to a spare tach, but I've got a Fluke DMM with which I can measure frequency.