Strange Tachometer Problem Fixed

[Oldslowandugly]

I have a fairly new Teleflex tachometer on my console. The motor is a 1993 Evinrude 48 SPL. It has a 4-Ampere unregulated alternator.

For the first time ever, the tach started reading 1000 or more RPM higher than normal at all speeds. I figured it might be the pole switch on the back of the tach. I went down today with my tools to test everything and work the switch back and forth in case it was crusty inside.

Before I started I opened the battery box and checked the level of the battery cells. It has been very hot and dry lately. The cells were pretty low and took almost a pint of distilled water.

Then I started the motor and to my amazement, the tach was reading perfectly. Then I remembered something my cousin told me. He said an outboard tach is basically an AC voltmeter. As the [engine speed] increases, so does the voltage, and the tach relates that as engine speed. Then I remembered something I read on here. I read that on an unregulated charging system, the battery becomes the regulator. So if my battery was low on electrolyte, and the alternator was trying to charge something that was not accepting the charge, the charging rate was way over specification. That high rate would result in the high tach reading. Does that make sense?

[jimh]

No. The tachometer is not a voltmeter. It is really a frequency meter. It counts the pulses in the alternator pulsating DC output.

A more likely cause for the behavior observed is that the pole switch fixed itself. The addition of water to the battery was probably coincidental.

The pole switch setting on the tachometer determines how the meter scale reacts to frequency of pulses; it calibrates the dial scale.

The water level in a battery does not affect the voltage. Battery cell voltage is determined by the chemistry of the electrolyte, not the amount of electrolyte. Actually, when you add water, you dilute the electrolyte, and the battery voltage tends to go down. This further contradicts your theory.

[Oldslowandugly]

Hmmm. Looking at the wiring, I see that the grey tach wire is attached to the alternator output yellow/grey wire. That would seem to indicate that the tach is seeing raw AC voltage before the rectifier can convert it to DC. Only the volt meter sees rectified DC voltage. Also, I never touched the pole switch. In my experience, a crusty switch fixing itself is unlikely. I have also seen an automotive alternator burn itself up trying to charge a battery that had a loose cable bolt. The alternator thought that the battery was dead and committed suicide trying to charge it. Low electrolyte level would be similar to a bad cable connection because the capacity would be diminished. I'm not trying to argue, just to understand what happened.

[jimh]

A tachometer does not measure voltage.

The signal fed to the tachometer is a half-wave rectifier tap of the AC output of the stator coil.

[jimh]

What was the battery voltage when the tachometer reading was out of calibration?

[Oldslowandugly]

The battery always reads 12 volts or better. It is only one year old. It is a combination starting/deep cycle. I keep a charger hooked up all the time the boat is at the slip. The charger is designed to switch to a trickle charge after reaching max charge. When underway, it usually sits around 13 volts. It can go as high as 14 volts. That is only when I am at sustained high speed running. At 5000 rpm it would see the max 4 amps of charging. Even then the tach read properly. Yesterday I went out fishing and at first the tach read high, then calmed down and worked perfectly all day.

[jimh]

Try moving the CAL control back and forth a few times.