Strange Tachometer Problem Fixed
Posted: Mon Jul 18, 2016 8:14 pm
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?
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?