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Replacement Tachometer

snowcruiser

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"I recently had to replace the

"I recently had to replace the tachometer (needle feel off) on my 140 I/O. I found a Mercury tach and installed it, but it only seems to read about 30% of the actual RPM's. The unit I took out had a selector on the back, but this one does not. I'm wondering if the connection point on the engine might be different, since I can't seem to change the pulse setting on the replacement unit. Anyone have any ideas on how to get this replacement tach to read the correct RPM's?"
 
"Most any boat tach has the se

"Most any boat tach has the selector for 4, 6, or 8 cylinders. There is no other connection at the engine, since all the tach is doing is counting the ignition pulses at the coil. There are 2 pulses per rev for a 4 banger, 3 for a 6, and 4 for a V-8.
You should likely return the tach you got and see if you can get an adjustable one, or one that is intended for a 4 cyl engine.
Rod"
 
"The reason I'm asking is

"The reason I'm asking is because the tach I took out has a 12V, GND, and P connection and the replacement one has 12V, GND, and ALT. So I was wondering if this particular tach may attach to the alternator instead of the coil?

Unfortunately, I purchased the replacement tach on eBay, so it's not so easy to just return."
 
"Jeff:
Google the brand nam


"Jeff:
Google the brand name of the tach and see if there are some specs. and/or instructions for the tach you bought. Is your coil 6 volts? That would explain the low RPM. Measure the voltage at the coil. You may have a 12 volt tach. Connect the new tach to your car engine with jumpers and see what it reads.

If your coil is 12 Volts, the alternator output to the battery and coil voltage are the same point electrically with the engine running. No luck on the tach? Take the cover off of the old tach and gently press the needle back on to the shaft in the correct position. If it reads correctly, put a pin head drop of red loctite on the needle and shaft connection and let it set overnight. It should work. Guy"
 
"I have since discovered that

"I have since discovered that the original tach is a Medallion brand. If there is a way to get this thing apart to glue the needle back in place, I can't figure it out. It seems to be sealed with no way to break in.

I doubt the coil is 6 volts, but I'll measure that tonight. Based on the information you guys have provided, I'm guessing that the replacement tach I bought must be only for a 8 cylinder engine, hence the low reading."
 
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