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  Tachometer: 1977 Mercury 115

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Author Topic:   Tachometer: 1977 Mercury 115
LOX of the LAKE posted 01-02-2007 05:59 AM ET (US)   Profile for LOX of the LAKE   Send Email to LOX of the LAKE  
Hi Guys,
I have a 115-HP Mercury, an oldie at 1977. Anyway I have a new tachometer, a Faria, but I can't get it to run. The old one has only two wires +ve and -ve and still works fine.

The new tacho has three wires, +ve, -ve and a signal. It talks of how many poles there are on the alternator for the signal input and has a little dial switch for the selection of the the poles. I have tried all the wires from the conrtol box to no avail. I see in the instructions there is mention of older models having to hook up to the stator before the rectifier.

I am getting about 2-Vdc from one of the wires but it doesn't seem to work. Do I need to chase something at the stator?. What am I looking for?

Cheers LOX of the LAKE

jimh posted 01-02-2007 09:14 AM ET (US)     Profile for jimh  Send Email to jimh     
Almost universally an outboard motor tachometer runs off a signal derived from the battery charging circuitry, and the replacement tachometer you describe sounds as if it is designed precisely that way.

Again, almost universally, the tachometer signal is carried from the engine to the helm location using a wire with gray insulation. This conductor should be connected to the tachometer input, which is often marked S (for sender or sensor).

The tachometer signal is generally derived as a half-wave rectified tap from the charging coil output. Here is a more elaborate explanation:

http://continuouswave.com/whaler/reference/PMA.html

LOX of the LAKE posted 01-02-2007 01:48 PM ET (US)     Profile for LOX of the LAKE  Send Email to LOX of the LAKE     
Thanks for that Jim, I'll get in to it and see how I go, thanks for the quick reply.

Cheers LOX

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