Re: [OMC-Boats] Instrument Question

From: Ethan Brodsky <brodskye@...>
Date: Mon, 2 Mar 2009 18:02:40 -0600 (CST)

On Mon, 2 Mar 2009, jd wrote:
> So just to clarify, as long as it's a matching set (oil sender/
> gauge water sender/gauge) than a person could go with any brand, not
> just Faria, right?

Barring any bizarre exceptions I can't think of, that's generally true.
The sender has a resistance that varies with temperature or pressure, and
the gauge has a needle that moves in response to changes in resistance.

Most old-school temperature and oil pressure gauges use single-wire senders
and are grounded through the engine. So the gauge is wired with +12V going
through a resistor on the gauge and through the variable resistor on the
sender to the engine block ground. The gauge senses the current through
this path and moves the needle accordingly. On modern systems, the sender
is typically wired through the engine computer, and the gauges are driven
over a serial bus from the computer.

For temperature gauges, make sure that they stick into the coolant stream -
installing them on an extension or a tee can lead to very delayed readings.

As for jdood's uncalibrated gauge, why don't you calibrate it yourself?
Take the sender out of the engine and use a small jumper wire to ground the
body of the sender to the engine block (make sure it's a good ground - get
something with an alligator clip or ring terminal, and ground it to a bolt
or a spot where you've scraped off the paint and corrosion).

Then dip it in a cup of boiling water and see what the gauge reads. You
know that is 100 C. Put a meat thermometer in the water as it cools off,
and record the gauge reading vs. the thermometer reading. Then relabel the
gauge with a sharpie (or take the glass off and print a new face -
sometimes we get fancy!). All you really need are two marks showing the
range of normal operation. Newer boats were designed to run at 160-180
degF, but I think the older OMC engines use 140-160 degF thermostats
(efficiency and emissions weren't such a concern back then).


<a href=""> Ethan Brodsky </a>
Received on Monday, 2 March 2009

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