1996 Merc 40HP 2 Stroke 4 Cyl Overheat Alarm Question

[mrmamiller]

I have a friend who brought an overheat sensor to me for a soldering repair (he had to cut wire to get a socket on the sensor). I know nothing about how Mercury does their overtemp/overheat alarming, so here goes. The owner has had an overheat alarm, but the engine was not "hot" to the touch. The motor is a 1996 Mercury "Classic", 40 HP, 4 cylinder, 2 stroke; serial is 0C268772. The friend ordered a new overtemp sensor but what he got appears top be a temp sender for a gauge rather than the overheat switch. From the manuals that were available for viewing on this site, there are both types of sensors on this setup. But, there is not very much information concerning the checkout of the overheat sensor. I was assuming that the sensor would make continuity at alarm trip point. I placed the sensor into boiling water and it remained as an open circuit. Afterwards, I was reading "somewhere" that the trip point is 300 degrees F; can this be right? I need to know: 1) what is the trip point for the overheat sensor, 2) does it "make" on alarm or "break" on alarm, and 3) what is the audible alarm for overheat and is it different from an oil alarm and in what way? Any chance that this is an alarm module error and if so, any checkout procedure? Thanks for any help that you can provide. Mark

 

[Texasmark]

Re: 1996 Merc 40HP 2 Stroke 4 Cyl Overheat Alarm Question

300 is too high. Try 195F-ish. A lot of stats start opening at 143F-ish (tolerances) and are full open roughly 10 degrees higher. The OT alarm for the Commander 2000 is a shorting of the sensor in the water jacket over the head (should be where it is on that engine) and is accomplished with a Tan or Tan/Lt. Blue striped wire.

When the sensor shorts out, it applies ground to the low side of the Alarm device (horn) which has 12v on the other side. The reset for the OT sensor is about 175F. The OT sensor is a SPST switch. To test it, just follow the wire from it to the first connection, break it and short the control side of that connection to the block. The alarm should sound. If you pulled it and put it in hot water and didn't get a short between the output wire and the case of it, it is not what you are looking for.

If you got an after market analog sending unit (the wrong part) which would go to the dash to operate an analog thermometer then it would have a varying resistance as a function of temp estimated to be roughly 35 ohms to 135.

HTH,
Mark

 

[jackle61]

Re: 1996 Merc 40HP 2 Stroke 4 Cyl Overheat Alarm Question

Just a quick note. My 93 Classic 40 doesn't have a thermostat. It is a "flow through" design. The only way to adjust the flow is to change the grommet on the discharge port.