False overheating alarm

I have a '94 Mercury 60 hp outboard. Took it to the lake yesterday and a couple of minutes after starting, (still on trailer) the alarm goes off. I immediately stop engine and check motor temp by placing hand on head. It is cool. so I check thermostat which is O.K. The pee tube is working fine. I disconnect the lead from the temp sensor in the head and the constant alarm stops. This was located at a pig tail connector in the engine compartment. Anyhow, I used the outboard rest of day without any issues checking the engine temp occasionally. My first question is the motor sensor supposed to be an open or a closed circuit. Beyond that, where do I go to further troubleshoot this problem. I realize the need to fix this issue.

The temp sensor that is wired to the alarm is a pretty basic piece of kit.

It works on simple resistance.

The "ground side" of the alarm module is wired to the sensor.

If the sensor heats up enough, it's resistance drops (to near zero) which completes the circuit to ground and you get the alarm (beeeeeep).

So, when the motor/sensor is cold you should have power coming into the sensor (tan/blue wire I think), but if you remove the sensor from the motor you should not have power at the threaded portion of the sensor itself (if you get what I'm saying here).

These sensor are prone to failure (due to old age as much as anything else) so if it is your sensor that is "toasted", it would not be considered "uncommon" at all.

You can always revert to the time tested 'hand on the left exhaust plate' method; if you can keep you hand there, it's not overheating. Or you could use an infrared temp gun.

Jeff

Basically all it is is a snap disc switch. Definately put in a new switch and see if the problem goes away.

Click to expand...

Thats correct as it does not read resistance, it either open or closed.

Is it possible the alarm is for something else?

No, it is only for two reasons; the first is engine overheating, the alarm buzzer will be constant. The second is to send an on off alarm if the oil reservoir gets too low. This information is from Mercury repair manual.

Thanks for the information. I understand now that it is a simple thermo-disc switch, (on or off). I have done some further evaluation and I now believe that the problem may be somewhere else in the wiring circuit. I removed the temp sensor switch from the engine, (just left it hanging). I checked the resistance from the outside surface of the sensor, (that contacts the engine block) and the lead wire from the pigtail. There is not any continuity, zero ohms. I then cranked the engine with my water muffs on and the alarm still sounds continuously. I figure there must something in wiring circuit that is allowing a ground connection. I am going to disconnect the low-oil sensor connections and see if this makes a difference since this must share a common connection to the alarm. I don't have a wiring circuit diagram for all the wiring so I will have to try some trial and error techniques.