Sending unit doubles use for alarm and temp?

[stephenspann27]

I stripped the motor (1985 Mercury blackmax 150) gauges, and controls from an '85 skeeter back in November. I had to strip it in a hurry. Luckily installing the motor and all the controls to my boat has gone pretty smooth so far. One thing I'm confused about though, there is a sending unit on the powerhead with a single wire coming off of it. I assumed this is for the overheat alarm, I removed an alarm from the skeeter. However, I was looking through my box of skeeter gauges and I found a temperature gauge. Is it possible that the sending unit can set the alarm off, and also send temperature readings to the gauge?

 

[Silvertip]

Not likely. A sender for a temperature gauge is a thermistor (temperature sensitive resistor) that changes resistance and hence gauge reading depending on engine temperature. The overheat sensor is a simple switch that closes at a specific temperature. In other words it is either on or off. To use the gauge sender to trigger the overheat alarm on a 1985 engine would require some extra electronics that your engine did not have. The engine likely had separate overheat and temperature gauge senders.

 

[stephenspann27]

Ok, I'll dig a little deeper for another sensor. I went to my storage and got the main harness that I cut out of the skeeter, I'll get it all laid out on the garage floor and see if I can make sense of it. If nothing else, I'll pull out what I think I is an overheat alarm sensor, and compare it to the one from my old motor.

 

[Silvertip]

All you need is an ohm meter to tell the difference between a temperature "switch" and a temperature "sender". A switch will measure either open (normal) or closed (hot). Your meter and a pan of water can be used for testing. A temperature sender for an outboard will measure 33 - 240 ohms (hot to cold) depending on ambient temperature.

 

[stephenspann27]

That's a good point. I know the current sensor doesn't have any ground, or else the buzzer would be going off right now.

 

[bruceb58]

That's a good point. I know the current sensor doesn't have any ground, or else the buzzer would be going off right now.

Many get their ground through the engine block so either type would have just one terminal. Wiring diagram in a service manual would tell you exactly.