Temp gauge "calibration" possible?

[Moody Blue]

I've got an automotive temp gauge and sender installed on my 1992 40hp Mariner 4 cyl. This motor runs so cool that the gauge just creeps into the lower portion of the dial. I'd like to artificially recalibrate the gauge to read more in the mid range scale so that I can see temp variations before they might become extreme. Any thoughts oon how to do this? The sender is the typical 1/8NPT probe that screws into the block and connects to the gauge with a single lead.

 

[Tom Scully]

Re: Temp gauge "calibration" possible?

Your outboard should not run so cool. Sounds like the thermostats are stuck open or missing.

 

[Moody Blue]

Re: Temp gauge "calibration" possible?

These motors do not have thermostats. Can't figure why they were never designed into this series of motor.<br /><br />How do these sending units work? Are they resistive?

 

[Silvertip]

Re: Temp gauge "calibration" possible?

Temp sensors are technically "thermistors" or temperature sensitive resistors. Resistance changes in relation to temperature. Most temp sensors are 30 - 240 ohms (or thereabouts) and require a gauge that matches the sensor. If your auto temp gauge starts at 120 degrees and ranges to 240 (as most do), outboard engine temperature will read on the lower end of the scale.

 

[Moody Blue]

Re: Temp gauge "calibration" possible?

Thanks for the explanation. Is it possible to fool the gauge by installing a resistor at the sense terminal? Would it be hooked up series or parallel? DOes resistance decrease or rise with temp? I hate always seeing the needle sit on the peg below 100 all the time. I've tested the sender by immersing it in hot water and the gauge read properly.

 

[Silvertip]

Re: Temp gauge "calibration" possible?

Here are typical cylinder head and water temp sensor resistance readings. Disconnect the wire from the temp sensor. With the engine cold, measure the resistance of the sensor (terminal to ground) with engine cold and warmed up. Here are some "relative" readings:<br /><br />Cylinder Head Temp Sensor:<br />60 degrees = 1195 Ohms<br />140 degrees = 192 Ohms<br />220 degrees = 46 Ohms<br /><br />Water Temp Sensor:<br />100 degrees = 450 Ohms<br />175 degrees = 99 Ohms<br />250 degrees = 29.6 Ohms<br />Note that the type of sensor and gauge you have will affect the sensor resistance. These are for reference and may vary somewhat from whatever type you have. Adding a series resistor will not work as it just makes the gauge read colder. You might try a parallel potentiometer.