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Oliver, Cletrac, Co-op & Cockshutt Tractors Discussion Forum
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Re: 1961 1800A gas - cooling - need advice

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Indiana Ken

04-16-2013 19:00:26

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Typically the coolant temp sensor is a variable resistor, as the temp decreases the resistance of the sensor will increase. For example, I am looking at one for a motorcycle I checked several days ago, at 212 F the resistance is 32 ohms, 180 F is 53 ohms, 140 F is 108 ohms and 86 F is 350 ohms. It is easy to check by placing the sensor in a cup of hot water and reading the resistance as it cools.

The sensor calibration (ohms versus temp) has to match that of your temp gage for the system to read correctly. As you can see there is a big change in resistance and for the gage to read full hot at ambient temp suggests another problem.

1) Perhaps your sensor is faulty i.e. grounded (zero ohms).
2) Perhaps there is a resistor used (now missing) to drop the voltage supply to the temp gage. Some times a voltage regulator is used.

As to the thermostat, if the engine was designed for one I always believe it should not be removed. The thermostat holds a backpressure on the system and can also help control/prevent pump cavitation. I have however, never understood how the flow can be too fast through the radiator such that there is not time to cool the water. The radiator is always filled with water and the is no difference in the time the radiator has to cool it regardless of the flow rate.

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04-16-2013 23:10:14

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 Re: 1961 1800A gas - cooling - need advice in reply to Indiana Ken, 04-16-2013 19:00:26  
Your motorcycle may have a resistor but not a variable one, mind. What you are describing is a thermistor.

However most automotive applications use a thermistor in which the temperature coefficient is negative with respect to temperature. Resistors, per se, have a positive coeffient wrt temperature. However a thermistor could be referred to as a negative temperature coefficent resistor, but thermistor is so much less of a mouthful.

The simple check is to short the meter-to-thermistor wire to earth - the gauge will peg on full scale. When cold the thermistor will be high reistance and the current through the meter a minimum value.

The voltage regulators for the guage supply are almost bullet proof and they regulate at 10.5 volts, so the meter would still not peg full-scale at cold, even if the full battery voltage was applied - the readings would be wrong, but that is all. Thermistors rarely fail totally.

As to the fallacy that no thermostat always leads to overheating, think upon this. When the thermostat is hot it will open virtually fully so as not to unecessarily impede water flow through the radiator - just as though it was not there...

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