Electrical troubleshooting on cooling systems

I had two cases where a piece of equipment was brought in for problems with the cooling systems. The gauge would read that the unit is overheating. I did some troubleshooting and found that the thermistor was no longer working properly. I tested it by removing it and comparing the value of resistance at ambient temperature to a resistance chart telling me what the resistance should be at that temperature.

Here's how it works. The unit will have a coolant temperature gauge. The gauge will have a power and ground like everything else but there's a twist. It also has a signal wire coming from a coolant temp sender. The job of the temp sender is to let the gauge know what to read for engine temperature. The sender is something called a thermistor.

In this scenario a thermistor is a resistor located on the engine whose resistance is reduced by heat. As the engine warms up the resistance reduces allowing voltage for the signal wire to increase which increases the reading on the gauge. In other words when the engine's cold the resistance is high causing the voltage for the signal wire to the gauge to be low. As the engine gets hot resistance for signal wire goes down increasing the signal voltage to the gauge.

One method for discerning whether you have a problem with the gauge or the sender is to ground the signal wire to the frame. If the gauge pegs over all the way then most likely there is a problem with the thermistor on the engine. You can troubleshoot further by removing the thermistor and testing it with your multi-meter. At a specific temperature there should be a specific amount of resistance. If your meter is reading a lot more resistance than the thermistor is supposed to have at that temperature then it should be condemned.

Typical thermistor in an engine with signal wire bolted to it

Lug with the S next to it is for the signal wire

coolant gauge on a Gehl skid loader