Fd3s:coolant gauge
The Third Generation RX7 Temperature Gauge
The following chart is the response of the third generation RX7 water temperature gauge. This gauge measures the resistance of a thermistor, which is in contact with the engine coolant. Thermistors change their resistance in response to temperature in a relatively linear manner. As the temperature of the coolant increases, the resistance of the thermistor decreases. The gauge's response to this change in resistance is not linear, it has three distinct slopes (sensitivities) with the middle range of the gauge being a slope of zero (no sensitivity). The lower resistance (higher temperature) is the most sensitive. I believe Mazda engineers did this to avoid having customers complain about their cars running hot. Unfortunately, the result is a gauge that will not move above "normal" until the car is near over heating.
My measurement technique was to disconnect the plug attached to the water temperature gauge sensor on the engine, and connect it to a resistor substitution box. The other wire on the box was connected to the negative terminal of the battery to complete the circuit. The resistor substitution box simulates the resistance changes that the water temperature gauge sensor undergoes in response to changes in coolant temperature. To record the needle movements I cut a 90-degree pie shaped wedge of velum slightly shorter than the length of the water temperature dial pointer. I taped this wedge to the dial face with the centerline of the wedge in the horizontal position. The apex of the wedge was located at the gauge-pointer pivot. I turned the ignition switch to the on position and dialed in a series of resistances on the resistor substitution box and marked the location of the tip of the dial pointer on the wedge shaped paper. I transferred the marks on the wedge to piece of graph paper and calculated the number of degrees from the center line each point was located. Points falling below the centerline were negative those above were positive. For example, if the pointer was exactly horizontal it was at 0 degrees.
