There is not a lot of information about the early temperature senders/gauges and some of that information is incorrect, which lead me down the wrong path. Maybe someone with a little more clout than I can confirm the information below and update some of the earlier posts in this thread. Hopefully this helps folks avoid the mistakes I made.
Operation of early (pre 9/72) temperature senders:
Much like the early fuel senders, the early temperature senders contain bi-metal voltage regulators and operate as a crude on/off (like really slow PWM) temperature dependent voltage regulator. They are NOT resistive temperature sensors, and trying to measure the resistance as a function of temperature will not work - the resistance is a near constant 35 - 40 ohms, regardless of temperature, as you are actually measuring the resistance of the heater wire wrapped around the bi-metal strip.
These sensors work in a similar fashion to the bi-metal strips wrapped in a heater coil that deflect the needle in the gauges. The gauge applies battery voltage though a ~25 ohm resistor (which is the heater coil on in the gauge itself) to the sender. This causes the bi-metal thermostat strip in the sender to be heated by a coil of resistance wire wrapped around it, and it bends away from a contact, breaking the circuit. As the circuit is now open, the bi-metal strip cools down and returns back to the contact, closing the circuit and starting the cycle again. The duty cycle (how long the circuit is closed relative to how long it is open) sets the average voltage through the sender. Now as the ambient temperature in the sender increases, it takes less time to for the bi-metal strip to heat up as it is already warm, and it takes it longer to cool back down, so the regulator spends more of its time in the open position, which decreases the duty cycle, and thus leads to a lower average voltage. In short, the sender is a temperature sensitive voltage regulator with a decreasing average voltage as the temperature increases.
This also explains why the needle on the early temperature gauges rests on the right (H) side of the gauge when the vehicle is off and deflect to the left. When you start the engine, the sender is cold and is closed most of the time, leading to the a higher average current through the gauge, which deflects the needle all the way to the left (C). As the sender warms up it spends more time open than closed, reducing the average current through the gauge, which then deflects less and moves back the right towards the Hot mark.
Hopefully this helps someone else figure out what is going on with their gauges. Next up my non working fuel gauge/sender combo.
Operation of early (pre 9/72) temperature senders:
Much like the early fuel senders, the early temperature senders contain bi-metal voltage regulators and operate as a crude on/off (like really slow PWM) temperature dependent voltage regulator. They are NOT resistive temperature sensors, and trying to measure the resistance as a function of temperature will not work - the resistance is a near constant 35 - 40 ohms, regardless of temperature, as you are actually measuring the resistance of the heater wire wrapped around the bi-metal strip.
These sensors work in a similar fashion to the bi-metal strips wrapped in a heater coil that deflect the needle in the gauges. The gauge applies battery voltage though a ~25 ohm resistor (which is the heater coil on in the gauge itself) to the sender. This causes the bi-metal thermostat strip in the sender to be heated by a coil of resistance wire wrapped around it, and it bends away from a contact, breaking the circuit. As the circuit is now open, the bi-metal strip cools down and returns back to the contact, closing the circuit and starting the cycle again. The duty cycle (how long the circuit is closed relative to how long it is open) sets the average voltage through the sender. Now as the ambient temperature in the sender increases, it takes less time to for the bi-metal strip to heat up as it is already warm, and it takes it longer to cool back down, so the regulator spends more of its time in the open position, which decreases the duty cycle, and thus leads to a lower average voltage. In short, the sender is a temperature sensitive voltage regulator with a decreasing average voltage as the temperature increases.
This also explains why the needle on the early temperature gauges rests on the right (H) side of the gauge when the vehicle is off and deflect to the left. When you start the engine, the sender is cold and is closed most of the time, leading to the a higher average current through the gauge, which deflects the needle all the way to the left (C). As the sender warms up it spends more time open than closed, reducing the average current through the gauge, which then deflects less and moves back the right towards the Hot mark.
Hopefully this helps someone else figure out what is going on with their gauges. Next up my non working fuel gauge/sender combo.
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