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Model 550 heater

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My 50 SD coupe has a model 550 heater unit. There is a capillary tube from the defroster duct to the water control unit under the dash. What is the purpose of that and how can I test it to see if it is properly "doing its thing"?

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My 50 Plymouth SD had a coil of tubing on the water control valve mounted on the firewall. I believe it was an early form of comfort control which regulated the flow of water when the engine coolant temp would increase on a long hill or decrease when less engine load was being used. I replaced the valve in the interest of reliability with a compatible new one I found by searching for modern heater control valves. The new one does not have any regulation but works fine as a simple push pull control valve.

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