It prevents full engine temp coolant from flowing through the heater core. It would have to help some in keeping the cab cooler (maybe less hot would be a better term) than if engine temp coolant was present. How much it helps??? Beats me. I've got one on my '38 and have run it both ways. I've not been able to tell a difference. Now I just leave it on all the time.

Last edited by Tiny; 06/20/2019 7:45 PM.
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