Kim, I haven't looked at the heating system on a pre-1990's model so maybe Clarke can jump in here. However, on the 1990's models, the dash switches turn on the individual heat exchanger fans but do NOT control any electric valves. On Tom's '90 model there is only one coolant circuit and the valve is controlled by the dash heater temperature switch. On my '92 model and several other '92-'93 models I have looked at, there are two coolant circuits. The one to the front heater is controlled by a valve that is controlled by the dash heater temperature switch. The other circuit feeds the water heater, under bed heater and the bay heater and is on unrestricted so that once the water in the engine heats up, those units will be feed hot water all the time, just as you under bed heat exchanger does.
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