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MG Car Repair/1979 mg midget heater


I'd like to know how the heater box works. I understand the controls on the dashboard, but I don't underneath the hood. I see there is a valve or handle on the left side near the heater box, as your looking from the front of the car. Is this little metal handle an "on/off" switch for the coolant to flow through the heater? I believe it does a quarter turn, but don't know which way is on or off. Also, with the air intake flex piping, where it connects to the heater box...should the open/close to be open for the heat to work, or should it be closed in order for it to work? I would think that having it open with the valve on the left side in the off position would give me some fresh cool outside air.
Anything would help me understand it better. I want to drive the car in cooler weather, and if need be, with the top up

Hi Dan,

The valve near the battery with the heaterhoses on it is a water conrol valve. This controls the coolant to the heater core. Used for "Heat" only.

The large butterfly valve at the heater box with the large flex hose attached is the control of the amount of fresh air that enters the heater box. Control on the far left of the dash. "ON" letes more air in and "OFF" closes off the air. Works the same for heat and cool.

If you don't have an owners manual jsut have someone opreate the heater control inside the car and you can see valve operate under the hood.

Move the water control valve lever toward the battery to open the coolant to the heater core and away from the battery to close the coolant for warm weather operation.


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Howard M. Fitzcharles III


MG from 1956 (USA versions only) up and Engine theory.


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

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