The MGB heater system relies on a network of hoses, pipes, a control valve, and cables to deliver hot coolant from the engine to the heater matrix and return it to the cooling circuit, with the driver controlling heat output and air distribution from the dashboard. When any part of this circuit fails, a split hose, a seized valve, or a broken control cable, the heater either stops working entirely or cannot be adjusted, and the demister becomes ineffective.
Valve & Pipework
The heater control valve regulates the flow of hot coolant through the matrix. A new heater water valve was introduced at V8 production number 2146 in December 1974. The heater valve gasket is available separately for reseating an otherwise serviceable valve. The copper pipe connecting the heater to the water pump is available alongside the metal heater pipe for 1968-onwards cars.
Hose clips in the correct sizes for heater circuit connections are available.
Control Cables & Knobs
Heater control cables are available in two lengths covering the heat and air control functions. A trunnion nipple kit is available for renewing the cable end fittings where the originals have worn or corroded. The heater cable clamp secures the cable outer correctly to the body. Heater control knobs differ across the production run and between RHD and LHD specifications.
The original choke-style pull knob marked ‘C’ was replaced by a new corporate knob with a fan symbol in 1970. The heater controls were revised again as part of the 1972 interior overhaul with illuminated rocker switches on North American cars, and the post-1976 rubber bumper dashboard specification brought a further revised knob design. Heat and air knobs are available for each of these periods, with US-specification knobs available separately.