Technical Tips

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Extract from March's MGOC publication - Enjoying MG
by Richard Ladds

Heater Removal and Refitting

An MGB heater in good condition will provide adequate heating and demisting. The heater system consists of; a heater matrix which acts like a small radiator, a water valve which allows hot water from the cooling system into the matrix, and an electric fan which can blow hot air to demist the screen or warm the footwells. If the heater matrix fails or should you decide to upgrade the matrix and blower, it becomes necessary to remove the whole heater unit casing from the car. This is because the bottom of the unit casing slots into a well in the bodyshell; to remove the heater matrix the lid of the casing has to be removed and this is only possible with the unit out of the car.


The electric blower fan is operated by a separate switch, while the heater unit is controlled by two dashboard or console mounted knobs. One is marked Hot, Warm, Increase, Off, and this is connected by cable to a valve on the cylinder head; opening this valve allows hot water to flow through from the cylinder head into the heating system. The other knob is marked On, Interior, Screen/Demist, Off, and this is connected by cable to a flap on the heater unit. Before removing the heater unit, check that all these controls are working correctly and that the heater valve on the cylinder head is operating. To check the operating of the heater flap control, lay down in the driver's side footwell to peer into the heater outlet. It helps to first remove the plastic cowl from around the outlet and you will need a torch to see inside. Resting the torch on the pedals provides good illumination. Twist the screen demist control to verify that the cable is connected and the flap is working.

Disconnect wiring and hoses

Before attempting to remove the unit, disconnect the battery or batteries, removing the earth terminal first of all. Disconnect the electric fan. Remove the top and bottom water hoses from the heater unit; have a suitable container ready to drain the coolant from the supply hoses and heater unit itself.

Disconnect control cable

Before the heater unit can be removed, it is essential to disconnect the control cable. The best approach is to disconnect the cable from the control knob end. The rotating heater control knobs are either in the dashboard or located in the transmission tunnel console.

To disconnect the cable from the screen/demist control knob, first gain access to the back of the control knob, either by removing the dash mounted radio/radio blanking plate on early models, or the fresh air vents, or the console assembly on later cars.

Photo 1 Spring off the control knob by locating the small hole and pressing in the spring retaining ball.

Photo 2 Undo the nut which holds the control assembly in place. Pull the assembly out from behind the dash to allow access to the cable connection.

Photo 3 Undo the inner and outer cable. Then disconnect the two hot air supply hoses which push into the bottom of the heater unit from deep inside the central tunnel.

Photo 4 Undo all the Phillips screws which secure the heater at its base and the single screw at the top. Now with the cable free you can withdraw the heater unit from the car. It will be necessary to lever and prise the unit from out of the recess in the bodyshell. After many years in position it will not come out easily.

Photo 5 Lever up the unit and pull it forward at the same time, in order to clear the rear lip along the top of the engine compartment. If there are any brake pipes located along the inside of this lip, take great care to avoid damaging them as you pull the unit away. It will require a considerable amount of force to free the unit, but once the old foam and rubber seals break away it should pull free. Gently withdraw the unit from the engine bay, pulling the attached control cable through the bulkhead behind the heater.

Photo 6 Spring off all retaining clips and lift up the lid from the unit. Now you can remove the old matrix and either refit a direct replacement or upgraded matrix. Note how the matrix goes in to the casing and the position of the packing foam. Take care not to refit the foam in such a way that it restricts the flow of air through the matrix.

Photo 7 The standard fan motor assembly fits through the hole in the lid and can be removed and replaced from outside the unit.

Photo 8 If you wish to fit the upgraded heater blower motor and blower assembly, now is time to do so, because the diameter of the uprated fan impeller is larger than the hole in the casing lid. Therefore, the fan motor has to be installed from outside the lid and then the impeller fitted on the inside of the lid. This can only be done with the unit removed from the car.

Photo 9 Included in the replacement matrix kit are a set of new foam seals and these must be glued in place with contact adhesive before reassembly.

Photo 10 When fitting the base foam, note the cut out area for the flap control and the position of the hole for the control cable.

Photo 11 Once the glue has dried, remember to make a new hole in the foam seal for the control cable to pass through.

Photo 12 Replace the heater unit lid and refit all clips.

Photo 13 Refit the control cable to the base of the heater unit, clamping the outer sleeve to the heater and fixing the inner cable to the flap control. Check that the cable fully opens and closes the flap, some adjustment may be necessary.

Photo 14 Replacing the heater unit. First work out the route of the cable back through the bulkhead and into the dash area. Thread the cable into the access hole, it is helpful to have an assistant working on the inside of the car to locate the cable and guide it up behind the dash. Avoid letting the cable curl up in some inaccessible area inside the bulkhead.

Final refitting

Carefully position the heater unit and ease the bottom into position. Once lined up, the unit can be manoeuvred firmly back into place. Replacement is generally much easier as all the surfaces are now cleaner. Try to avoid damaging the new foam seals when re-positioning the unit. Before replacing any fitting screws, check that the cable is following the correct route and will reach the control knob. Refit the cable to the control knob mechanism, remembering to adjust it and to check its operation before fixing the assembly back in the dash.

Reconnect the fresh air hoses and face level vents if fitted. Next and most importantly refit the two flexible hot air/demister hoses to the bottom of the heater unit. These two hoses feed into the base of the heater through the inner tunnel area beneath the dashboard. To gain access to this area, remove the trim panel and hose clamps. This will enable you to squeeze your hand inside the tunnel to push the demist hoses into position. Failure to reconnect these hoses correctly, will result in ineffective demisting.

Test the system

Refit both water hoses to the heater, reconnect the heater fan and replace all the fixing screws. Remember to top up the coolant level in the radiator. You can now reconnect the batteries and start the engine. Run the engine with the radiator top cap removed to allow the coolant to circulate and expel any trapped air. After several minutes check coolant levels and then replace the radiator cap. Allow a reasonable time for the engine to warm up and then switch on the heater, carefully examine all the hoses for leaks, especially around the clips. Switch on the fan to verify the operation of the demist function to ensure that you have reconnected the hot air pipes correctly.

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