The complete heater unit assembly is catalogued in four handed variants: MGF RHD, MGF LHD, MG TF RHD, and MG TF LHD. A complete heater assembly includes the matrix, the blower motor, the wiring harness, the resistor pack, the air distribution lever, and the housing, all as a single supplied unit. Complete assembly replacement is rare given the cost and the behind-fascia labour involved, but for cars where multiple internal components have failed together (matrix, blower motor, and housing damage, for example) it can be more cost-effective than individually replacing each part. More commonly, the individual components are replaced as needed while leaving the housing in place.
Heater Matrix, With Dedicated Bleed Screw
The heater matrix is the small radiator-like heat exchanger inside the heater unit. Hot engine coolant flows through the matrix tubes while the blower motor pushes cabin air across the fins, transferring heat into the airflow. Matrix failure typically presents as a coolant smell inside the cabin, misted or greasy-filmed windows from vaporised coolant, or a wet passenger footwell where coolant has leaked from the matrix onto the floor. A leaking matrix should be addressed promptly, the coolant loss reduces the main engine cooling circuit and in the mid-engine MGF/TF, coolant loss is more consequential than on a front-engined car given the long pipe runs.
A dedicated matrix bleed screw is catalogued to bleed trapped air from the matrix after refilling, a step that is often overlooked during DIY work and produces persistent weak-heater complaints until performed correctly.
Blower Motor and Fan, Handed LH and RH
The blower motor and fan assembly forces air through the heater unit and out to the cabin vents. It is catalogued in handed LH and RH variants, ordering the correct one requires knowing the car's steering position. Motor failure causes complete loss of forced ventilation, some airflow will still occur through the ram effect at driving speed, but demisting relies entirely on the blower being functional. A squealing or grinding blower motor indicates worn bearings and should be replaced before complete failure (which typically happens in winter, when demisting is essential).
The motor is accessed from behind the dashboard, an involved task on the MGF/TF given the dashboard construction, though manageable with patience.
Fan Speed Resistor Pack, Most Common Fault
The fan speed resistor pack controls the blower motor on the lower fan speed settings, the resistor drops voltage progressively for speeds 1, 2, and 3, while the full-speed (speed 4) setting bypasses the resistor and runs the motor at full 12 volts. Resistor pack failure is the single most common heater electrical fault, producing the classic symptom of a blower that works only on the highest speed with all lower speeds completely dead. The fault is usually caused by the resistor coils overheating over time, particularly if the blower motor bearings have dragged and forced the motor to draw excess current. The resistor pack is a single specification across all MGF and MG TF cars, simple to order, and typically accessible without major dashboard removal, one of the quicker heater repairs.
Wiring Harness and Distribution Lever
Two heater-unit-specific wiring harnesses are catalogued: one for MGF cars and one for MG TF cars, reflecting the different connector arrangements between the two models. The wiring harness carries the blower motor feed, the resistor pack connections, and the illumination feeds to the control panel. An air flow distribution lever, the internal mechanism connecting the distribution cable to the flap inside the heater unit, is catalogued individually. This is often ignored in heater diagnostic work but can be the cause of "distribution control doesn't work" symptoms where the cable operates but no air reaches the intended outlets.