Heater Matrix

The heater matrix is a small radiator through which the coolant runs and warms the air which enters the cabin. On the S1, it is located on the driver's side, near the battery. The heater matrix is mounted vertically to the side of the blower fan unit.

When the matrix fails, coolant leaks out of it's pipes and into the heater system, resulting in a fine mist entering the cabin through the ventilation system and misting up the windscreen. With a badly failed matrix, a micro-climate can form in the passenger footwell with mist rising and falling whilst driving.

Replacing the heater matrix

This is a tricky job due to the location of the matrix and the position of the screws holding it in place.

First, empty the system of coolant. This can be done by first unscrewing the coolant reservoir tank cap, then removing the drivers-side wheelarch liner (remove the wheel first!) and undo the radiator feed hose. Drain into a bucket and dispose of safely. The radiator hose and wheelarch liner can be refitted as this side of the car doesn't need to be accessed again.

Next remove the plastic radiator surround with a T10 screwdriver and remove the battery.

The matrix/blower assembly is held in place by 4 screws. Remove the metal blower air hose and the hoses into and out of the matrix itself. One of the screws is just inside the outlet from the blower where the hose attached. Two are into the bulkhead below the radiator behind the cylindrical green ceramic terminal block. And the fourth is closer to the battery again into the bulkhead below the radiator. It's easiest if you remove the green ceramic terminal block - it's simply held on by the metal at each end which can be squeezed together and pulled out of the center of the green cylinder.

The wire which controls the hot/cold flap sits on top of the assembly. This should be removed. The sprung metal clip can be levered off and the S-shaped end of the wire can be removed from the control armature.

Once the matrix/blower assembly is free, it may still not be possible to get the unit out of the vehicle. The blower is connected to the matrix by a metal strip and nut both on the top and bottom. As the matrix/blower assembly is now free and has a certain degree of rotation within the car, the matrix can be separated from the blower by undoing the nut and rotating the metal strip about 45 degrees.

With the matrix separated from the blower, you can now remove the matrix from the car. You may need to rotate the matrix around and extract it upside down.

The matrix unit is held within a plastic case which is riveted and can be glued on the bottom. Take care to remove the rivets without damaging the plastic too much - you only need to remove one pair of rivets as this will allow the plastic to bend enough to remove the matrix.

Clean out the inside of the plastic case before putting the new matrix inside and either riveting or glueing the case back together.

Re-assemble matrix/blower unit and then screw it back into the chassis - this may need two pairs of hands and a lot of patience due to the location of the screws. A telescopic magnetic device can be useful to retrieve dropped screws! Re-attach the green cylindrical terminal block, re-fit the hoses into the matrix and replace the battery.

It may be easier not refitting the plastic radiator surround just yet as it is easier to get to the radiator bleed screw later on.

Finally add coolant back into the header tank and follow the instructions in Bleed the cooling system.