Remove the lower dash panel on the side of the instrument panel that houses the Blower Fan Final Stage or fan resistor (See Photo 7). Unplug the resistor pack and check for power from the control head. Most likely, you will find power and ground, and by using a jumper at the proper terminals, the fan can be energized (consult an accurate wiring diagram) which will eliminate the fan motor as the failure.

Inspecting the fan resistor for any indication that the potting material has melted or the resistor wires have pushed out of the housing will probably reveal the failure.

1. 1. Heater does not put out heat. This one can involve a number of different components. Assuming that the cooling system operation has already been confirmed as being OK, start checking the temperature of the hoses at the control valve assembly (See Photo 8). With a dual-zone heater system, the valve will have two valves and three hoses going to the heater core, which is divided into separate sides. Using a non-contact temp probe, the temperature of all of the hoses into or out of the valve should be about the same, with the engine at operating temperature and the heater on full heat.

If there is any difference in temperature between the hoses, then start eliminating components that could affect that temperature difference. As long as the engine is running and the heater is turned on, the auxiliary coolant pump should be running, and the control valves should be oscillating, you can verify this with a stethoscope or some other listening device.

Heater cores can fail on any car, but I would eliminate all other possibilities before going to that point. I had one instance a few years ago, where just before tearing down a BMW to do a heater core, I decided to visually check the operation of the control valves, and found the entire control valve full of some kind of silicone jelly.

1. 1. Air doesn’t change from floor to defrost. Control of the various flaps and doors to channel air from the heater core or evaporator is done by small stepper motors. By checking output signals from the connector at the control head (output signal should change as the buttons are pushed), the problem can be traced to the control head or a mechanical problem, or stepper motor failure.

The flap motors are sometimes difficult to access, but can be easily removed to manually check the operation of a flap or door, or replaced. The biggest problem here is access, as some are buried deep in the wiring and dash components. Just allow sufficient time to diagnose and repair. Removing a flap or door motor to allow manual operation will determine whether the problem will require a dash removal to repair or replace a broken door or flap (See Photo 9). Debris falling down the defrost ducts is the most likely cause of door or flap failure. Nothing else can be done once that happens.

1. 1. Defroster output is insufficient. If you have verified that the operation of the blower fan is good and the flaps are moving, the first place to look for restriction is the cabin or pollen filter. On 5 Series cars, this is easy as they are located in the plenum area under the hood, and can be accessed by releasing the retainer wires and lifting the cover.

On some later model 3 Series cars, the filter will be under a small narrow cover in the center of the plenum area, or there may be two small “micro filters” under the plenum cover at the inlets to the blower fan housing.

On earlier 3 Series cars, the filter is in a housing section between the blower fan and the mixing chamber, in the center of the dash. This filter is accessed through a small cover on the right side, under the dash. The steps to remove this filter are straightforward, even if the location is very difficult to reach.

A. Remove the lower cover plate under the dash on the passenger’s side. You may need to disconnect the under-dash courtesy light if it’s clipped into this panel. On some cars, you will need to remove the glove box to have enough room to lower the relay panel.

B. Unscrew the auxiliary relay panel; usually two vertical screws and one horizontal going toward the firewall. Pull the panel down and as far to the right as possible.

C. Turn the release knob on the filter cover; it’s located on the right side of the climate control system housing just back from the firewall, near where the fan resistor is located. You may need to remove the resistor harness to get the cover out.

D. Grab the tab of the old filter and pull it out. The filter has a hard plastic surround, but it’s notched and will actually fold into three sections to facilitate removal. The new filter can be “cracked” to install, or you can work it into place carefully on most models without bending it far enough to separate the sections. Make sure any large debris is removed from the heater housing before installing the new filter.

I can’t stress enough the need to understand how the various parts of the system work before trying to fix anything. Just like a jigsaw puzzle, it’s easier to finish if you know what it’s supposed to look like in the end.