I have a 1990 911 Carrera C2. the heater stopped working last week. The A/C works fine at all fan speeds but as the temperature control approaches 27 degrees or higher, the fan shuts off. Hot air will come through the vents if you put your hand up to it, presumably just from the heat exchanger. Turning the temperature dial back down causes the fans to work normally again at all fan speeds. The fuses are ok. It doesn't seem to be a problem with the servos because the driver, passenger, upper and lower vents all work in unison and appropriately at lower temps. What's wrong with my car? I talked with a porsche mechanic and he thought that the most likely possiblity (without seeing my car) was the rear blower motor relay. What is anyone's experience with that and how easy is it to replace on your own? Thanks a lot for any suggestions.

 

The A/C system and heating system on air-cooled 911s are almost entirely distinct, sharing only controls and ductwork in the cabin. The A/C fan is located in the trunk, while the heater fan (a.k.a. engine blower fan) is located in the engine compartment. What you've explained about the A/C fan seems normal, as it should turn off when the heater comes on. Therefore, I'd concentrate on the engine blower fan and related components.

Air circulation around the heat exchangers is from two sources. At high RPMs, the engine fan blows fresh air into the engine cooling duct system, some of which is diverted past the heat exchangers, through the air boxes/valves, and (if the air boxes/valves are in the appropriate position) into the cabin. When the heater is turned on, the engine blower fan boosts air flow through the heat exchangers and into the cabin. This boost is not noticable above 3,500 RPM when the engine fan takes over.

On your vintage of 911, the air boxes/valves (located under the car in the wheel wells) are automated and run by servos. On the older vintages, they are controlled by two red levers located in the cabin immediately behind the emergency brake lever. The fact that you can feel warm air at all probably indicates the air boxes and servos are functioning properly.

That leaves the engine blower fan and controls. Engine blower fans are subjected to relatively high heat, and used intermittently, so are prone to premature failure. To check, test for power at the blower motor connector when the heater is on. Power present at connector = new blower motor required (Pelican quotes $260, and its pretty easy to install).

The rest of the engine blower system is typically trouble free. Having said that, if there no power at the blower connector, check the blower motor relay and fuses. On the older vintages at least, these are located along with the engine management system inside the engine compartment. There is also a switch that controls power to the relay. On older vintages, there are two switches at the base of the red levers (one for each lever, and wired in parallel). Sorry, but I'm not certain of the location or specific wiring on newer vintages.

Anyway, hope that helps.

 

That is brilliant. Thank you. I'm going to check the charge on the connector and the fuses this weekend. Apparently the 1990 911's don't have the red switches or the boxes in the wheel wells. The heat just comes up from the manifold to the front of the car. But that was also enlightening about the separate fans and knowing that it is appropriate that the A/C fan shuts off when the heater is employed. At least something's working as it should!

 

How do you know if the relay needs to be replaced?

 

Relays are interchangeable. To test the heater relay (located inside left rear of engine compartment), replace temporarily with another relay from the main fuse/relay panel in the luggage compartment. If the heater motor works with the new relay, that was the problem.