944 HVAC Operation and Malfunctions
Explains the operation of early and late-model 944 HVAC systems, and covers the most common malfunctions: full heat at all temperature settings and intermittent heat.
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HVAC Operation
Early 944s (Pre-1985.5) and 924S
The early 944 has separate heating and A/C systems that supply air flow to a common mixing chamber. Each system has its own housing and blower. Air flow through the system is controlled by flappers operated by Bowden cables. The heater control valve is also controlled by a cable. When problems occur with heat output — either no heat or excessive heat — the cause is usually a broken or disconnected cable.
Late-Model 944s (1985.5 and Newer)
On late-model 944s, the A/C evaporator and heater core are contained in the same housing. Temperature control is automatic, governed by three temperature sensors. Once a temperature is selected, it is maintained automatically by the temperature control unit (TCU), which repositions dampers to control air flow through the unit. One sensor monitors outside air temperature. All cabin air flows first through the A/C evaporator, then is directed either through the heater core or past it (via flappers) and into the cabin.
The photos below show the late-model 944 HVAC component locations.
Full Heat at All Temperature Settings — Pre-1985.5 944s
On early 944s, a full-heat condition is unusual but does occur. It is typically caused by the Bowden cable for the heater unit damper coming loose. Locate the Bowden cables on the back of the heater controls in the centre console and follow them to the relevant damper. There should be three cables: one for the heater unit damper, one for the A/C unit damper, and one for the heater control valve (which passes through the firewall). The heater unit damper is behind the centre console; the A/C unit damper is on the passenger's side. Trace the cable for the heater unit damper and look for a disconnected or broken cable.
Full Heat at All Temperature Settings — 1985.5 and Newer 944s
Several problems can cause a permanent full-heat condition on late-model 944s.
1. Damper linkage failure (most common)
The first and most common problem is a failed damper linkage in the driver's footwell (left-hand-drive cars). Stick your head into the footwell and look up toward the centre console. You should see a small-diameter metal rod resting in a white plastic support, held in place by a metal retaining clip. On later 944s, two damper linkages are present instead of one, and they are located behind a plastic cover. If a plastic clip on the linkage is broken, the linkage cannot operate the damper, causing the system to fail to full heat. German cars are generally designed to fail to full heat — a deliberate safety measure to prevent the occupants from freezing in winter.
The first photo below shows a linkage that has come loose from its support because the metal clip is broken. The second shows the linkage correctly seated with a new support and clip.
2. Heater control valve or vacuum line fault
The heater control valve is vacuum-operated and located at the rear of the engine near the firewall. A vacuum line runs from the valve to a connection at the firewall where the heater hoses penetrate. Check this line for correct connection and signs of cracking.
Near the heater hose connections there is another vacuum fitting connected to a rubber "Y" connector. One branch goes to a small check valve attached to the large brake-booster vacuum line at the rear of the firewall. The other branch passes through the firewall to a plastic vacuum retention canister next to the battery. Inspect all these lines for connection and cracking.
If all vacuum lines are intact, remove the heater control valve and test it with a vacuum tester.
A failed vacuum solenoid valve can also prevent the heater control valve from opening. Remove the solenoid and apply 12 VDC to verify it opens and closes properly. The solenoid is visible in the HVAC unit right-side photograph above.
3. Temperature Control Unit (TCU) failure
A failed TCU can cause a full-heat condition. The TCU is expensive and difficult to test without specialised equipment — replace it only as a last resort after all other causes have been ruled out.
Intermittent Heat (Heat Drops at Idle)
If the heater works normally while driving but the air from the vents turns cold at idle, air in the coolant system is the most likely cause. Air pockets in the system often accumulate in the heater core. At higher engine speeds the water pump develops enough head pressure to push coolant through the core; at idle the air pocket blocks flow.
Vent the coolant system using the COOL-02, Coolant System Draining, Filling, and Venting procedure.