I have a 1999 Ford Taurus with climate-control air conditioning and have not been able to get the heater control door to operate. I have replaced the door control motor and it still doesn’t work. Do you have any suggestions as to what might be causing the problem?

­- Ron Francisco, Tulsa, Okla.

It sounds as if your Taurus is equipped with the optional Electronic Automatic Temperature Control (EATC) air conditioning/heating/ventilation system. EATC and systems similar to it have been in use by many manufacturers dating back to the middle 1980s. More recent systems typically share information with other vehicle modules and have scan tool diagnostic capability.

Pressing buttons on the Taurus’ Integrated Control Panel (ICP) results in a network signal being sent to the Remote Climate Control Module (RCCM). This module computes the necessary functions – air door position, blower speed, A/C clutch engagement and temperature blend – to achieve the desired outcome. Commands are then sent to these components to make

Both EATC and the base model air conditioning/heater system use an electronic blend door actuator to control temperature. The blend door directs air through or around the heater core, which is a small radiator-like device deep inside the duct work.

If hot air is desired, all ventilation air is directed through the heater core. If cold air is desired, air is diverted around the heater core. Warm air is created by blending a bit through and around it.

Does your Taurus by any chance blow warm air all the time? If so, Ford service bulletin 02-23-6 calls for replacement of a sticking climate control stratification door, which is a secondary temperature blend door, to remedy the problem.

Now let’s talk about the blend door motor (actuator). This gadget operates much like the control servos found on a radio-controlled model airplane or car, except it is attached to the temperature control door, rather than a rudder, steering system or aileron. The blend door actuator contains a reversible electric motor and a position sensor. If a change in cabin temperature is desired, the RCCM sends a signal, driving the motor in a given direction. When the position indicates the specified position has been reached, the motor is stopped there. During vehicle operation the blend door motor is fairly busy, continually attempting to align actual and desired temperature.

Why might your blend door actuator not operate? It’s possible the RCCM is failing to send the necessary command, due to an internal fault or input error, the wiring between the RCCM and actuator is corrupted, the blend door is stuck, or the actuator is faulty. Diagnosis will require the use of a pro-level scan tool and first-rate (Ford or after-market copies) wiring diagrams and flowcharts.

Fortunately the system has built-in diagnostics, and a helpful trouble code might be lying in wait, providing a path to a solution. There is a silver lining to all this module and scanner business. System reliability is improved, and with the right diagnostic tools, troubleshooting is easier.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose.