By Jeff Boskowitz
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“This system uses a manual heating/cooling system, but the temperature is controlled by the blend door actuator (BDA), which functions much like an electronic temp control system.”
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In the beautiful winter weather of the northern states (yeah sure, you betcha, we love all those MINUS 20 degree days in Minnesota), the customer's complaint may be “erratic heating” or “no heat.” During the summer, regardless of location (hot is still hot), their complaint changes to “erratic cooling”
or “no cooling.” Weather aside, the problem is essentially the same.
This system uses a manual heating/cooling system, but the blend door actuator (BDA), which functions much like an electronic temp control system, controls the temperature. The BDA has a more precise temperature control than the ones that use a cable (which is prone to binding or slipping) to move the temperature selector.
The BDA controls the heat/cooling by monitoring a feedback signal from the temperature selector and electrically adjusts the blend door by means of an electric motor to satisfy the request. These days, when most manufacturers follow the philosophy that “cheaper equals better,” the components in the BDA have proven to be both durable and effective. As strange as it may sound, if this system has a weakness, it is in its strength. When a technician calls for diagnostic assistance on this problem, the problem always ends up being a broken blend door. Electrically, we have yet to see a component failure.
Diagnosing this system is extremely easy once you get a feel for how it works. This unit is nothing more than a potentiometer.
It has three wires on the rear of the temperature control knob: a reference wire, a signal wire and a ground wire. The reference voltage and ground are provided by the BDA.
To diagnose:
1) Verify thermostat and/or AC compressor operation.
2) Remove the temp control head to gain access to the three wires.
3) With the temperature control head connected and key on, connect one lead of a digital voltmeter to the red/light green wire and the other to the red/white wire to verify the 9-volt reference signal and ground to it.
4) If OK, find a good ground and back-probe the yellow/light green wire to watch the voltage signal change while turning the temperature control knob. This voltage should sweep from approximately 1.5 volts in the full cold position to around 8.5 volts at full hot. If the voltage sweep is correct, the BDA should do its job and drive the door to the appropriate position.
5) If not OK, the problem lies at the heater plenum. There could be a fault in the actuator itself, or a problem inside the heater case. And from our experience, we find it is usually the heater case.
The actuator mounts to the right side of the heater case and has a D-shaped drive from its drive motor that attaches to the blend door. Technicians watching the operation of a functioning system have told us that when the actuator drives the door to either stop, they can physically see the door bow from the torque applied by the motor. This will cause the shaft of the door to eventually fail.
6) At this point, drop the lower panel on the right side of the dash for access. While rotating the temperature knob, if you can hear or feel the motor moving, odds are that the shaft on the blend door is broken and the door will need to be replaced.
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