I was recently reading a posting by Tore from Bergvillfx.com concerning the mercurial workings of the heater blower fan in the engine compartment and found it very interesting:
"The rear fan(s) has two purposes.
- One is to help blowing air through the heat exchangers onward to the front fans to fan improve the heating of the cabin.
- The second function of the rear fan is to do additional cooling of the heat exchangers/engine when in normal operation.
- Low speed if the fan speed knob on the CCU is below 2.
- Fan settings above 2 will set the rear fan speed to high.
The operation of the fan is also dependent on the temperature setting, i.e.:
- if [ the cabin] temp is set to minimum, the rear fan will not start since no heated air is needed to the cabin.
- However, on minimum [cabin] temp setting, the fan could automatically be started by the [engine] cooling function.
In [engine] cooling mode, the rear fan runs at two speeds depending on the reading from the engine compartment air duct temp sensor. Heated air is dumped out through the two rubber spring-loaded dump valves under the engine. On low speed, the airflow may seem to come out of only one of the dump valves."
Also, Tore's website https://www.bergvillfx.com contains a vast treasure trove of information on how the Climate Control Unit (CCU) in the dash controls all the other aspects of the Heating, Ventilating & Air Conditioning system (HVAC).
Also, there are two types of actuators used in the HVAC system:
- Vacuum actuators both in the engine compartment and behind the CCU in the dash, that are known failure items. Look in the middle of this page for more information on servicing them, and;
- Servo-Motors that move the various duct valves in the system, there are five of these. Look here for more information on servicing them.
If you found this page informative return to the main page and bookmark it for future 993 related servicing, repair, and upgrading guidance & information.
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