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post #2 of (permalink) Old 04-10-2014, 04:44 PM
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Location: Delray Beach FL
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The two cooling fans are fitted beneath the radiator or (with a.c.) condenser/radiator package, and the coolant re-circulation pump is mounted below the header tank. Both the fans and pump are controlled by the engine management ECU using data provided by the engine coolant temperature sensor mounted in the back of the cylinder head.
The cooling fans are switched as a pair, and will operate at half speed (connected in series) when coolant temperature reaches 208f on rise, and switch off at 201f on fall.
The fans will also operate at half speed irrespective of coolant temperature if the a.c. is switched on and the compressor is running.
If coolant temperature rises to 217f, the fans will switch to full speed (connected in parallel), reverting to half speed at 208f.
The fans will also run at half speed, unless high coolant temperature dictates otherwise, when the a.c. is switched on and the compressor is running, or if the engine management system detects a fault with the inlet air temperature or coolant temperature circuits.
At road speeds in excess of 85 mph (135 km/h), equating to the fan stall speed, all fan functions are switched off.

A trinary switch fitted into the top of the receiver-drier senses the pressure of refrigerant and allows
system operation only within a pressure range of 2 to 32 bar (29 464 psi) in order to prevent system damage from too high
a pressure, or from compressor oil starvation damage caused by too low a pressure. A third switching point is
used to engage the two condenser fans at half speed at pressures over 17.5 bar (see also sub-section KH.5).
An additional safeguard is provided in the form of a high pressure relief valve in the condenser inlet pipe, which
opens at 38 - 41 bar.

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