Re: [DML] Re: (wasHeat test) Cooling Fans

["G Ryerson" <TRyerson@xxxx>]

Sorry for being so windy here, but that is the way engineers are.

Regarding the installation of a cutoff switch in the cooling fan circuit:

In order to understand why most, if not all, cars are arranged to have the
electric cooling fans on all the time when the air conditioning it is
necessary to consider the following.

The radiator size in a given car is determined by the maximum amount of heat
it will ever need to get rid of: Death Valley on a hot day in July for
example. The water in the engine cooling system can store up a great deal of
heat without changing temperature a lot. This characteristic will take care
of many situations where there is minimal air flowing through the radiator
from the motion of the car. These considerations mean that under almost all
other conditions the radiator is actually much larger than is necessary. The
fans then, are provided to take care cases of extreme cooling needs. To
leave them running all the time would result in excessive current usage,
excessive wear on the fan motor(s), and could result in overcooling the
coolant under certain conditions.

When we add air conditioning into the picture we have a somewhat different
situation. In this case the air conditioning is constantly ( or almost
constantly ) transferring heat from the interior of the car and into the
outside air through a device called the condensor. The AC condensor is
usually mounted in front of the cooling system radiator. This is because
good efficiency of the AC depends on the condensor getting a large amount of
the coolest possible air. Therefore the electric cooling fans are left on at
all times when the AC is on.

Interestingly there will many times be no apparent ill effects if the
electric cooling fan(s) are off when the AC is on. What happens in these
cases though is that the temperature of the AC condensor rises a LOT:
sometimes to as much as 300 degrees fahrenheit or more. What this results in
is VERY high pressures in the high pressure side of the AC system. Some
systems, but not all, have high pressure cutoff switches to prevent damage
in these cases. What you will experience when the high pressure cutoff
operates is that the cold air output will become warm / hot over a period of
30 seconds or so due to the AC compressor being shut down.

In any case it is a bad thing to be running the AC without the electric
cooling fans running.

George Ryerson
Akron Ohio ( Yellow '83 )