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Old 04-02-2002, 10:50 PM
Arthur Dalton Arthur Dalton is offline
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Join Date: Aug 1999
Location: Florida / N.H.
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Which is the norm.
The high side pressure switch has a cut-in point that has to be met in order to turn the low fan on. It is a safety switch.
The design of the cut-in point spec. is to let the a/c do it's job
until there is a demand to keep excessive high side pressure down.
[ air across the condenser does this]. This happens when the ambient temps and humidity are high enough to tax the system.
So, it is quite possible to have normal a/c and NO fan.
An example would be a/c operation on a cool day.
However, if you do have taxing conditions [ hot/humid, etc] and the fan does not run, before condemning the sw ,
[ assuming the fan runs with the jumper test], it is very possible to be JUST short on freon. This shortage will limit the compressors ability to ever reach the pressure switch's cut-in point.
So it is important to check freon level [ either with the high-side eye-sight or better yet, gauges].
But , you say "my a/c seems to work, so I must have freon".
True, but not enought to trip the switch, but still enough to cool on a NORMAL day.
The engine coolant fan [ high speed] kind of works the same way, but is eng. temp sensored. Again, it is a safty device and only comes on when the cooling system is TAXED beyound normal load. [ hill climb, towing, etc.]
I think a lot of the confusion on the fans comes from the fact that the a/c is actually a physical load on the engine and it's operation in itself will cause the engine coolant temp to rise enough to trip the coolant high speed fan. [ around 105-107C].
A fairly good DIY a/c high side pressure test is to let the car idle on a fairly warm day with a/c on and watch for low fan to cut in. This may take a few minutes. If it does not , take a peek at the eye-sight on top of the drier. Lots of bubbles/foam ???= low freon
That's the basics.....
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