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A/C HiPressure Switch


System switch A/C system

I was looking for a way to turn the cooling fans on and off, when in use for the A/C system.

I went to an A/C shop and explained what I was doing.

The man at the shop showed me his catalog, which showed a switch that uses hi-side pressure only to operate it.

The switch is from SCS Frigette #211-191.

What I did was to get another relay mount from the wrecking yard from an 87 Olds just like the one that I use to operate the cooling fans.

It is wired as follows:

  1. power from the battery (fused)
  2. Power to fans
  3. Wire to hi-side sensor then to ground
  4. Power wire tapped into the compressor clutch
  5. Not used

The relay has the numbers on the bottom of it

The system switch works by the pressure in the hi-side line.

As the car slows down the pressure starts to rise with reduced air flow through the A/C condenser, then the cooling fans come on to artificially draw more air through the A/C system which lowers the hi-side pressure.

The cycling of the fan causes the hi-side pressure to stay in an acceptable range, which lengthens the life of the compressor. It also causes the A/C system to maintain lower cabin temperatures. The best part about the use of this switch is that it mounts onto the hi-side service port with little or no loss of refrigerant. The reason that this system will work is that when you turn on the A/C system, the compressor is always on when the A/C system is in use.

hi-side line from the factory

switch/sensor as mounted on the hi-side line

  1. Make sure that you look into the threaded end for a valve depressor so that it can read the freon pressure



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