A/C high pressure switch question

[lindbergh]

I overhauled by A/C system and replaced everything and switched over to R134a. With my A/C on and not moving my fans unnoticed by me failed. The high pressure switch did not turn the compressor off and the high pressure port opened evacuating the freon. Since the high pressure and low pressure switch are new with the A/C overhaul, any ideas why the pressure switch failed? I am kinda doubting I got a defective one, is there any way to test? Thanks.

[Jonathan]

When you said "your fans unnoticed by me failed" are you referring to your cooling/radiator fans up front? If your cooling fans did not come on but your A/C compressor clutch engaged, then something is wrong with the fans circuitry (among other things).

If your cooling fans are not coming on when the A/C compressor clutch kicks in, then this needs to be addressed (first, second, or at some point before the system as a whole will work properly).

You can test your cooling fans as to whether they are coming on when they should in a couple different ways. You can jumper the connectors to the otterstat and this will trick the car into thinking the coolant is hot and should turn the fans on. The other way is to just turn the HVAC control knob to MAX A/C. Normally you will hear the cooling fans turn on when you do this and the compressor clutch clack in. This will not happen if you are low on Freon though, so you may need to jumper the connectors to the low pressure sensor up at the front right wheel. This particular test you can do with the car off but the key turned to accessories. This is important because you don't want to run the compressor with low Freon and you don't want to run it without the high pressure sensor shutting it off either.

I've not experimented with the high pressure cut off switch/sensor myself, but my assumption would be that you could also jumper those connectors to simulate a high pressure condition and see if the clutch disengages when you do it (with the low pressure sensor jumpered if necessary).

As far as I know there is not a direct connection between cooling fans and compressor high pressure shut off. It's two separate fluid loops.

This may be a long shot, but we used to have a term called FME in the nuclear industry. It stood for "foreign material exclusion." Simply put, don't let any foreign material like a rubber glove or wrench get falling into a pipe or vessel, etc. It also meant to ensure the shipping or packaging caps are taken off the piping connections or instrument ports when doing maintenance or installing new equipment. Any chance on this new A/C compressor that the end of your high pressure sensor/switch has the plastic cap still on it? And thus blocking the sensor from reading the pressure?

[Bitsyncmaster]

It's my understanding the high and low pressure switches are wired in series. So the high pressure switch would normally be closed until you hit the high pressure setting. I don't know the specification of the switch or the port but would think the switch should open well below the port blowing. My guess would be around 350 PSI for the switch and 450 to 500 PSI for the port

[DMCMW Dave]

I'd suspect a wiring error. The high pressure switch was a later factory add-on, and it moved once (early cars don't have it at all, most cars have it mounted near the receiver/dryer, very late cars have it mounted at the lower left end of the condenser). Yes the two switches should be wired in series so that very low or very high pressure shuts off the compressor.

[lindbergh]

The high pressure switch is normally closed and opens on high pressure. I disconnected the plug from the high pressure switch simulating a high pressure condition and the compressor switched off as it should. So the circuit appears good so either the switch is bad or is there something else I am missing? Thanks.

[Jonathan]

The high pressure switch is normally closed and opens on high pressure. I disconnected the plug from the high pressure switch simulating a high pressure condition and the compressor switched off as it should. So the circuit appears good so either the switch is bad or is there something else I am missing? Thanks.

Would you be able to post a photo or two of the switch and area around the compressor and wiring to it?

[David T]

Would you be able to post a photo or two of the switch and area around the compressor and wiring to it?

If the fans were not running while the A/C compressor is engaged then the high side pressures will get very high. High enough that it *should* trip the high pressure switch BEFORE you pop the safety pressure release. If the high pressure switch and the pressure relief valve are not coordinated the pressure relief can blow before the safety switch can shut the compressor off. Another cause is if the system is overfilled with refrigerant or if the orifice tube gets plugged up. Keep in mind the switches and valving was not set up for -134. -134 runs at higher pressures and you must not use as much as -12 in weight.