How do you test the AC low pressure sensor?

[greggbrat]

The Air Conditioning on my 87 RX is not working. Here is what I have done/check thus far:

 

1. Checked the freon pressure. Pressure is good. Can also see in the receiver/dryer look glass and see bubbles.

 

2. checked the A/C fuse

 

3. Checked for proper resistance on the AC Relay

 

4. Hot wired the A/C Compressor to make sure it was working (and it blew cold air when it was hotwired)

 

5. The auxillary A/C fan (ie. the extra cooling fan on the radiator) does not come on.....even with the compressor hot wired)

 

I am thinking the low pressure sensor on the receiver/dryer might be bad. How do I test it? Is there a proper voltage/resistance that should be seen from the low pressure sensor? Can the low pressure sensor be replaced without evacuating the freon? (note - The low pressure sensor has 4 wires)

 

Does anyone have any other ideas on why the compressor is not getting voltage?

 

Thanks,

Gregg

[subaruloverbrad]

it would have to be vented. that much i do know. in order to remove it. my 82 repair manual tell a little, but dont know if it would help you much. as for sub fan there are a few inline fuses, and a relay that should be checked,as for location, im sorry for i do not know. It is a high current device and i have had to replace my relay a couple of times. my book also says on auto trans models there is a water temp sensor also that is connected to the air conditioning clutch circuit. remember i am going by a ea81 book,yours will most likely be the same, but maybe not. wish i could help you more

[greggbrat]

looking at the dryer/receiver it appears the system would have to be vented to change it out. But I still cant tell if its bad. And why are there 4 wires? I check the resistance on the two pairs..........one pair is open to the pressure switch and the other pair is closed. I tried jumping them but to no avail. I need to know what the correct resistance is before I can determine if it is bad. The wiring schematic was no help.

[RedLance]

Well, I am trying to remember how my XT is set up. I know there is more then one switch that has to work for the compressor to run. There is the low pressure switch, which is only there to keep the compressor from running with no freon in the system. No freon in the system, no oil gets to the compressor, bye bye compressor.

 

 

 

There is also an over pressure switch, in the low side line, and if it is faulty, it won't work either.

 

 

 

Now, since your one switch has 4 wires, it might be these 2 switches combined. That would explaine why one set of wires is shorted, and the other is open.

 

Now, take all this with some salt, since I'm doing it all from memory, but on my crown vic, you can unscrew the switch, because it has a shrader valve(tire stem valve) under it. I haven't replaced any of the Subaru switches before, so I don't know for sure if they have the valves or not.

 

I hope some of that helps.

 

RedLance

[greggbrat]

I dont think the switch has a shrader valve under it. If you were to look at the switch on my 87 RX you would see what I mean.

 

I am guessing there are 3 switches total. The one on the accumulator/drier acts as a dual switch for the low pressure side of the system. It checks for low pressure and pressure that is too high.

 

There is also another switch built into the compressor that is checking for pressure that is too high on the "high" side of the AC system.

 

But I need details, details, details to go any further. I cant exactly trace the wires through the wiring harness and the chilton/haynes wiring diagrams are useless.

[NoahDL88]

Pull the plug on it, the low pressure sensor. and then jump the connection with a paper clip, if you have the fittings to test pressure check to see for pressure, if you don't have a pressure gauge, just check to see how cold the air is coming out of the vents on a 2 or 3 setting, not 4, it should be around 40-50, anything less than 50 is good. by jumping the sensor, or the wires to it rather, you can see if all the other components are working. the subaru in question unless its a loyale will have R-12 which is about 15 bucks an ounce, rather than the new R-134a which is about 2 dollars a pound.

[greggbrat]

Noah,

 

Its a 4 wire connector going to the pressure switch on the receiver/dryer. I tried jumping it every which way but it didnt seem to have any effect. Its pretty much impossible to tell why there are 4 wires......I am only guessing that there is both a high and low switch in the single unit. I need details......details, details, details!!!!!

 

I found some R12 replacement freon for about $4 a can (which is equivalent to a pound of R12).......not too shabby. According to the EPA they can not be mixed......but I wont be telling the EPA what I am doing.

 

I already checked my pressure and its perfect. I dont have a temp guage but the A/C was blowing wonderfully cold. I just cant get the compressor to come on unless I jump it with 12 volts.

 

Does anyone have knowledge of teh

[subiemech85]

on my 85, disconnect one of the two wires on the hi? line in front, and the cooling fan turns on with key turned to on position it has 2 spade connectors

there's also another switch that has 2 red wires with a boot and round connectors

on the compressor, there is the cycle checker, the clutch switch, and about 3 wires in a barrel

[RedLance]

If you haven't already used that R12 replacment stuff...I'd advise against it. I was shown a hose from a system with some sort of "drop in replacement" and the hose looked like a fire had run though it. It was all sorts of messed up on the inside.

 

 

So...just make damn sure you know what your putting into it. Some of them are mostly propane too...something I'm not sure I want in my A/C system.

 

On mine, I went ahead and had new hoses made with barrier rubber, and flushed all the components and refilled it with R134a...seems to work ok.

 

RedLance