Why does my car 2 pin ac high pressure switch socket always have 12v on one lead

[Amadneon123]

Hi,

I checked the voltage on the lead of the high pressure switch socket, and it reads 12V on the lead, even with the a/c button and blower off. This causes the compressor to engage all the time and only disengage when the pressure switch sensor trips (which takes a very long time) and im afraid this will damage the compressor. This happened out of the blue.

According to my friend car which is the same model as mine, the high pressure socket shows no voltage reading when a/c and blower is off, and shows 12V only when a/c and blower is on. Unfortunately, my car is not very well documented regarding its a/c and wiring so its hard to diagnose it by myself.

From my observation on the a/c piping, the car only has high pressure switch and no low pressure switch. My question is, why does it shows 12V all the time? Does it have to do something with the a/c switch burning, closing the circuit permanently?

According to this diagram i found on a general a/c manual, the high pressure switch should only get voltage when all the switch before it is closed.

Any help is greatly appreciated.

[Amadneon123]

This diagram

[JohnHere]

No mention of the make, model, year, and engine type/size of your vehicle? So at this point, any suggestions will be guesswork at best. You alluded to previous owner(s), who might have modified the wiring for reasons not known to any of us. I suggest subscribing to Mitchell's for one month (see the "sticky" near the top of this Forum) for your vehicle's wiring diagram, which will help in making sense of this.

[Amadneon123]

Hi John,
Thanks for the reply.
Im the first owner, it's a local market Malaysian made car, Proton Exora and i dont think Mitchell have the wiring schematics to this car. It's impossible to find info regarding this specific car wiring because in my country everyone is not good with tools so they dont troubleshoot themselves and just send the car straight to the service center. I have tried asking other owners of the same car on a local forum, and none of them seems to understand what im talking about. And to add to that, the car manufacturer doesn't provide any schematics at all. Simply said, its not a well documented car.

I need to know how does a car's high pressure switch socket gets 12V, with this information i can further narrow down the scope of troubleshooting myself.

[JohnHere]

According to my friend car which is the same model as mine, the high pressure socket shows no voltage reading when a/c and blower is off, and shows 12V only when a/c and blower is on. Unfortunately, my car is not very well documented regarding its a/c and wiring so its hard to diagnose it by myself.

First, have you verified that the system has the correct refrigerant charge and pressures? If the pressures are off, the PCM could be commanding that circuit to remain energized all the time.

From my observation on the a/c piping, the car only has high pressure switch and no low pressure switch.

It could be that you're looking at a binary switch, which should shut-off the compressor if the high-side pressure gets too high or too low.

My question is, why does it shows 12V all the time? Does it have to do something with the a/c switch burning, closing the circuit permanently? According to this diagram i found on a general a/c manual, the high pressure switch should only get voltage when all the switch before it is closed.

To begin troubleshooting, I would verify that the refrigerant charge and pressures are correct and, if so, go on from there. Can you get at the terminals of the A/C and blower switches to test them, using a VOM, to determine whether the switches open and close properly? One or both of them might be defective. The trouble could also be something else, like the A/C amplifier--or more likely, a low refrigerant charge.

[Amadneon123]

According to my friend car which is the same model as mine, the high pressure socket shows no voltage reading when a/c and blower is off, and shows 12V only when a/c and blower is on. Unfortunately, my car is not very well documented regarding its a/c and wiring so its hard to diagnose it by myself.

First, have you verified that the system has the correct refrigerant charge and pressures? If the pressures are off, the PCM could be commanding that circuit to remain energized all the time.

From my observation on the a/c piping, the car only has high pressure switch and no low pressure switch.

It could be that you're looking at a binary switch, which should shut-off the compressor if the high-side pressure gets too high or too low.

My question is, why does it shows 12V all the time? Does it have to do something with the a/c switch burning, closing the circuit permanently? According to this diagram i found on a general a/c manual, the high pressure switch should only get voltage when all the switch before it is closed.

To begin troubleshooting, I would verify that the refrigerant charge and pressures are correct and, if so, go on from there. Can you get at the terminals of the A/C and blower switches to test them, using a VOM, to determine whether the switches open and close properly? One or both of them might be defective. The trouble could also be something else, like the A/C amplifier--or more likely, a low refrigerant charge.

Sorry, i forgot to address the obvious information first in the post, here's the readings

R-134a
Low side: 43 PSI
High side: 192 PSI

About testing the blower and A/C switches, im facing some difficulties as the switches are enclosed in a plastic housing, and are connected by multi pin sockets. I don't know which pin to test. I think i have to borrow the whole A/C panel from someone and plug it in to confirm if the problem was the switches.

[Amadneon123]

The A/C panel

[Amadneon123]

I just noticed i can open the switches as there are screws there. I'll have to do some testing first. will update soon.

Just to confirm, does the compressor only turn on only if there is request from the switches inside the cabin? Can a faulty ecu cause the compressor to run?

[tbirdtbird]

The wiring diagram you provided shows both a hi and low pressure switch. They may be combined into one switch.
You have not mentioned exactly what behavior you are trying to correct.
Does the compressor come on at all?
If so, does one pipe get hot and the other cold?
Does the evap fan work?
Does it ever blow cold air? If so, what is the temp at the vents.
How old is the vehicle?
etc.
Not enough info
As John has mentioned the most common issue with auto AC is low charge.