How to Test why A/C wont work: 96 GQ

[The Turdinator]

Im over having to have 2 x 100 air con lol.

Have had my car for about 5 months now and the A/C has never worked. You press the button on dash and nothing happens or lights up. I have tested to see if there is power at relay and there is but not sue how they work or what is meant to have power at it , as 2 points have power. all fuses are ok. Someone told me that the motor has a protection on them so that if the gas is low they shut down?Can you test to see if motor starts by shorting it out or test to see if there is gas in the system.
Any help would be appreciated CHEERS

[Bigrig]

Peacemaker did this exact test for me a while back - id reckon he (and probably Yendor) would be able to fill in the blanks for this one mate - maybe send them both a PM ...

[MudRunnerTD]

There is a pressure switch on top of the dryer unit behind the drivers headlight. If there is no gas it won't let the system power up. It is positive only. Discounect and bridge for a second or two only, if the aircon loads up you have no gas. Simple 20 second test.

[BigMav]

Mate have a read of this thread.

http://www.nissanpatrol.com.au/forum...-working-96-GQ

I made a post a couple from the top which explains pretty much all the electrical side of things. If you need to know anything specifically then give us a yell once you have tried everything there. About the protection you are talking about. This is the trinary pressure switch. How it works, there is 3 set pressures (can't remember them off hand) lets just call them low pressure set point, medium pressure set point and high pressure set point. This switch is in your aircon line and is electrically in series with the compressor, and also has a seperate feed to the condensor fan so has two internal switches. When the pressure in the line is below the low pressure set point the switch to the compressor is open therefore no power getting to the compressor, if the pressure at the switch exceeds the low pressure switch point the switch then closes supplying power to the compressor. Now the next bit is a bit tricky and you need to understand a little about refrigeration theory but basically as the temperature of the condensor increases (for example, sitting at the lights with no airflow) the pressure in the line increases at the same time. So the pressure then reaches medium pressure set point while sitting at the lights, this closes the second internal switch which provides a feed to your condensor fan (the small one in front of the radiator) and attempts to cool down the condensor so the pressure at the switch will lower and cut the fan off again (ie, once you are moving again from the lights) . Lastly, if all this fails, lets say the condensor fan is broken and you are sitting at the lights for a long time or in traffic. The pressure at the switch continues to increase until it reaches the high pressure switch point which opens the switch which feeds the compressor. On top of all that the thermostat under your dash in the evaporator switches on and off the compressor according to the temperatures at the evaporator core. Obviously there is a fair bit too the whole system but if your completely stuck let me know and I'll try and point you in the right direction.

[Yendor]

Make sure your interior fan is working correctly and there is a belt on the A/C compressor.

Then I would do the test as per MudRunnerTD post and listen/see if the compressor clutch cuts in.

If it cuts in when you bypass the pressure switch then the system is either fully out of gas your very low and you will need to take it to a licensed A/C tech who will perform a leak test on it.

Also don't leave the pressure switch bypassed for a long period of time, there need to be gas in the A/C system to help circulate the oil, if there is no gas in the system the A/C compressor can run dry.

[BigMav]

Don't think that just because you bypass the switch and it runs you are low on gas, it could be a faulty switch which is quite common on these. Kinda why I went into detail to explain how the switch works. You'll need a set of gauges to diagnose further. Yendor is right though only short the switch out while you are testing. and I wouldn't short it out unless I was certain on how much gas was in it. A/C compressors aren't cheap to replace at all.