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Intermittent problem

pomelo on Sun April 04, 2010 11:45 PM User is offline

Year: 1994
Make: Nissan
Model: Sentra
Refrigerant Type: 134a

I am having an intermittent problem with my a/c. It seems to run fine every once and a while, but I feel room temperature air some times. It also seems to take longer to cool. What controls the compressor cycling on my vehicle? Does it have a low pressure cycling switch or a different control? If it is the latter, is there anyway to test if it? From the research I have done, it seems like it could be an air gap in the clutch, but I haven't been able to stop the car when it isn't working to check if the clutch is engaged. What testing procedures should I do?

mk378 on Mon April 05, 2010 12:10 AM User is offline

When it's an intermittent problem, you can only do meaningful testing while the system is not working. Japanese economy cars almost all have TXV systems that cycle on evaporator temperature, not system pressure. So there is a temperature sensor or switch involved, but the clutch gap is the more common problem. If you can reach the compressor easily go ahead and try reducing the gap some and maybe the problem will just go away.

pomelo on Mon April 05, 2010 9:57 PM User is offline

I can get it to not work to work regularly now. I tried tapping the pulley when it wasn't working to no avail. It does have a thermo control device in the evaporator. What should I do as far as testing? The compressor shuts off after it runs for a period of time. Here are the things I think could be keeping it off:

-Thermal cutout at the compressor
-Thermo control
-dual pressure control

I tried jumping out the harness at the dual pressure control and it did not run. I didn't know the control went back to the thermo control. I wouldn't have jumped it out if I knew. I'm still not familiar with this type of device. I don't think it is the pressure control, however.

I'm leaning towards the thermal cutout at the compressor. It is kind of tight at the compressor. I wouldn't want to stick my hand in there to do some tests with the engine on. How can I test the thermal cutoff switch that is attached to the compressor? I guess I could test for 12 volts at the harness. It seems like it is a dual wire harness. I'm not sure what the reasoning is behind this. I thought most just ran one 12 volt wire and the ground was obtained near the compressor to complete the circuit. I could check to see if I'm getting 12 volts at the compressor. If so, then it is more than likely the thermal overload. I could also pull the a/c relay and check to see if the computer is sending voltage to the a/c relay coil. I could jump it out directly at the relay and see if the compressor comes on.

If I'm not getting 12volts to the a/c relay coil, it seems like it would have to be the thermo control or dual pressure switch. Does anyone know how to test the thermo control?

What steps would you take in troubleshooting the problem? What would you do first, second, etc?

Any help would be greatly appreciated.

Also, if it is the thermal cutout at the compressor, what are the possible causes of it opening? I know it could be just faulty, but it does seem like it would be opening and closing. Therefore, it is opening and closing on temperature. I just don't know if it is opening at too low of a temperature. What would cause a compressor to overheat?

pomelo on Mon April 05, 2010 10:02 PM User is offline

I didn't think about asking, but does the thermal cutout at the compressor automatically reset?

TRB on Mon April 05, 2010 10:51 PM User is offlineView users profile

Are you sure it has the proper amount of refrigerant? I would not be jumping he different electrical connections.

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pomelo on Tue April 06, 2010 8:42 AM User is offline

Yeah, I regretted jumping out the dual pressure switch. I looked at the wiring diagram and it does just have open and close contacts. The compressor clutch should have engaged when I jumped out the pressure switch if it was a refrigerant issue. I will check for continuity when the clutch isn't engaging today. I'm heavily leaning towards the thermo control amplifier. I think it shuts off the compressor when the temperature in the evaporator drops past a certain temperature and then it never cycles back closes again. It seems pretty difficult to check the voltage at the compressor. I'm probably going to check to see if the ecm is sending power to the a/c relay coil. If it isn't, I know that it isn't the thermal protector. By process of elimination, it would have to be the thermal control amplifier. I didn't know 94 vehicles used these types of controls. I thought it would just have a basic cycling switch.


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