Question about 1995 Mercury Grand Marquis

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Air conditioner fails intermittently

The air conditioner on my 1995 Mercury Grand Marquis cools well, but when the outside temperature is high, it will often stop cooling after about 10 or 15 minutes of operation. It will start working again if I turn the system off for several minutes. I have checked the refrigerant level, and it appears to be normal (about 40 psi on the low-pressure side when operating).

I suspect that a limit switch of some sort is malfunctioning. There is a switch on the low-pressure line and another on the high-pressure line. Replacing both switches myself (if possible) would probably be more economical than taking the car to a shop, where I am likely to be charged $100+ just to check it out before repairs even begin.

Questions: What is causing the problem? What do the two switches do? Can either or both of them be removed and replaced without discharging the refrigerant?

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It is not going to be in the low,or high side switch,the most likely problem is that the high side pressure is getting to high,check for any debris in the condenser,and radiator,and check the fan clutch out,the fan clutch maybe the problem,if it never turns on,or if it quits turning to soon,and does not re-activate,a set of gauges to see the high side reading would help in the is situation.

Posted on Jun 06, 2010

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  • Havaproblem Jun 06, 2010

    Thanks. I had hoped that the problem had a simple solution, but apparently it does not.

    The radiator is clean, but I cannot gain access to the evaporator coil inside the dash.

    I doubt that the compressor or the compressor clutch is the source of the problem because the system cools well when it is working. If I had to guess, I would say--as you suggest--that the high-side pressure is getting too high, but the question is "Why?"

    One other observation: I tried leaving the system on to see whether it would start cooling again, but it did not. It started cooling only after I turned the system off for several minutes. Does this observation have any significance?

  • Ronny Bennett Sr.
    Ronny Bennett Sr. Jun 06, 2010

    Yes they can be replaced by unscrewing them,with out loosing freon,change them if you want to,but this is not the problem,unless the ac compressor is cycling off,if it is ,it would be the low pressure switch,the high side cuts the compressor off also,but only if the high side pressure is too high.

  • Havaproblem Jun 06, 2010

    I think that the compressor is probably cycling off, but I do not know why. The problem occurs only intermittently, and while I am driving, so it is difficult to determine what is causing the problem.

  • Ronny Bennett Sr.
    Ronny Bennett Sr. Jun 06, 2010

    You need a set of ac gauges on the car,and let it run,to see what the pressures do,it the fan clutch,is not pulling enough air threw the condenser,then the high side switch will cut it off,because of the head pressure and the gauges would tell this.Also try wiggling the low pressure switch connector as the ac is running to see if it cuts off.

  • Havaproblem Jun 07, 2010

    OK, thanks. To do what you recommend, I will need to purchase a set of gauges, but given the rates that repair shops charge, they will probably be a relatively modest investment. In the meantime, I can try wiggling the connector to the low-pressure switch. I will keep you posted.

  • Ronny Bennett Sr.
    Ronny Bennett Sr. Jun 07, 2010

    ok just get back with me

  • Havaproblem Jun 08, 2010

    The manager of the local Midas shop, whose advice has been helpful in the past, said that he has seen the pressure switches go bod on Ford vehicles but that he thinks that my problem is being caused by a defective cooling fan clutch. I intend to replace the clutch this weekend.

  • Ronny Bennett Sr.
    Ronny Bennett Sr. Jun 09, 2010

    This is good,if the high side gets too high,like I was saying,the high side pressure switch will cut the ac compressor off,but gauges will confirm this,changing the switches is not the answer,they seem to be doing there job,also,if the orifice tube is stopped up with trash the low cycle switch,will cut the ac compressor off,as low on freon will also.



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