In most A/C sytems, a pressure switch is used to cycle the fan on or off. There are too many different control systems for "one diagnostic fits all". For example, some systems have two rediator fans, others have only one.

However, you have probably answered your question: Over-filled the system. Under normal operation, when the overall pressure drops below a certain value, this signals that the system is getting too cold and the fan and/or compressor clutch is deactivated.

When the pressure rises, this indicates that the refrigerant is gettting warm and needs to be cooled, hence, the fan is turned on. If the system is overfilled, the pressure is too high at all times, and the pressure switch in on all of the time. Futhermore, if the pressure switch got overpressured, it may have been ruined. These are diaphragm operated, and these can be bent out of shape.

So the first step is to fill to the proper amount and see if that fixes the problem. If that does not work, then the system requires a complete diagnostics to chase down the problem.

Over filling can damage the compressor as well. The compressor must never recieve liquid. The incoming flow should be a gas. Liquid in the compression chamber can cause hydraulic lock.

thanks for the info. after i overcharged the system, i had to add another can of r-134a over the summer. i have a small leak somewhere. when the system gets low i can hear a hissing sound in the vents and the performance starts to drop. i have had this problem for 3yrs now.
last summer the compressor seized and I got lucky and just replaced it with a used compressor and added another can of freon and it seemed to work fine. i still have a leak though. i thought perhaps the original compressor was the cause of the leak but it does not seem that way.
i am fairly sure the system used to cycle on/off even with the "bad" transducer. lately it runs all the time. do i want to spend $$ to have it diagnosed? a $90 transducer might be a cheaper way to start.

On one hand I don't even want to respond to this thread, but really I have to. Where all all of the enviorment friendly posters? Is it OK for an individual to waste refrigerant, and dump it to the atmosphere year after year?

People who are not completely trained and equipped to do AC (refrigeration) service should NOT be trying to do it. We had to reduce the useage of R12 because of this exact kind of problem, and the EPA restricted purchase to only those trained and equipped to properlly service the systems. They should have done the same thing with R134. It's a greenhouse gas, that will contribute to global warming just like carbon dioxide will.

First and foremost, your AC system has a leak. You need to find and repair that leak. The coolant fan operation is controlled by a switch (sensor/transduecer) on the high side, and is pressure(temperature) related. With gages hooked up, and the correct scan tool to monitor the inputs to the PCM, (or AC control head depending on the exact system in your car) a technician can easily determine why the fans are not cycling, or if they in fact should not be depending on conditions. In fact, many of the systems have two speed fans, and the operation of the different fan speeds will vary because of ambient temperatures and system heat loads.

yes, i know i am a lazy sob. replacing the transducer will not fix the leak. since the transducer is failed to a point that the fans always run, i think that might also prevent the compressor from cycling. maybe thats why the compressor seized last summer. i did look at the new transducer at the dealer, it has 3 wires with splices so you can hook it up to several yrs models. i pulled a used 1 off a junkyard car and the plug was different than on my car. figures. nothing is easy.

Your compressor may have seized if it was designed for R12, and the R12 was replaced with R134a.

Also, R134a has a different pressure-temperature chart, therefore the pressure/temperature controls should be re-selected for a particular refrigerant

John G is correct. If you want this system to work, first fix the leak and then take it to an expert versed in converting an R12 system to R134a. The expert may be able to find the leak faster than you can. Don't surprised if his estimate is seemingly high.

The '96 Dodge is 10 to 11 years old. You may wish to reassess if the repair is worth it. But please, do not keep pouring more refrigerant into a leaky system.

the system is a R134a as a bit of info so it has not been retrofitted. not sure how the discusion moved from R12 vs R134a. when I overcharged it several yrs ago, I got a big puff of gas close to the compressor. I read that the compressor has a blow off valve so I assume thats what popped. the car has 2 belts. a sepentine and a v-belt for the a/c so it would be easy to bypass the compressor if it ever seized.

The very reason people that don't know what they'er doing
shouldn't be playing with an A.C. system in a car. Any A.C. for that matter. You blew open the safety valve on the
compressor. That proves my point. That big Puff of gas was
not gas but compressor oil mixed with a LITTLE bit of gas.
Your old compressor probably seized from lack of oil as it leaked away. The new one will do the same untill you get the leak fixed. Was the replacement compressor contaminated ? Was the oil changed ? The hissing sound is flash gas in the evap. due to a low charge. Charge yourself $75.00 an hour for your labour as you play around with your car tring to fix it, then after the third day find out what it would cost you to send it out and have it fixed in a few hours. How much money did you lose ? We are not allowed to touch any freon including 134 unless certified. good luck.

Indeed, inpart of driving are car issues. That's why im here. By the way, i give my apology for reviving this thread. Just wanna share my previous experience and hopefully, i could get response from here. Ok, this is my current dilemma, I have a 4X4 car of some japanese make model year 2000 and recently I started having some serious problems with the AC. I took it to some 3 electrical shops and they all told me the same story that I have to change the compressor. So I decided to take it to the dealer who, after about 5 days, gave a list of things that require changing from compressor, to condenser, to evaprator , to piping and some seals at a total cost of 1300 OMR . Now, I am thinking of taking the car back to the car electrical shops, but I do not know any one who does this kind of work and who is trust worthy. I am also considering changing the whole AC system with a generic type of AC system such as AC delco, bosch .. etc but I do not know who can do that. Hope someone here share his ideas...thanks

Source i've already visited: South Dakota Auto repair