Air conditioning issue


#1

I have 97 Mitsubishi Galant. I have a problem when I run the AC. It runs for a while, 10-15 minutes then the trouble starts. The immediate symptom is that the rpms drop by a notch or two and the power decreases momentarily. This happens about every 5 seconds or so. The AC gets progressively weaker until it blows lukewarm air. I’ve had some work done recently- The alternator was replaced, the thermostat was replaced and a leak in the AC was repaired. The AC is fully charged.



I opened the hood to see if I could find anything. I checked the belt for the compressor and it was taught. I discovered that the rpm dip coincides with the fan blowing harder. First there is a click then fan speeds up then slows down. It does this every time the rpm drop happens.



Any ideas?


#2

Don’t be too sure that the A/C is fully charged.

From what you describe it sounds like the A/C compressor is cycling on and off. If it is cycling on and off fairly quickly then the system is low on charge.

How long ago was the A/C leak fixed? You should bring it back to have them recheck the system.


#3

Makes sense- I’ll get it checked- Thanks


#4

Not only is willey probably dead-on about the system being incompletely charged and leaking…

But there’s probably been some air and moisture introduced into this sytem. Either through a leak or due to incomplete evacuation when you had the system worked on. A small amount of moisture seems to be building up and freezing slowly, eventually causing a restriction between the evaporator and the compressor.

The click you’re hearing, associated with the RPM drop and fan activation, is the compressor clutch cycling on and off. Normally on this type of A/C system this would just indicate a low charge. But if the system is operating for 10-15 minutes and blowing cold air before it begins of have problems, I would suspect a moisture problem. As the system pressures stabilize and the low side gets to its coldest, moisture that has been trapped within the system, circulating through the system, freezes in the cold refrigerant, and becomes ice. This ice begins to clump together, eventually forming a restriction. This restriction causes a noticible drop in cooling ability, and will eventually trip the dual pressure switch on the receiver and cause the system to turn off. After a moment, the pressure equalizes enough for the compressor to turn back on. The pressure changes sharpy again, and the compressor turns off. The dual pressure switch also turns the fan on and off.

An A/C shop with the proper test gauges will be able to confirm this diagnosis, or provide another. In any case, it needs to become their problem now. Whoever did the work on your A/C may very likely have fouled up. When you take the car back and the shop gives you a diagnosis, post it back here before making any decisions (if they’re costly) so we can see if the shop is just trying to cover it’s own tail.

-Matt