Mercury Sable has Screwy A/C

[lawnmowerman]

Seems my A/C blows room temperature air and will occasionally blow cool air. Sometimes it blows cool air, sometimes it doesn't. I have looked at forums, had a friend look at it, and we are both annoyed. What could be the problem? I don't believe it is electrical or fluid wise since i have checked all connections and no leaks seem apparent anywhere. The compressor engages when the A/C is on. The sensors, relays, and fuses have all been checked multiple times. Any ideas guys?

[umina]

are you sure your system is topped off with R134a (if you have a 96 or newer). I had similiar issue, cured by adding 1 and 3/4 cans of R134. The air gets cold when the compressor kicks in, then when the clutch stops spinning, you get not so cold air. Mine was cycling on and off very frequently to blow cold air. Get the pressure tested with the air on and then you will know if you have a leak, or just need fluid. R134a has smaller molecular size than R12, so all R134a systems will leak, its just a matter of how much and how long it will take before the system needs a recharge. Some cars have tighter systems than others, but this is totally normal.

[lawnmowerman]

I'll give that a try. Hopefully that is the problem and that i don't have a serious leak somewhere.

[lawnmowerman]

I apparently have an electrical problem between the compressor and the PCM with my A/C. For some reason, other than pressure, it will not turn it on correctly. Therefore, rather than messing around with that, i intend to go around it. i'm thinking about installer a simple microcontroller to control the clutch so the compressor will engage when i want it to. however, i do not know how often to engage the clutch. Does anyone know a typical A/C clutch cycle in a specific time interval i know this is pretty open ended but under normal operating conditions is there a specific on-off type deal going on?

[rrclyoung]

the manual has the cycling rate.......it depends on outside temperature and system charge.......I will post it when I get home to take a look at the manual...but I think a good Idea is to fix it the right way and not to do a "work Around" it may be low charge causing the quick cycling.

[reekor]

Place a gauge on the low side of the A/C. If it's not coming on or shutting off when the pressure is or falls below 25psi everything is normal and you just need a recharge. Your PCM does not control your A/C unit, it is turned on and off via pressure switch. If your A/C is working at times and not, it sounds like a low charge and your system is falling below 25psi. Refrigerant expands with heat, the hotter the day is the more pressure your system will have and this might explain why on some day’s its working. When your compressor engages it cools the refrigerant and the pressure falls.

Originally Posted by lawnmowerman

I apparently have an electrical problem between the compressor and the PCM with my A/C. For some reason, other than pressure, it will not turn it on correctly. Therefore, rather than messing around with that, i intend to go around it. i'm thinking about installer a simple microcontroller to control the clutch so the compressor will engage when i want it to. however, i do not know how often to engage the clutch. Does anyone know a typical A/C clutch cycle in a specific time interval i know this is pretty open ended but under normal operating conditions is there a specific on-off type deal going on?

[rrzimmer]

Replace your a/c relay in your engine compartment. You can get one from any parts store for about $6.00. It only take a minute to change.