I have a 1999 F-150, 4.2 6cyl. Haven't used my a/c for two years. I 'm no mechanic especially on these newer vehicles so I charged the system today with a diy kit. The chincy meter showed 35 psi. Everythig was fine and blowing cold for about 5 minutes then started blowing hot in an instant. The compressor appears to be running and the metal line with the Low inlet stem valve is cold to the touch so there is refrigerant in there. It's just not blowining cold in the cab.

Any ideas where the fault may be and how I troubleshoot it?

Thanks very much in advance.

 

Go to the HVAC section in the forum and see the FAQs. You'll find information there to get you oriented.

 

Thanks Poject. I looked in the hvac faq but didn't see anything listed that seemed to help.

 

I pulled the cab's control panel and the knbos are not broken. I Don't think I touched the controls when it switched from cold air to hot and I was on driving on flat ground so I doubt the problem is a stuck or flappy blend door.

 

If my ac clutch is slipping, would I get this phenomenon? If so, would I not hear it (which I don't) when I have the hood open or see the belt slip periodcially (which I don't)?

 

Cold to the touch or dripping water from it? Have you check the levels again since it began blowing warm?

 

Just cold to the touch. Nothing dripping. I checked the oressure again and got a ~ 30-32 psi reading. Not 100% sure I can trust the gauge though.

 

Cold to the touch is one thing. The refrigerant properly saturating and thus removing or condensing the vapor in air molecules, when it drips, is the best indication of a proper charge.

Try adding just a little bit more refrigerant, see if the air cools down. Get it back to 35 psig, (gauge). If it does then you've pretty much narrowed down your problem to a leak.

 

That woud be a pretty fast leak. I'll get a better gauge and top up with some more 134a. Thanks.

 

Tonight I ran her and at first she was blowing warm but after cruising a litte whie she started to blow cool but not cold. I checked the low pressure line again and it was sweating pretty good in the evening air. I'll top up again tomorrow morning.

 

If you over-charge it won't cool properly either.

 

Correct. Can also cause floodback which will inevitably kill the compressor.

An accumulator would protect the compressor in this instance.

 

Sounds like a bad blend door in the dash to me.

 

She sat over night and it appears to be working today although it isn't super hot out today-maybe 80F. The pressure reading is 40 psi so I chose not to add any more 134a. I was getting 35 psi from the the gauge on the A/C Pro can. I suspect the can's gauge was off calibration.

I have a theory what went wrong though. I was reading a website yesterday on charging an ac system. It adivsed to invert the refrigerant can upside down to drain some liquid into the system if the pressure is "low, low" whatever that means. Since I started with 0 psi, I followed the advice. I suspect that was the problem. The compression quit when it hit the pocket of liquid. Over night the liquid may have turned gaseous so it works today. That's my logic any way. Am I all wet or am I on to something? Another theory is moisture in the system from non use these last two years.

 

Close. The compressor changes the pressure and the temperature of the refrigerant while it's in vapor, not liquid, form. Technical term for a compressor is "vapor pump."

Low pressure low temperature vapor travels to the compressor in a larger diameter tube. The compressor then squeezes that vapor into a much smaller tube, now under very high pressure and much hotter, and now travelling through the condenser.

With air flowing over these tubes in the condenser, cooling it off, the high temperature high pressure gaseous refrigerant condenses into a liquid, then is shot to the evaporator where with the assistance of the blower fan the refrigerant becomes saturated as it extracts heat from the air flowing over the tubes of the evaporator, saturated being in both vapor and liquid state, the fan cooling the refrigerant enough for it to eventually fully change state. By the time it leaves the evaporator it is full vapor or should be, on it's way back to the compressor to begin the cycle all over again as a low pressure low temperature vapor.

In other words, without the system running there is no change of state. Only a few refrigerants, those with POE (synthetic blend) oil, can we charge a system with refrigerant in liquid state, bottle upside down. In residential that would be the new R410-A. Try that with R-22 for any length of time and the compressor will die.


Also, there could be air in the lines. Glad it's working, though.