My daughter's 1999 4.6L F150 stepside has started blowing hot air when the AC is turned on. The compressor seems to cycle on and off ok. The outside temp when I tested it was about 70F, with about 20% relative humidity, and all the vehicle doors were open.
I turned the truck on and let it warm up. I then turned on the AC, and measured the temperature coming out of the panel vent at 120F. All of the knob functions (floor, defrost, panel, etc) cycle fine, but it only blows hot air. Both the large and thinner aluminum tubes running into the firewall were cool. I measured the temp with a remote thermometer, and one of the tubes was about 10 degrees cooler than the other one. I don't remember which was which now, but according to the Haynes manual, the aluminum tube that was supposed to be slightly cooler was the correct one.
A friend hooked up gauges to the high and low pressure sides, and he said they were where they should be. The fuses under the driver's side dash were ok, although I didn't have a way to check the black square relay next to the fuses (Number 5 relay, I think, had something to do with the AC). We put a couple of clamps on the 2 heater hoses under the hood, and the blowing air temperature dropped maybe 5 or 10 degrees. After shutting the truck off, I could hear the normal pressure release noise under the hood for a miunte or so. Any help would be greatly appreciated. Scott.
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Monday, May 4th, 2009 AT 12:14 PM
