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A/C will blow hot and cold air


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jkintexas
New User

Jul 29, 2008, 10:02 AM

Post #1 of 2 (1837 views)
A/C will blow hot and cold air Sign In

2002 Ford Thunderbird. 3.5L V8. 51k miles. As the only person in the vehicle, I routinely keep the two vents for the passenger closed so more AC will come to me. I've noticed that the inside vent will begin to blow warm to hot air after about 3 - 45 minutes of interstate driving (or about 50 - 60 miles). If I open one of the passenger vents the heat usually all blows out of that newly opened vent and the two driver vents again blow cool (not cold) air. Adjusting the temperature up or down has no impact.

I have never had the AC serviced. Also I'm not a technical person - just would like to have some idea before taking it to the Ford dealer.

Thanks in advance for your help!


Tom Greenleaf
Ultimate Carjunky / Moderator
Tom Greenleaf profile image

Jul 29, 2008, 1:19 PM

Post #2 of 2 (1821 views)
Re: A/C will blow hot and cold air Sign In

This sounds a lot like evaporator icing. There's a bit too much "residence" time for air going thru evap and what should be water dripping ices up. It still shouldn't do that as it's the same effect as lowing the fan speed.

I'm about sure that's what will be found. It could and probably is just a tad low on refrigerant in conjuction with low side cut out being just a degree or two too low which is adjustable as measured in PSI on low side swich which would confuse most anyone and this one may not be adjustable.

The reason for different temps on opposite side is that when refrigerant is low it has finished cooling in the evaporator which should be all the same temp and the ductwork is just picking up the differences OR the ice is slowly melting away with more air flow - hard to say which.

The odds are the best fix is just to add the specific # of ounces needed and not too much. My guess is 3-4 of the 28oz OE spec (chart if correct) has leaked out over the years which is not unusual in conjunction with the cut out being just ever so slighly off which when charged fully would give you slightly cooler output temps at the risk of what's happening.

A pro shop/tech should handle this without making a big deal of it from what I can see. With pressures taken it will show the flaw which again I think is so minimal it's not worth going nuts over. Basically a "boost" with known pressures should do OR an evacuate and charge measuring ounces out then put back correct charge and the amount out will verify what I think.

Another test for this is to let it do it again - this time use second highest fan setting and it will do it faster and pull over an shut off system and watch for a huge amount of water (plain water) as the ice melts down. Also can just shut system down while driving and let it melt for a few minutes and it will probably come back on cold.

________________

Sorry for the book on this. Bet you are wondering how it could be icing when too low on refrigerant? Reason is it's evaporating completely too early in the loop. Overcharged it would "flood" evaporator and hence blow warmer air all the time then it's best. It needs to be just right,

T







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