A/C question

[syn-]

Just replaced the hvac controls on my 97, the air starts out cold then after about 10 maybe 15 min it gets hot (like engine heat hot). I haven't used the a/c in noticeable amount of months (I want to say 4 to possible 6). Why does this happen? Do I need to recharge my freon?

 

[syn-]

Ok, so it does not take 10-15 min its more like 5 or less now.

 

[budwich]

well first you need to ensure that your compressor is running and "fast cycling" (low freon). Then, if all is "good", it is likely you have a blender door issue... lots of posts, common problem.

 

[syn-]

my compressor is cycling, I can hear it cut on and off, and see the change in rpm's. There is no cold air coming out any more, but it did once I replaced the controls.

 

[budwich]

sorry for the wording on the previous post.... IF it is "fast cycling", then it is low on freon. The "blender door" is a common issue. If you change out the controls, make sure your vacuum connections are OK.

 

[syn-]

Not sure what the case might be here. I bought some 1394a to see if I can charge the system up, but from what I can tell it looks to be full. the low side where the a/c recharge adapter is supposed to go on is a hose or is it metal? Also, if the compressor is kicking on and off quickly, is this fast cycling?

 

[Evil Eye]

Yes, that is fast cycling. It means you are low on freon. You probably have air in the lines and/or a small leak.

 

[syn-]

here is the kicker it will blow cool air, but once the truck is in motion it makes a thud near the glove box (as if the vent was suddenly closed by something) and its straight hot air with engine heat.

 

[daffy]

That sounds more like a hvac control problem then, maybe a vacuum problem.

That sound (if my guess is correct) is the blend door closing, thus throwing you to all heat.

Try this, you say it's fine parked, leave it parked and stick your hands behind heater control panel, and start wiggling hoses, you may find a bad one.

 

[syn-]

now when you say behind the control panel, I take you mean unbolt the hvac controls and wiggle the wires behind it? I also want to state that the reason I replaced the controls is because the right knob (a/c on, max a/c, defrost, etc.) was dim lit. Once replaced it was fine and bright, though even before the controls were replaced it would only blow hot air.

 

[Evil Eye]

I agree it sounds like a blend door problem.

 

[TheJackal]

The blend door is electrically controlled. A vacuum leak wouldn't cause that problem.

Try running the A/C on "Max A/C." If you still feel hot air, you're either low on refrigerant, or you have a vacuum leak somewhere. (Max A/C shuts off coolant flow to the heater core via a vacuum-controlled valve.)

 

[budwich]

IF your compressor is "fast cycling" then the "blender door" problem isn't the likely issue (my bad on a post a few back about the vacuum... as stated correctly is electrically controlled).

 

[Evil Eye]

I guess I should clarify. He sent me a PM and he described that it's about 3 seconds on and 3 seconds off, which isn't fast cycling.

 

[budwich]

well... that's pretty fast to me.... and that kind of cycling isn't going to help the situation. Another test is to "short" the low pressure switch and see if the compressor stays on (of course, don't do this "permanently" ... just a quick test). The compressor should stay on and if you have "any" freon left in the system, you should get some degree of coolness. Anyways, it is likely that he has a "few onion skins".

 

[Evil Eye]

Yeah, you're right. That's a good suggestion. We need to know how much freon is in the system and go from there. Since he doesn't have a set of gauges that's a good next step. I'm still wondering about this "thud" behind the glove box... Sounds like a vacuum issue, but that wouldn't explain the heat, unless he also had a blend door issue.

 

[syn-]

Ok, did some weekend testing and the truck cools fine in park, its actually a pleasure to sit and cool down. though once in drive it sounds like the blend door shuts and I begin to get hot air. the only thing I can think of is where the hvac switch was positioned when I first installed it. it wasn't in the off position when I first got it, though I did switch it to the off position before installing it. Also that cycling is at the beginning of starting the truck and turning on the a/c, if I go to drive down the road (there is a wallgreens up the street). it will make its thud and begin letting the heat in. I turn the a/c off, park the truck, turn it off, start it back up, and turn the a/c back on its cold (and the cycling isn't as fast). though as soon as I put it in drive, and start moving. the thud returns and so does the heat.

 

[budwich]

OK.... so IF your AC (compressor) is running as "expected" then you likely have the "infamous" blender door problem which means its busted mechanically and you are likely hearing it "free flap" as a result. There are posts on how to address this.

Having said this IF your compressor is "fast cycling" at any point, this is not good and is likely an issue also.