Air conditioning blows hot at highway speed.

[Sailing_Faith]

Ok,

So this really is a weird one. Ac works fine driving around town, but when on the freeway or driving over about 45mph it only blows hot air.

This is not the blower motor or resistor pack, since the fan blows just fine... I am wondering if maybe the freon pressure is high? Kinda weird, since it was working fine.... And I have not added any.

So, what you think? I suspect somehow the high pressure switch is getting triggered... Ever seen one go bad?

[chevymaher]

Kinda sounds like a vacuum issue. Door to the heater is opening. I would check the pressure in the system tho first.

I never heard of a high pressure switch going bad on anything ever. But that don't mean there isn't a first time.

[SilverBullet1997]

Sounds like a vacuum issue to me too

[Sailing_Faith]

Thanks, I will take a look. I just replaced the main vacuum line from the manifold earlier this week... I had replaced the others already but will take another look at them.

Might hook up a light to the compressor clutch to see if it is disengaging when the hot air blows... Hope it is a simple vacuum hose though.

You guys are the greatest, getting ready to drive from CA to FL and then MA and back... Having AC on the highway is going to make a huge difference!!!!

[Mmusicman]

Typically, when I experienced the conditions you describe, it's because I was low on freon... not enough in system at highway speeds.

Yes, certainly the compressor will kick off for two reasons, excessively low or excessively high pressures. Low of course is usually just low freon.

High pressure however, will shut it down too and can be caused by several things... overcharging, blockage, or insufficient air over condenser coil. I mention the condenser coil because I had charging issues back when I had issues with my electric fans. They did NOT pull enough air at idle across the condenser while charging... causing my high-side pressure to go unnaturally higher than it should have, giving the appearance that it was fully charged according to the gauges. Then when I revved it up and held the throttle... the high side would practically go off the scale and the compressor would of course shut down... even though I was actually low on freon.

I learned a trick from an automotive A/C guy... run a water hose across the condenser while charging to lower condenser temp and simulate proper cooling while cruising. This really exposed that I had a serious issue with my inadequate electric fans not pulling enough air across condenser, causing me to get erroneous readings while charging. I did finally resolve this issue with a much more powerful fan that had a much larger shroud as well. It wasn't just important to pull air... but that it needed to be pulled across ALL of the condenser area too! Problem solved.. now I'm seeing correct high pressure readings while charging... without the need for the water hose. I was now able to charge the system fully and correctly, and resolve my cruising temps too.

Just thought I share my experience! Living in Florida... A/C is EVERYTHING!!! It has to work.. and work right! I've been at both extremes... under and over-charged! In each case, the rest of the system was working correctly.

I can now sit in 90-100 degree temps idling with my A/C on full, and it keeps me cool. My A/C compressor also remains constantly on while normal highway cruising.

[sfeaver]

Check the vacuum check valve, or replace it. Does the airflow go to the defroster instead when at highway speeds? Is it only when stepping on the gas a bit?

From what you said it starts blowing warm air but just wanted to double check. Do you have A/C gauges to get readings?

[Sailing_Faith]

Thanks to all,

First, the vacuum question: I don't think this is it. I looked at all the hoses, I have replaced them all since I got the van.... That was a bunch of miles ago. Had an idea, and one of my stops today I switched off the AC about 2 minutes after the engine had run... I was dumping and clearing codes so I had the ignition on.

The vacuum accumulator had enough vacuum after about 2 minutes to switch the door over, so I suspect that my visual inspection was right... Would also seem to rule out the check valve. (?)

I have gagues, but they are on the other corner of the country right now. I do have one of those cheap gagues that read the low side only, that are hooked up to a charge valve. I will check the pressure, but the weird part is it only blows hot on he hwy, so I will have to run it up and use Music's hose trick to see what is going on.

Thanks for the help, and helpful suggestions!