1996 AC troubles....forced air fades slowly from vents

[72freak]

1996 C3500. I'm guessing a blend door thing or shorting out switch but here are the symptoms.

AC comes on and blows cold. Blows freely out of the vents. 5 or 10 minuets in you start to feel warm and realize no more air is coming out. You can hear the blower pushing air but its not exiting anywhere. Not from feet area or defrost or vents. The air is trapped and you can feel the cold air escaping somewhere barely. If you turn the switch off and wait a few minutes and then turn it back on it will blow great out of the vents as it should and then it slowly fades again.

The system is fully charged, AC is blowing cold, fan speeds are working and compressor is staying on.

Anyone who has experienced this what is the problem? THANKS!


Before I get asked...and it will happen...yes, the air pressure in my tires is full, my flux capacitor has been replaced, my headlight solenoid is good and my rocker arms do not need adjusting. LOL

[sntrym95]

Ah but did you disconnect the battery and flush the coolant before doing any work at all? :P

[speedygonzales]

Sounds to me like your evaporator coil is freezing up. But you say your charge is at full. Maybe crap has accumulated in your orifice tube? Or there is a partial blockage some where in the system. Have you check high side pressure when the problem occurs?

[72freak]

Originally Posted by speedygonzales

Sounds to me like your evaporator coil is freezing up. But you say your charge is at full. Maybe crap has accumulated in your orifice tube? Or there is a partial blockage some where in the system. Have you check high side pressure when the problem occurs?

As mentioned, the air is still cold and blowing. The refrigerant system is not failing. It is the directing of the cold air I am having trouble with. it is either a blend door issue that is a very common problem or a controller issue controlling the blend doors and they are always going bad also. I have just never had to deal with it and was looking for some responses from folks who have had the same issue to shed some light on the subject.

[Tropical Snowstorm]

If I understand your post it's an air flow, not A/C operation that's the issue. I'd look in to the blend door, from what you're saying it sounds like it is getting stuck between two positions and blocking the air to the vents. Once you turn it off the pressure releases and the door opens back up until it slowly closes from the air flow. Never had a GMT400 apart in there, only heater cores, so I could easily be wrong but that's where I'd start.

[72freak]

Originally Posted by Tropical Snowstorm

If I understand your post it's an air flow, not A/C operation that's the issue..

Yep...agreed..I thought I was pretty clear on that too.

Originally Posted by Tropical Snowstorm

I'd look in to the blend door, from what you're saying it sounds like it is getting stuck between two positions and blocking the air to the vents. Once you turn it off the pressure releases and the door opens back up until it slowly closes from the air flow. .

Thanks for the input.

[ChevyTech]

Originally Posted by speedygonzales

Sounds to me like your evaporator coil is freezing up.

I agree.

[72freak]

Originally Posted by ChevyTech

I agree. Sounds to me like your evaporator coil is freezing up.

I am no pro on AC so I will ask. The symptoms of the evaporator freezing up would probably be the warm (not cold) air blowing through the vents....correct?

It does not make sense to me that a frozen up evaporator would stop the air flow coming through the vents all together

Do I just not have an understanding of how the system works or is my description of the problem lacking in detail?

[speedygonzales]

Originally Posted by 72freak

AC comes on and blows cold. Blows freely out of the vents. 5 or 10 minuets in you start to feel warm and realize no more air is coming out. You can hear the blower pushing air but its not exiting anywhere. Not from feet area or defrost or vents.

Right there you said the air is not exiting from ANYWHERE including places it should not! So how could it be a blend door issue?

Originally Posted by 72freak

As mentioned, the air is still cold and blowing. The refrigerant system is not failing.

If it's freezing, what would you expect? Remember, if it is freezing, it's usually because the refrigerant is flashing off too fast. So it's colder than normal.

Originally Posted by 72freak

I am no pro on AC so I will ask. The symptoms of the evaporator freezing up would probably be the warm (not cold) air blowing through the vents....correct?

NO

Originally Posted by 72freak

It does not make sense to me that a frozen up evaporator would stop the air flow coming through the vents all together

How do you blow air through a block of ice?

Originally Posted by 72freak

I have just never had to deal with it and was looking for some responses from folks who have had the same issue to shed some light on the subject.

We have seen this before, we gave the best answer for the symptom and you still don't like the answer.

Originally Posted by 72freak

Do I just not have an understanding of how the system works or is my description of the problem lacking in detail?

The answer is yes, you don't know the system. You attitude precludes anyone helping you.

[72freak]

Thanks for the input. I will get it to where is is not working and run it to my AC guy to check before digging into the ductwork and control unit.

[ChevyTech]

Originally Posted by 72freak

...I am no pro on AC so I will ask. The symptoms of the evaporator freezing up would probably be the warm (not cold) air blowing through the vents....correct?

It does not make sense to me that a frozen up evaporator would stop the air flow coming through the vents all together

Originally Posted by speedygonzales

...No...

...How do you blow air through a block of ice?...

Exactly !!!
I agree with Speedy completely.

Originally Posted by 72freak

...Do I just not have an understanding of how the system works or is my description of the problem lacking in detail?

Originally Posted by speedygonzales

We have seen this before, we gave the best answer for the symptom and you still don't like the answer.

The answer is yes, you don't know the system. You attitude precludes anyone helping you.

The symptoms fit a frozen evaporator perfectly.
Air will not flow through ice.

[speedygonzales]

I'm not sure about this but I would think that if the evaporator was freezing, there would be a ton of air coming out the evaporator drip tube that normally would not.

Also, I would think a bore scope could look into the drip tube to view the evaporator.

Anyone correct those 2 statements please.

[Kim57]

I've been having a similar problem with mine after I changed out the blower motor.
I thought I screwed something up when I changed it.
Whats the fix for this problem.
thanks
Kim

[Boog]

These R134 systems are fairly finicky about their pressures. If it's undercharged the evaporator can ice over enough to stop air flow, period. I've been there. By turning the truck off for a few minutes it allows the ice to partially melt again allowing air to pass through the evaporator until it ices over again.
Before tearing anything apart you should at least have the pressures checked.

[ChevyTech]

If the evaporator is dirty, less air flows through it, making easier to freeze up.

When the system was designed, a calculation was made on how much freon flowing through how big of a evaporator would give the best cooling without freezing up. If the evaporator is dirty freezing up becomes easier, and if the temperature is not real hot, then freezing up is much easier.

Good thread with photos on dirty evaporators.
http://www.chevytalk.org/fusionbb/sh...php?tid/282054

Re: Kim57
Did you make sure the blower motor was turning the correct direction?

[speedygonzales]

Got confirmation about air out the drip tube. So, check the air coming out the drip tube when the fan is first turned on. Then let the system come to a stop as you mentioned and then check the air coming out the drip tube. If it's a great bit more, viola. You have the answer.

BTW, a few companies make an evaporator cleaner spray foam that you squirt in the drip tube. I used it before to get rid of the must smell.

[72freak]

Originally Posted by speedygonzales

Got confirmation about air out the drip tube. So, check the air coming out the drip tube when the fan is first turned on. Then let the system come to a stop as you mentioned and then check the air coming out the drip tube. If it's a great bit more, viola. You have the answer.

BTW, a few companies make an evaporator cleaner spray foam that you squirt in the drip tube. I used it before to get rid of the must smell.

I had 2 issues. You were basically correct. I haven't had the truck long and at some point the compressor was hotwired to the AC switch. It was running all the time in AC mode and not cycling thus freezing up. I do also have a intermittent faulty control panel. I/we did a little temporary controller swap-a-roo, and put the AC wiring back to stock and so far no issues. Time will tell.

Thanks

[Kim57]

Originally Posted by ChevyTech

If the evaporator is dirty, less air flows through it, making easier to freeze up.

When the system was designed, a calculation was made on how much freon flowing through how big of a evaporator would give the best cooling without freezing up. If the evaporator is dirty freezing up becomes easier, and if the temperature is not real hot, then freezing up is much easier.

Good thread with photos on dirty evaporators.
http://www.chevytalk.org/fusionbb/sh...php?tid/282054

Re: Kim57
Did you make sure the blower motor was turning the correct direction?

Yes.
My symptoms started a little while after I changed my blower.
I've noticed since the temperature has gotten hotter around here it's not happening.
Thanks for the link to the write up.
I will have to do that and see what I find as my truck is pushing 20 years old now.
Kim

[ChevyTech]

Originally Posted by Kim57

Yes.
My symptoms started a little while after I changed my blower.
I've noticed since the temperature has gotten hotter around here it's not happening.
Thanks for the link to the write up.
I will have to do that and see what I find as my truck is pushing 20 years old now.
Kim

You're welcome!

[ChevyTech]

Originally Posted by 72freak

I had 2 issues. You were basically correct. I haven't had the truck long and at some point the compressor was hotwired to the AC switch. It was running all the time in AC mode and not cycling thus freezing up. I do also have a intermittent faulty control panel. I/we did a little temporary controller swap-a-roo, and put the AC wiring back to stock and so far no issues. Time will tell.

Thanks

With the AC hotwired, so the compressor stays on and cannot cycle, will cause a evaporator freeze up.

You got it under control.

[72freak]

Originally Posted by ChevyTech

With the AC hotwired, so the compressor stays on and cannot cycle, will cause a evaporator freeze up.

You got it under control.

All tests so far say I do...long drive in near future.