71 A/C Problem
Thread Starter
Instructor
Joined: May 2006
Posts: 135
Likes: 0
From: Hot Springs Village AR
71 A/C Problem
My 71 factory air works great initially. After 20-30 minutes of continuous driving the air coming out of the vents is no longer cool and the air flow is greatly reduced. I can hear that the blower is still functioning at high speed, but the air flow is just not coming out of the vents. If I stop and shut the car off for five minutes or so, it will again function normally for 10-20 minutes.
It seems that an air diverter door is closing, causing this problem. I know these doors open/close using vacuum. However, why would I lose vacuum only after 20-30 minutes of operation? Power brakes, headlamps and wipers all function normally when the A/C problem exists.
Any suggestions?
It seems that an air diverter door is closing, causing this problem. I know these doors open/close using vacuum. However, why would I lose vacuum only after 20-30 minutes of operation? Power brakes, headlamps and wipers all function normally when the A/C problem exists.
Any suggestions?
Racer
Joined: May 2002
Posts: 372
Likes: 11
From: Abilene TX
I also have a 71, but it is a long way from a working A/C. I have a 72 Chevy truck that does the same thing as your car. I believe that it is freezing up. I haven't had it looked at yet. It does blow cold and air volume is good, but decreases as it runs. I will also look for an answer.
Race Director
Joined: Dec 2006
Posts: 13,182
Likes: 99
From: West Chester OH
2020 C6 of the Year Finalist - Unmodified
Cruise-In IX Veteran
St. Jude Donor '08 thru '13
A shot in the dark
Is the drain tube from the evaporator clear? If not, the condensation that normally drips onto the pavement will build up and block air flow.
After you turn it on, check to see if it's dripping. Hmmm, does that sound right?
After you turn it on, check to see if it's dripping. Hmmm, does that sound right?
Thread Starter
Instructor
Joined: May 2006
Posts: 135
Likes: 0
From: Hot Springs Village AR
Yes, I have condensation dripping out. Replaced that rubber check valve some time ago.
Thread Starter
Instructor
Joined: May 2006
Posts: 135
Likes: 0
From: Hot Springs Village AR
Thread Starter
Instructor
Joined: May 2006
Posts: 135
Likes: 0
From: Hot Springs Village AR
The fan is running at high speed but very little air coming out the vents. Sounds like all the air is in the plenum.
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Burning Brakes
Joined: Dec 2005
Posts: 1,098
Likes: 1
From: west babylon new york
litle a/c
check to make sure your rad fan blade is not on backwards ,put a napkin or paper towel on front of rad and race engine slightly, it should suck paper torwards rad if not fan is backwards or wrong it happens ; what is happining is the cond is not getting cooled properly or there may be too much oil in the system and the freon is not circulating hope this helps
Thread Starter
Instructor
Joined: May 2006
Posts: 135
Likes: 0
From: Hot Springs Village AR
check to make sure your rad fan blade is not on backwards ,put a napkin or paper towel on front of rad and race engine slightly, it should suck paper torwards rad if not fan is backwards or wrong it happens ; what is happining is the cond is not getting cooled properly or there may be too much oil in the system and the freon is not circulating hope this helps
However, this still doesn't explain where the air flow is going with the blower on high speed. It's not coming out of the vents, heater ducts or defroster ducts.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Here is a simple test:
Disconnect the electric plug to the a/c compressor.
-Set you a/c controls just as you do when you have the problem.
-Drive the car for your 20-30 min and see if your air flow stops.
If your air flow does not stop, then you can believe that your evaporator is freezing and blocking the air flow. You have ice on the fins, not on the outside. Usually a sign of either low freon or a malfunctioning xpansion valve. You can also put a thermometer on the pass vent and close all the others to see what vent temp you get.
Disconnect the electric plug to the a/c compressor.
-Set you a/c controls just as you do when you have the problem.
-Drive the car for your 20-30 min and see if your air flow stops.
If your air flow does not stop, then you can believe that your evaporator is freezing and blocking the air flow. You have ice on the fins, not on the outside. Usually a sign of either low freon or a malfunctioning xpansion valve. You can also put a thermometer on the pass vent and close all the others to see what vent temp you get.
Thread Starter
Instructor
Joined: May 2006
Posts: 135
Likes: 0
From: Hot Springs Village AR
Here is a simple test:
Disconnect the electric plug to the a/c compressor.
-Set you a/c controls just as you do when you have the problem.
-Drive the car for your 20-30 min and see if your air flow stops.
If your air flow does not stop, then you can believe that your evaporator is freezing and blocking the air flow. You have ice on the fins, not on the outside. Usually a sign of either low freon or a malfunctioning xpansion valve. You can also put a thermometer on the pass vent and close all the others to see what vent temp you get.
Disconnect the electric plug to the a/c compressor.
-Set you a/c controls just as you do when you have the problem.
-Drive the car for your 20-30 min and see if your air flow stops.
If your air flow does not stop, then you can believe that your evaporator is freezing and blocking the air flow. You have ice on the fins, not on the outside. Usually a sign of either low freon or a malfunctioning xpansion valve. You can also put a thermometer on the pass vent and close all the others to see what vent temp you get.
Racer
Joined: Dec 2006
Posts: 417
Likes: 40
From: Swansea IL
Expansion Valve Problems
I would bet money that the problem is your expansion valve is stuck open. Too much freon is flowing through the evaporator and causing the temp in the evap to drop below freezing.
When you turn the system off, the ice on the fins of the Evap. melts and the airflow returns. You can check that by seeing all the water run out from under the car. Probably no water is running out when the system is operating and no cold air is coming out.
If you put gauges on the system, odds are that the low side pressure will be below 35 psi. and hence the temp is below freezing.
Bob
When you turn the system off, the ice on the fins of the Evap. melts and the airflow returns. You can check that by seeing all the water run out from under the car. Probably no water is running out when the system is operating and no cold air is coming out.
If you put gauges on the system, odds are that the low side pressure will be below 35 psi. and hence the temp is below freezing.
Bob