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My new to me 95 XL 4.9 has AC. While driving highway speed on the Max AC setting the air flow changed from the vent to the defrost then back again. It only happened twice on a recent local trip. Only happened on the highway. Is this a vacuum line issue?
Very common issue caused by the white vacuum line to the recirculation door. It tends to crack or disintegrate if you stare at it too long. Highly suggest to replace all of the vacuum lines under the hood. Your truck uses R134a refrigerant.
As previously stated, it was R-134a from the factory. The servicing sticker says which refrigerant is in the system also the high/ low ports are different sizes for the two refrigerants. For R-12 you screw the hose onto the fittings while the fittings for R-134a are quick connect and bigger.
As previously stated, it was R-134a from the factory. The servicing sticker says which refrigerant is in the system also the high/ low ports are different sizes for the two refrigerants. For R-12 you screw the hose onto the fittings while the fittings for R-134a are quick connect and bigger.
I thought R12 had the same size quick connect fitting on the.... high side.... and then a different sized low side buut otherwise changeable fitting. were they screw in?
I worked on my 81 F100 that had R12 enough to remember but I am pretty sure they were screw on.
I have converted to R134a and added the adapters so it now has the quick disconnects.
If you still find the door moves when the motor is under a heavy load, pulling a hill, you may want to check the 1 way valve between the firewall and the vacuum side. It should be on the manifold vacuum supply side.
The supply comes from the manifold and tees, 1 going to a vacuum can and the other to the cab.
My 81 dose the same thing and I am sure the new check valve is bad.
Dave ----
Very common issue caused by the white vacuum line to the recirculation door. It tends to crack or disintegrate if you stare at it too long. Highly suggest to replace all of the vacuum lines under the hood. Your truck uses R134a refrigerant.
The vacuum line I referenced above:
I checked this line under the hood and it is in really good shape. Still flexible and I could not get it to crack by flexing it. Maybe where it comes into the cab? I've got to see if I can find it under the dash I guess. What's the best access point?
Remove the glove box to gain access the bundle of vacuum hoses. Another common problem spot, really surprised the white line was not broken, is the HVAC vacuum reservoir mounted to the evaporator/blower box is cracked or not connected. There is also a check valve in the supply line.
Remove the glove box to gain access the bundle of vacuum hoses. Another common problem spot, really surprised the white line was not broken, is the HVAC vacuum reservoir mounted to the evaporator/blower box is cracked or not connected. There is also a check valve in the supply line.
Both items are in the diagram below.
HVAC Vacuum Reservoir
Here is the Check Valve
What happens if that red vacuum line connecting to that T is broken and what happens if that check valve dont work properly? Say you have a cracked case, would a bad check valve be a vacuum leak then but wouldnt be if it worked?
That was the best picture I could find for the check valve. From the factory that is a black vacuum supply from the manifold port tree.
If the red (for this scenario) vacuum line was broken then yes it would be a vacuum leak. If it was completely open to atmosphere the HVAC system would always default to blowing out the defrost ducts. If the check valve is bad, this has happened in recent past, then the HVAC would work as it is supposed to until you are under load and the manifold vacuum drops. That check valve is supposed to close when the manifold vacuum drops below what is stored in the HVAC reservoir. With that check valve open the HVAC reservoir will empty very quickly which in turn will cause the HVAC system to default to blowing out the defrost ducts. Release the throttle, vacuum build up again, then the HVAC system will return to whatever setting is selected on the dash controls.
Someone here had this very issue and very methodically troubleshot the vacuum circuit. Turned out to be that crazy little check valve was bad.
That was the best picture I could find for the check valve. From the factory that is a black vacuum supply from the manifold port tree.
If the red (for this scenario) vacuum line was broken then yes it would be a vacuum leak. If it was completely open to atmosphere the HVAC system would always default to blowing out the defrost ducts. If the check valve is bad, this has happened in recent past, then the HVAC would work as it is supposed to until you are under load and the manifold vacuum drops. That check valve is supposed to close when the manifold vacuum drops below what is stored in the HVAC reservoir. With that check valve open the HVAC reservoir will empty very quickly which in turn will cause the HVAC system to default to blowing out the defrost ducts. Release the throttle, vacuum build up again, then the HVAC system will return to whatever setting is selected on the dash controls.
Someone here had this very issue and very methodically troubleshot the vacuum circuit. Turned out to be that crazy little check valve was bad.
This sounds like my issue. I only noticed it on the highway, so let's say under heavy load. I'll try to make it recur before going after that check valve.
On my old 1992 F350 w/5.8L engine the HVAC reservoir was cracked. Someone tried to fix it with epoxy but failed miserably. Very similar symptoms until I used the one of the chambers on the plastic dual chamber vacuum reservoir on the passenger side inner fender as a replacement. The Air Injection system been previously removed so one of the chambers was not needed anymore. I used it as a substitute reservoir for the HVAC system.
The next thing if you find the check valve is bad is a replacement part number...
On my old 1992 F350 w/5.8L engine the HVAC reservoir was cracked. Someone tried to fix it with epoxy but failed miserably. Very similar symptoms until I used the one of the chambers on the plastic dual chamber vacuum reservoir on the passenger side inner fender as a replacement. The Air Injection system been previously removed so one of the chambers was not needed anymore. I used it as a substitute reservoir for the HVAC system.
The next thing if you find the check valve is bad is a replacement part number...
My hvac will occasionally do the thing you mentioned "under load" but it actually doesnt follow a rhyme or reason, and i usually just turn the **** and let off the throttle to increase vacuum and then it resets to where it should be, but its weird, because i did try and wait out the issue once and it corrected itself after a minute. the switch to defrost then back to vent, that is. odd. it doesnt happen often in the cold, it happened more in summer.
Right now I have about 4 vacuum caps on various parts of this god damn vacuum mess because when doing the pcv hose I accidently bumped into the vacuum elbows and lines and broke a bunch. nothing seems to be affected despite this but its... weird...
Very common issue caused by the white vacuum line to the recirculation door. It tends to crack or disintegrate if you stare at it too long. Highly suggest to replace all of the vacuum lines under the hood. Your truck uses R134a refrigerant.
The vacuum line I referenced above:
Randy, it turns out you were right. I inspected the tube more closely and found a spot where it had broken open. I was able to cut out the exposed piece and run it out of the loom closer to the actuator so it was a very easy repair. Thanks for the tip!!
