Reaper19

I just replaced all of my A/C system and it is cooling to 40-45 degrees in the plenum, but cold air comes out of the vents at 68 degrees when testing today with outside temperature at 84 degrees. the pressures are great 237 High and 36 Low using R134a

Testing the vacuum from the hot water shut off switch behind the glove box to the hot water shut off valve. I am getting 18 inches.HG of vacuum when the HOT/COLD selector is moved to HOT and none when moved to cold. Does anyone know if I am supposed to have vacuum in Hot or Cold selection?

Just trying to see if maybe the vacuum lines are backwards? I can also feel both heater hoses getting very warm while in COLD near the heater core as well as probing the heater core and getting back over 100 Degrees adding warm air to the mixture.

Thanks

'75

The water shut off valve should get vacuum to stop the water flow.

Reaper19

Thanks, I will need to play around with the vacuum lines on the switch until I get vacuum in COLD instead of HOT.

Street Rat

The first hose on the vacuum switch is your vacuum source from the engine. It should be solid black in color.

The second hose (middle port) controls the water shut off valve in the engine compartment. It should be black with a white stripe on it.

The third port is a vent and should remain uncovered.

Dave J

What year car are we referring to here?

Street Rat

He has a 1981

Reaper19

I have a 1981 Vette. As mentioned this is puzzling the A/C is blowing 40-45 degrees in the plenum now, but I can feel the heater hoses still getting hot which seems this would be the source causing heat through the vents and my air in the vents is coming out 68 degrees.

I attached a few pictures of what the hot water vacuum switch looks like in my car. The first one with the lever in the HOT position and the second with the lever in the COLD position with the button pushed in. I have the two hoses mentioned as well, one with a white stripe that attaches to the hot water vacuum valve and the black stripe hose attached to the vacuum source.

There are three ports as Street Rat mentioned on my switch. Testing with a hand vacuum pump and when the hoses are attached the only time I got vacuum to the shutoff valve is when the lever is set to HOT. I tested the switch with the hand pump but cannot get vacuum to the valve in COLD unless the vacuum source is connected to the port nearest the front button on the far right in pictures and the valve vacuum hose in the middle .

If anyone knows which ports the hoses should be attached on this switch and if the selector switch should be in COLD to allow vacuum to the valve I would appreciate any help. Or does anyone know a definite test for the switch to see if it is working properly? I am researching and read over my shop manual and AIM book, but various diagrams contradict each other because it looks like there are two variations of the vacuum switch, the one I have and another with a large button head and black in color.




Thanks

7T1vette

The Chevrolet Chassis Service Manual for 1981...or whatever similar document is was called at that time...is the manual you need to show you all the electrical, mechanical and vacuum connections and logic on your A/C system (and other systems, for that matter). AFAIK, there is no other manual/document that shows the same info as the Chevy Service Manual.

Dave J

The black large button head valve is the one I am most familiar with. Have you tried testing the shut off valve with your vacuum pump? If not, maybe it would be prudent to see if it is indeed working as it should. Does the valve close with or without vacuum?

I would next vacuum test the valve at each port. Connect to each port, and test each individually. Then do the same with the other ports blocked closed. Once you have that information it is very easy to diagnose which hose goes to what port.

On the 75-77 a/c cars, (the large black valve), the middle port gets the vacuum source hose, and the lower (farthest from the plunger) gets the white striped hose to the shut off valve. My guess is yours is the same, but lets make sure.

Your statement about contradicting diagrams is indeed very true. In my research I have found far too many incorrect diagrams. Bubba worked overtime here.

Reaper19

I'll try testing the shut off valve as you suggested tonight by applying vacuum and see if it works by shutting off the hot water or if no vacuum works, sounds like the best first test. I may need to remove it to confirm it is working and see if the plunger is moving up/down. I also thought about installing a manual valve temporarily to see if that works and the A/C stays cold while the vacuum valve is out

I tried testing the switch with the vacuum pump last night, but no definitive answer yet for the correct hose connections accept there is vacuum to the valve when connecting per shop manual/AIM manual. Funny though, they show both switches in the manuals . Talk about bubba writing these. I'll need to do some more testing and report back.

Thanks! This forum is the best!

Reaper19

I have the COLD/HOT vacuum switch and hot water valve working thanks to forum member SwampeastMike. He sent me some instructions on how to troubleshoot the vacuum switch and in 20 minutes found the vacuum lines were incorrect. After correctly connecting the vacuum lines to the switch I now have vacuum to the hot water valve when set to COLD and no vacuum in HOT setting. I can see the temperatures drop now when set to COLD and rise when set to HOT. Getting close to having all A/C and heater issues resolved now.

It turns out if you look at the pictures of the vacuum switch in the thread, the port nearest the button on the right is where the Black Striped hose (vacuum source) should be connected and the next port to the left in the middle is where the White Striped vacuum hose to the hot water valve gets connected. The third port on the far left is the Vent.

Taking the vette out to cars and coffee tomorrow at the beach and give it a good test.

Thank you everyone for helping me resolve this issue!

Reaper19

Thanks Street Rat for the info and getting me pointed in the right direction.

Street Rat

Sounds like you got it figured out. Congrats.

Reaper19

Well it appears the Hot Water Valve is sticking now and not allowing flow through the heater core for heat and seeping fluid through while closed, dang. I ordered a new hot water valve for my 81 since the one on the car now is not correct, it is only 2 years old but I think it is for an earlier model since the heater hose is on the side closest to the exhaust manifold instead of along the fender.

The new valve arrived last Wednesday but I have not been able to install and do more testing due to Hurricane Matthew coming through our area in Florida and we had to evacuate.

I'll get a chance to work on it next week sometime, I hope, and tweak everything to get the Max A/C out of the vents. I am also going to flush the heater core while I have the valve out just to be sure nothing is stuck and hopefully the new valve cuts off the flow of antifreeze correctly. If the temps are still 40-45 degrees in the plenum and 68 at the vents, it looks like more troubleshooting.

Thanks everyone, I'll give an update afterwards.

Hopefully others from the Southeast fared well during and after the storm.

Reaper19

Got a chance to install the correct Hot Water Valve today and the HOT/COLD shutoff is working correctly. I now get HOT and COLD through the vents as expected.

Still getting 40 degrees in the Plenum, but only 68 degrees at the Vents when the AC is on

Anyone have any other ideas to try? All systems are working as they should, somehow I am losing COLD AC air from the plenum to the vents.

bmotojoe

If you pull the fan resistor from your evaporator housing in the engine compartment look back towards the firewall you will see the heater door operated by the heater control cable. When you slide the heater control full cold the door should completely block access to the heater core side. When you slide it to hot the door should open and direct fan air directly to the heater core. Make sure your heater cable is adjusted correctly.
When MAX A/C is selected, the engine compartment plenum vacuum actuator should close the door and the re-circulation vacuum actuator door (behind lower right kick panel) should open.

Reaper19

Thank you for the suggestions.

Checked the heater door and it is definitely closing in either selection HOT/COLD and I can feel the temp change cold/hot. The engine compartment actuator is closing the outer fresh air door and lower door is opening. Puzzling, the cold air is being lost from the plenum to vents someplace?

I am going to take a mirror tomorrow and check if there is any blockage in the front of the evaporator. The side looking through the resistor hole is very clean and spotless. But the plenum is ice cold 40 degrees.

This is very puzzling

Street Rat

Wow. You sure are having a hard time with this car. Unfortunately I think you need to pull the heater/ac case from the interior of the car and see exactly what the hell is going on in there.

Good luck Reaper. You have to find the problem eventually. Stick with it.

Reaper19

I will keep at it, thanks. I took the car out today for a long ride and the new hot water valve works great, no hot air in the car at all when on cold and only allows hot coolant through when on hot and no longer while in cold setting. No more hot feet or warm air through vents.

Now I need to find if warm mixture is getting in and not allowing all the cold 40 degree AC air to get through the vents. Funny the plenum valve, kick panel valve and all others are working correctly. The blend door also seals up over the hot/heater core side which I can see through the resistor hole.