Why is A/C defaulting back to defrost vents under load?
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Why is A/C defaulting back to defrost vents under load?
On my 98 I've had this problem of a/c defaulting totally to the defrost vents under load before and it was either the gray and black hose coming from the vacuum ball and the heater valve. That's replaced a couple years with an nos factory harness. Then I had a problem after that with the heater valve diaphragm leaking. It started doing it again after about 2 years so I changed it out again. Either it's leaking vacuum out of the box or there is something else is wrong. I did a smoke test to the vacuum hoses going into the cabin and didn't notice any leaks. I had a Motorcraft heater valve on there that lasted about 2 years and I switched it out with an off brand GPD and I'm thinking it's leaking right out of the box new. It also could be my vacuum pump is bad. I pumped up the heater valve and it wouldn't hold vacuum. I know these hand vacuum pumps go bad quick. Is there anything else I could check before swapping out the heater valve again?
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The check valve will maintain. vacuum in the system when manifold vacuum decreases with open throttle. It just does not let the system go to no vacuum when the vacuum source has no vacuum. Many have a reservoir connected to manifold vacuum, get pulled to a vacuum as the engine runs with high vacuum,and then the check valve built-in keeps the reservoir at vacuum when the engine is under load or turned off. The reservoir can provide a few power brake assists, and then be back to standard brakes.
Look at the vacuum routing for a reservoir & check valve. they may be combined in that case would likelyi have two connections to the reservoir, one to manifold, the other to the system that uses the vacuum.
tom
Look at the vacuum routing for a reservoir & check valve. they may be combined in that case would likelyi have two connections to the reservoir, one to manifold, the other to the system that uses the vacuum.
tom
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On my 98 I've had this problem of a/c defaulting totally to the defrost vents under load before and it was either the gray and black hose coming from the vacuum ball and the heater valve. That's replaced a couple years with an nos factory harness. Then I had a problem after that with the heater valve diaphragm leaking. It started doing it again after about 2 years so I changed it out again. Either it's leaking vacuum out of the box or there is something else is wrong. I did a smoke test to the vacuum hoses going into the cabin and didn't notice any leaks. I had a Motorcraft heater valve on there that lasted about 2 years and I switched it out with an off brand GPD and I'm thinking it's leaking right out of the box new. It also could be my vacuum pump is bad. I pumped up the heater valve and it wouldn't hold vacuum. I know these hand vacuum pumps go bad quick. Is there anything else I could check before swapping out the heater valve again?
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From: North Georgia
I checked all those lines and did a smoke test on them. There weren't any leaks that I could see or that failed the smoke test. Even the ones going into the cabin. I didn't see any smoke coming out when I filled the lines going into the cabin. I did notice today when I decided to try a different mode. I put it on A/C instead of Recirc. It doesn't default to the defrost when it's in A/C mode. So it's something to do with the Recirc circuit.
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From: WA
A broken check valve leaks internally. There wil be no smoke present. Your symptoms match a bad check valve.
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Fleet Mechanic
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It should be in line with the hose to the vacuum reservoir tank from the intake manifold. The check valve stops reservoir vacuum from leaking back in to the intake system when the intake system loses vacuum at open throttle.
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I've replaced the ball in the past. I think I replaced it when it wasn't bad. I replaced it with a new Ford one. I think that's the only way you can still get it. Is there a way to test the vacuum ball to see if it's work right? I don't see anything in the Ford manual about testing it. I don't want to buy another one if I don't really need it and it's actually maybe something else wrong. I've got a vacuum pump. I know they sell new vacuum line and ends in case my hose is messed up.
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From: WA
Might be a check valve in the "ball". Most vacuum based systems have check valves. Because at WOT there's very little vacuum in the intake manifold. The brake booster, the air controls, etc. The ones that don't are using intake vacuum as a trigger or control for certain functions, like EGR.
With a check valve the system is essentially closed. So if the check valve is working and vacuum still leaks down that means there is a leak somehwere else in the system. Could be anywhere from the engine bay to one of the controls behind the dash panel.
Anyway, if you find out that the ball has a check valve but it's not checking anymore you can install one in line on the hose that goes to the ball check valve. Might be why there are Dorman check valves on Rockauto.
With a check valve the system is essentially closed. So if the check valve is working and vacuum still leaks down that means there is a leak somehwere else in the system. Could be anywhere from the engine bay to one of the controls behind the dash panel.
Anyway, if you find out that the ball has a check valve but it's not checking anymore you can install one in line on the hose that goes to the ball check valve. Might be why there are Dorman check valves on Rockauto.
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I'd rather just get a new vacuum reservoir. I've replaced the black and gray vacuum lines going from the vacuum ball and heater valve into the cab. It was a new Ford vacuum harness because I had a similar problem a few years ago when the original gray vacuum tube kept falling apart. That fixed that. At that time I believe I replaced that vacuum reservoir so it's a few years old. I just can't believe it would be bad again already. It's an original part. I read and it does have an internal check valve in the vacuum ball. I don't know if the rough riding my truck does now lessens the life of it, since it sits on the fender apron. They're not too bad for new ones, but I'd kind of like to test it first to be 100%. I'm trying to find a correct way to test it.
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