7.3 stuck on defroster

Brucesearly99 · Nov 18, 2015

Like the title says my defroster is stuck on. You can turn the knob to the vents but it still blows out the defroster. I put manual hubs on about a year ago, so I don't know if that has something to do with it or not. Any help would be appreciated. Thanks

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79jasper · Nov 18, 2015

Vacuum pump.

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griz700rocket · Nov 18, 2015

Try unplugging the solenoid that controls the auto hubs. The hubs start to leak when they wear out and there's no need for it with manual hubs. That was the cause on my 01 7.3 that did the same thing anyway.

Arisley · Nov 18, 2015

Plug the lines that run from the vacuum pump down to where your auto hubs used to be. With no hubs there, they are leaking and not allowing your defrost door to work, it takes vacuum that is not there with those two lines open to atmosphere.

Brucesearly99 · Nov 18, 2015

Is it easier just to plug them at the pump? Or does that feed a lot of other stuff also?

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mrit · Nov 18, 2015

I cant remember which fuse it is, but check the fuse that control that circuit. I had one randomly burn out 8-9 years ago, Replaced it with the same size fuse and its been good ever since.

jngreen · Nov 18, 2015

Can you hear your pump running? Do you hear a buzzing sound with the key on and engine off?

Mine did the same thing and I plugged mine at the vacuum canister. I think I prevented the vacuum from going to the malfunctioning solenoid. I think I just rerouted some hoses so that the canister ran back into itself instead of going to the solenoid.

griz700rocket · Nov 19, 2015

Just unplug the wires from the solenoid. It's much easier than messing with the vacuum hoses and does the same thing.

dsberman94 · Nov 19, 2015

Two screws and a bit of silicone and the hub lines are blocked and the issue is gone.

Arisley · Nov 19, 2015

If those hoses are open to atmosphere, you will get no vacuum to HVAC doors. It will default to defroster. You can plug them at the wheel, or remove them and plug them at the pump.

dsberman94 said:
Two screws and a bit of silicone and the hub lines are blocked and the issue is gone.

Not even that hard. A couple of vacuum plugs from your favorite parts store. Remove hoses and install plugs. Or just simply cut them off, leave some length, shove a couple golf tees in the ends of the hoses. No silicone needed.

jngreen · Nov 19, 2015

griz700rocket said:
Just unplug the wires from the solenoid. It's much easier than messing with the vacuum hoses and does the same thing.

Not if the solenoid is stuck open like mine was.

dsberman94 · Nov 19, 2015

Arisley said:
If those hoses are open to atmosphere, you will get no vacuum to HVAC doors. It will default to defroster. You can plug them at the wheel, or remove them and plug them at the pump.

Not even that hard. A couple of vacuum plugs from your favorite parts store. Remove hoses and install plugs. Or just simply cut them off, leave some length, shove a couple golf tees in the ends of the hoses. No silicone needed.

I didn't do silicone on mine. But I thought about it. I just put two appropriate sized screws in the plastic hose connector and my blend door works just fine. Lots of ways to get to the same outcome. One thing that bugs the crap out of me is when someone buys a truck and the PO just started unplugging random chit.

Brucesearly99 · Nov 19, 2015

jngreen said:
Can you hear your pump running? Do you hear a buzzing sound with the key on and engine off?

Mine did the same thing and I plugged mine at the vacuum canister. I think I prevented the vacuum from going to the malfunctioning solenoid. I think I just rerouted some hoses so that the canister ran back into itself instead of going to the solenoid.

Yes my pump is running.

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dsberman94 · Nov 20, 2015

Probably constantly.

jngreen · Nov 20, 2015

Brucesearly99 said:
Yes my pump is running.

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Well that is an easy fix then, all you have to do is plug the vacuum leak. I would assume that your solenoid is stuck on like mine was, because I doubt that you are driving around with your 4x4 switched on. That is what that solenoid controls, it is supposed to send vacuum to your auto hubs, which you no longer have. You now have a vacuum leak bad enough for your blend door to stay in the default position. If you fix the leak, you will fix the problem with the defroster staying on.

Arisley · Nov 20, 2015

Arisley said:
Plug the lines that run from the vacuum pump down to where your auto hubs used to be. With no hubs there, they are leaking and not allowing your defrost door to work, it takes vacuum that is not there with those two lines open to atmosphere.

Arisley said:
If those hoses are open to atmosphere, you will get no vacuum to HVAC doors. It will default to defroster. You can plug them at the wheel, or remove them and plug them at the pump.

Not even that hard. A couple of vacuum plugs from your favorite parts store. Remove hoses and install plugs. Or just simply cut them off, leave some length, shove a couple golf tees in the ends of the hoses. No silicone needed.

jngreen said:
Well that is an easy fix then, all you have to do is plug the vacuum leak. I would assume that your solenoid is stuck on like mine was, because I doubt that you are driving around with your 4x4 switched on. That is what that solenoid controls, it is supposed to send vacuum to your auto hubs, which you no longer have. You now have a vacuum leak bad enough for your blend door to stay in the default position. If you fix the leak, you will fix the problem with the defroster staying on.

Same thing I said twice. It has been a simple fix since these trucks were built. No reason to make is complicated.

jngreen · Nov 20, 2015

Arisley said:
Same thing I said twice. It has been a simple fix since these trucks were built. No reason to make is complicated.

Even then, It shouldn't be leaking unless the solenoid is bad, the 4x4 switch is on, or the pump is not running (which he just stated it was). I do agree that he needs to plug the leak somewhere, but like someone mentioned before, just unplugging the wires running to the 4x4 solenoid is not going to fix the problem.

Dflippin08 · Nov 23, 2015

My truck did this last year. Someone had kicked a main vacuum connector under the dash on the pass side.

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bordo380 · Aug 11, 2016

Well I'm waking this thread back up. My 7.3 X is doing this too. I plugged my 4wd vac hoses at the fenders and checked all the fuses and I'm still stuck on defrost.
I've had it a year and it's never had the tubes running to the hubs and it always switched fine.
That pump on passenger side is running non stop. https://vimeo.com/178559868

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