I have narrowed down an HVAC leak to the back of the air door selector in the dash control unit... I can push the rubber backing that has the wires running into it to augment the pitch of the hiss to confirm this is the location. It is only blowing air out of the main vents will not switch to floor or defrost. All others work fine.

With that said, what part is this and what do I need to replace? Would the switch itself be the cause of the leak, or are the vacuum hoses going into the switch (or rubber seal) responsible?

TIA

 

Attached is a pic

 

Used to be able to buy this part separately, but I can't find anything recently. Probably need to buy the whole control unit. About $72 at RockAuto.com.

 

I pulled the unit off and have the exact location of the hissing sound... It is the black vacuum hose on the back of the switch. I can get it to stop by covering that hole on the rubber seal.

Would my logic be correct that the lack of a proper functioning switch cause it to blow only to the vents? My understanding was there was a fail safe to default to defrost/floor if the actuator failed, so it could only be the switch right?

Any other things it could be before I buy a new unit?

 

I've just recently gone through HVAC troubleshooting twice on my truck. Here's some of what you should know:

1. The black hose on the connector is what provides the engine vacuum to the switch. You want that connector to seal up tight against the switch so there's no loss of vacuum. If you're hearing hissing coming from that switch then most likely the problem is a bad rubber diaphragm inside that switch which is leaking vacuum instead of routing it to the proper vacuum actuators in the dash.

2. To test that the vacuum actuators are working properly, open the glove box all the way past the catch that keeps it in. Look for the end of the vacuum lines behind the glove box. There's a rectangular white plastic connector in there that all the vacuum lines go to. Take the connector off. Find the nipple on the firewall side of the connector that supplies vacuum; it's the one that the black hose goes onto. Using a short piece of small vacuum hose manually connect the vacuum supply nipple to the various other nipples. Make sure the truck is running so there's engine vacuum. When you make that manual connection you bypass the problematic vacuum switch in the HVAC control unit. If each of the vacuum actuators works and the air flow changes as it should then you will know the vacuum switch in the control panel is bad.

3. You may be able to take the vacuum selector switch apart and clean the rubber diaphragm. When you take the vacuum connector off the back of the switch there will be six nipples on a plate on the back of the switch. That plate can be unsnapped from the switch. Underneath is a metal retainer disc and the rubber diaphragm. You may very well find it saturated with automatic transmission fluid. That's usually the cause of a leaking switch. Try cleaning the rubber diaphragm with Simple Green or another mild solvent that won't degrade the rubber. Once it's clean and completely dry you can reassemble it and see if it will work.

Good luck!

 

I did notice moisture, is that an underlying issue? I have a manual transmission FYI... I cleaned up the disc and rubber vacuum re-router and re installed hoping to fix the issue, no dice. I could get it to stop hissing when I applied back pressure to the switch, but I could not get it to stop consistently after keying off and releasing the vacuum. I think the spring just weakens up enough over time to keep it from sitting flush.

I replaced the spring with a heartier cone spring at the hardware store... Everything seems to be working perfectly. I am going to keep it as is for now and hopefully this is a long term solution. Now i need to worry about what is causing the moisture in the first place.

Upon looking on the forum I see that the fluid is from the vacuum switch at the top of the transfer case.

 

Usually when fluid gets into the HVAC controls it's because it gets sucked through the vacuum lines from the transfer case. The vacuum switch on the top of the transfer case has a seal that's supposed to keep the fluid from getting to the vacuum but that seal fails and allows fluid into the lines. Another component of that failure is the transfer case input shaft seal failing and allowing ATF into the transfer case from the transmission.

Since you have a manual transmission you probably don't have ATF going into the transfer case, however, the transfer case probably uses ATF as a lubricant. If you have the standard three-button switch on your dash then you have the transfer case with ATF in it. The four-button switches are for the automatic transfer case which uses AutoTrak fluid instead of regular ATF.

If I were you I would definitely pull the vacuum switch from the top of the transfer case and check to see if there's any ATF in the connector or the lines. If that's the case then change the switch.