Hi,

I've been having a hiss lately inside dash when I turn on either A/C or Heat -
IT SEEMS LIKE'S it's literally coming from right behind Switch itself.

Here are some notes:

1 ) All actuators have been tested using mighty vac ( 4 ) - Pass, as in they pull and then let go naturally ( not sure why mighty vac won't hold pressure, never has , as it has a release trigger for vacuum)

2 ) Defrost does not go to full power

3 ) A/C works

4 ) Fan Speed Increase/Decreases Normally when selecting

5 ) I disconnected black wire that has straight connector ( by gas pedal ) and that made no difference - not that I expected it too.

6 ) All connections to actuators look good.

7 ) Drivers side under hood "3-way" hose connector looks fine. No Leaks.

8 ) I followed the hose that goes from brake booster area behind the distributor but kinda don't know where it ends up??? Engine was too hot, I can find the answer but thought I'd mention it.

So is it really possible the switch itself could leak at the back ? Do they ever fail?

If so, assume you just pull dash bezel off front, then access two screws to pull that "3" switch assembly out.

 

There is a cluster of vacuum hoses that plug directly into the back of the heater controls. So its very possible to have a hiss from one of those hoses directly behind the controls.


to get to the controls you will have to remove the panel under the dash on the drivers side then remove the dash panel under the steering column. then you can get to the 2 hidden bolts and the one above the cluster for the dash bezel. after you remove those 3 screws you have to very gently pull it off. Theses things are easy to crack and not cheap to find.....ask me how I know

 

Ok, thank you for letting me know. That's what I figured. I wish I could diagnose it with more certainty without having to rip it apart. Can these switches be fixed at all? I doubt it....

thanks for detailed instructions on how to get dash apart. I was so hoping a pod was dead as i have 4 of those.

I'll keep the post updated..

 

before you completely take the dash apart you may be able to check some of the vacuum hoses by dropping the glove box down and checking the hoses you can see. Just push the little metal clip in the upper left corner and the glove box should swing down out of the way.

 

thanks, i took all the panels underneath off first. Then found out the glove box easily comes out hoping that the pod there was culprit, I see the two of them there.

I also spent some more time under the hood, I followed the one vac line down to the transmission. Well I followed it to behind the engine but I recalled it must go down there....as I've read other posts of tranny fluid coming up that line to the 3 way plastic piece into the hvac system.

I also followed the 2nd of the rubber hoses that connects to the drivers fender area about 5" inches away. It seems fine however I can't actually see where it plugs into due to a bracket being in the way.

The other line goes to the engine - i believe it's the PCV valve off the drivers valve cover. This must be what supplies the VACUUM to the heater assembly. There must be one line under the dash that runs to the "switch" which supplies full engine vac. Then I assume as you move the switch it routes different vacuum to perform the function required. vents, down, below. etc.

So at this point

1 - under hood looks fine - the lines
2 - vac pods appear okay however I'm not testing that they hold vacuum
3 - running the car and changing the switch to different positions , i don't see a lot of vac pod movement so it like the vacuum isn't even getting to the various pods.
4 - this leads me to believe that the supply line to the switch could be affected.

Hence why the next step it to pull the dash carefully as instructed and access that switch, hopefully I guess seeing a vac hose fallen off? doubtful... so it most likely is the switch - $45 on ebay in usa - $140 canadian from parts store -

Gotta get this fixed so defrost blows on high for winter.

 

At this point I'm lead to believe it's behind the switch, does any know if the connector with the hoses plugs into the back of the switch? How do they connect?

I haven't removed it yet for inspection, but was curious what to look for if I buy used.