My 1996 was missing the know to select the direction of heat like the defroster or the floor. When I put the replacement in and turned both ***** (temp and position) to the off position i can hear a hissing sound from behind the dash. The hissing only occurs when in the off position or the first position (recirc)

Anybody heard about this or have any ideas.?

 

normal. at least for my 1995 it is. anytime I turn the **** to switch where the air gets directed to, I hear this. I think its just the vacuum opening or closing vents to direct the air.

 

Vacuum leak. (no, really, its a snake. ) Pull the HVAC controls, leaving everything hooked up, start the engine, see if you can find the leak.

 

What exactly do you mean by "Pull the HVAC controls"? I haven't done a lot of work with AC before.

 

The control head in the dash. The **** that controls where the air blows is vacuum powered. There is a bag o' snakes mess of vacuum lines there. One may have broken, or, it may simply not be sealing well.

 

Thanks I'll check it out.

 

I have not been able to get behind dash yet and check vacuum lines that suggested by HeyYou. I have another question regarding controls. Why would my compressor engage for about 5 seconds every minute when heater is on?

 

Because refrigerated air carries less moisture.... The compressor is enabled in almost EVERY setting... Strikes me as kinda silly, but hey, I wasn't consulted when they designed the system.

 

Yup... cycle the A/C to lessen the humidity in the cabin air (moisture condenses on the coil and drains out the bottom), and avoid drowsy winter drivers.

 

Well mine doesn't do this. Only time the compressor comes on is ac and defrost. May be a software flash or newer pcm???
You have a vac line leaking or a bad actuator. The actuator that closes the air door is to the right upper side of the dash. You really cant get to it unless you pull the dash. You may be able to get your arm in there by removing the stereo. Where do you hear the hiss come from? Left right or center of the dash?