AC mode switch / vacuum troubleshooting

[DMC5180]

Actually had it running when I was testing...

Weird.

Oh and I meant vacuum is required to close the floor vents.

[Rich_NYS]

I'm working on my HVAC mode selectons, and have an issue that seems to only affect airflow to the footwell as well as the selections being a bit wonky.

I tested all modes, heat & cold. I'm looking for some help diagnosing, and help with understanding what to expect with each selection.


Max, Norm, and BiLevel: [HEAT setting] Center vent (low flow) and Driver's Door .....[COLD setting] Center Vents & Doors (high flow)

Vent: [HEAT setting] Center vent (low flow) and Driver's Door .....[COLD setting] Doors (High flow)

Heater: Only blows out the doors on HEAT or COLD setting

Defrost: [HEAT setting] Windshield (low flow) and Driver's Door .....[COLD setting] Doors


Observations:
-No air out the footwell vent on any selection
-Slight hiss on the Heater selection
-Defrost seems low-flow

Thanks!

[Rich]

There's a lot going on here.

First, the airflow/ducting velocity at each outlet isn't supposed to depend on whether you choose HEAT or COLD on the temp knob. That knob only controls the air temp so I can't figure out what's causing different flow situations with just a temperature adjustment. So I will set that aside for now.

Second, if you're getting low/zero flow to the floor ducts in HEAT mode and you hear a hissing sound in that mode or any mode you probably have a worn-out mode control switch. Simple to rebuild it if you take your time.
Houston and others have the parts. LINK: Example parts

Recommending you start with that. A bad mode switch can affect more than one of the duct control vacuum motors. A hiss from the A/C control area is almost certainly a sign of a bad/worn mode switch diaphragm.

Extra hint: Whenever you check your face level vent flow be sure both of the vent thumbwheels are in Open mode.

I'm working on my HVAC mode selectons, and have an issue that seems to only affect airflow to the footwell as well as the selections being a bit wonky.

I tested all modes, heat & cold. I'm looking for some help diagnosing, and help with understanding what to expect with each selection.

Max, Norm, and BiLevel: [HEAT setting] Center vent (low flow) and Driver's Door .....[COLD setting] Center Vents & Doors (high flow)
Vent: [HEAT setting] Center vent (low flow) and Driver's Door .....[COLD setting] Doors (High flow)
Heater: Only blows out the doors on HEAT or COLD setting
Defrost: [HEAT setting] Windshield (low flow) and Driver's Door .....[COLD setting] Doors

Observations:
-No air out the footwell vent on any selection
-Slight hiss on the Heater selection
-Defrost seems low-flow

[Ron]

Refer to the .PDF in post #2. Disconnect each line going to the switch and confirm they will hold vacuum, except #1 (black=supply). A leak involving port #5 (black/Red) could cause a lot of what you are reporting. So could the flap it operates being stuck. If all OK, use the chart at the bottom of the PDF to check switch operation using a cigar etc. (Or see Lou.)

[Rich_NYS]

Great help, thanks guys.

I made sure the center vents are open and shined a light inside to confirm the flap was opening & closing.

Saturday I'll do as Ron suggested, and will also prepare to replace the switch diaphragm Rich mentioned.

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[Rich_NYS]

I have a lot of things apart; working on my wiper issue, and HVAC.

I checked all the hoses, they all appear to hold vacuum, and I can hear most of the actuators working (others I can hear/feel changes in the airflow when the selector is changed.

Tomorrow I'll go further with the testing, but I also noticed the hissing noise is fairly significant (albeit brief) when I turn off the car, so that seems to be indicating the switch as Rich mentioned.

[Rich_NYS]

I'm pretty sure it's the mode switch, I have it out where I can manipulate it and if I squeeze it to where it doesn't hiss it seems to function properly.

I'll rebuild it and see how it goes.

[David T]

If the mode switch hisses it is worn out. Even if you can get it to be quiet, it can still be leaking, it just isn't making any noise. Most of the time, if you have a problem with the actuators and the mode switch is hard to turn or is hissing, if you rebuild it, all of the problems are fixed. The other common problem is a vacuum hose that got disconnected. Just be careful to put the wires and hoses back correctly. Make notes or take pictures. Be gentile with the plastic parts. They are brittle and can break easily. On one car I fixed the vacuum feed hose was pinched under a console support but if your mode switch is hissing you've got vacuum.

[Rich_NYS]

Thanks David, I'm going to rebuild the switch this week and will post results.

[Rich_NYS]

Rebuilt it today; I think it's better (everything seems to work,) but now whistles on the heat setting and hisses worse than before. WTH.

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