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Discussion Starter #1
Had our first overnight low in the 50's this morning, so I decided it was time to test the operation of the heat/defroster, before the colder weather later in the year in my 2003 Impala. the heat was good and warm, BUT it only comes out the floor and dash ducts, it won't come out of the defroster ducts. What is the issue, and what do I need to do, to fix it?
 

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The mode switch is a rotary vacuum valve that directly applies vacuum to the appropriate vacuum actuator. It sounds like it's not switching vacuum to change from vent to any other mode.
 

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^ With a vacuum leak/break, does the hvac system default to defrost like Ford? Underhood vac. line breaks were common on my Fords.
 

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Not sure if there's a default mode for a vacuum loss.
There's two causes for improper air delivery:

1. Vacuum applied to the wrong actuator.

2. Vacuum leak.
 

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Discussion Starter #5
All I know, is when I had it in the defrost setting it was coming out of the floor and dash ducts, not the defroster ones. I've not touched anything on the HVAC system
 

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2 things to look at:

Is vacuum present at the back of the HVAC control head?

Does the mode knob actually rotate the vacuum switch on the back of the HVAC control head?
 

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@ My 2003 Impala, did you resolve your issue?

No. I can feel the mode switch detents as I moved it between the different mode selections. Still comes out of the dash vents, and floor only.
You need to remove the control panel mounting screws and pull the control panel out enough to see the back side. Test the switch operation again with the engine running. You should hear vacuum as you change the mode switch positions. The purple vacuum line is the vacuum source. If you don't hear vacuum as you change modes, trace the purple line toward the engine and test for vacuum at each connection point with the engine running.
 

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Discussion Starter #11 (Edited)
You need to remove the control panel mounting screws and pull the control panel out enough to see the back side. Test the switch operation again with the engine running. You should hear vacuum as you change the mode switch positions. The purple vacuum line is the vacuum source. If you don't hear vacuum as you change modes, trace the purple line toward the engine and test for vacuum at each connection point with the engine running.

How much of the lower dash trim do I remove to get at the control panel mounting screws?
 

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You need to remove the control panel mounting screws and pull the control panel out enough to see the back side. Test the switch operation again with the engine running. You should hear vacuum as you change the mode switch positions. The purple vacuum line is the vacuum source. If you don't hear vacuum as you change modes, trace the purple line toward the engine and test for vacuum at each connection point with the engine running.

How much of the lower dash trim do I remove to get at the control panel mounting screws?

See below for the IP Bezel and HVAC control head removal and replacement steps.



Instrument Panel
Cluster Trim Plate
Bezel Replacement

I/P Bezel Removal Procedure

1. Apply the parking brake.
2.Using a small flat-bladed tool, remove the
bezel from the ignition switch cylinder.
3.If the vehicle is equipped with a column
shift, position the transaxle shift control
indicator to 1. Keep the key in the ignition
switch cylinder.
4. Adjust the steering wheel for access.
5. Remove the steering column filler panel.
6.Remove the instrument panel (I/P) fuse
block access hole covers.
7.Remove the screws from the I/P cluster trim
plate.
8. Starting at the right end of the I/P cluster, grasp the trim plate and pull rearward.
9. Continue working around the I/P cluster trim plate until all of the retainers are released from the I/P trim pad.
10. Disconnect the electrical connectors from the hazard switch and from the traction control switch, if equipped.
11. Remove the I/P cluster trim plate.

Heater and Air Conditioning Control
Replacement

HVAC Control Module Removal Procedure

1. Remove the instrument panel trim plate bezel.
2.Remove the retaining screws from the HVAC control module.
3. Pull the HVAC control module out from the instrument panel.
4.Disconnect the electrical connectors (1)
from the HVAC control module.
5. Disconnect the vacuum connectors (2) from
the HVAC control module.
6. Remove the HVAC control module from the instrument panel.

HVAC Control Module Installation Procedure

1.Connect the electrical connectors (1) to the HVAC control module.
2. Connect the vacuum connectors (2) to the HVAC control module.
3.Install the HVAC control module into the instrument panel.
4.Install the retaining screws to secure the HVAC control module.
5. Install the instrument panel trim plate bezel.

I/P Bezel Installation Procedure

1.Position the IP cluster trim plate to the I/P trim pad.
2. Connect the electrical connectors to the hazard switch and to the traction control switch, if equipped.
3. Press the IP cluster trim plate into the I/P trim pad until the retainers are fully seated.
4.Install the I/P cluster trim plate screws.
5.Install the I/P fuse block access hole covers.
6.Align the bezel retainers to the tabs on the ignition switch cylinder.
7.Install the bezel to the ignition switch cylinder pressing into place until fully seated.
8.Install the steering column filler panel.
9. Return the steering wheel to the original position.
10. If the vehicle is equipped with a column shift, position the transaxle shift control indicator to Park and remove the key from the ignition switch cylinder.
11. Release the parking brake.
 

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Discussion Starter #13
Refer to the attached documents for the IP Bezel and HVAC control head removal and replacement steps.

Thanks! I did turn the mode switch with the engine running, did not hear any vacuum as i moved the knob to the different positions. So now I gotta look under the hood and see where the vacuum connections are at
 

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Discussion Starter #15
Any luck in resolving your issue?

Nope, I checked vacuum lines from the floor to the engine compartment and they were all ok. Looking like I will have to go ahead and pull the trim piece out and look at the back of the mode switch. Have to replace the fan switch anyhow as it only works on position
4, and 5
 

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There's a vacuum reservoir tank attached to the back of the HVAC control module. Check it and the lines to the control head for vacuum.
 

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Discussion Starter #19
Update: Was looking deeper under hood yesterday and found a vacuum line broken. I slipped a piece of hose over it, and now the mode selector works. Just have to fix the blower speed switch as it only works on positions 4 and 5
 

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Update: Was looking deeper under hood yesterday and found a vacuum line broken. I slipped a piece of hose over it, and now the mode selector works. Just have to fix the blower speed switch as it only works on positions 4 and 5
Kudos on fixing the vacuum leak.

The speed control issue is usually what's called the blower motor resistor. In some cases, they actually contain resistors; in other cases, it contains a more sophisticated electronic circuit for controlling the fan motor speed. Either way, as long as you get a "blower motor resistor", you should be fine.

Looking on Autozone's website, most of them are around 40 bucks, but there are two outliers: one at 12 bucks and another at 94.

The resistor is located near the fan, up above the passenger's feet, against the firewall.

HTH.

Doug

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