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Discussion Starter · #1 ·
On my 2004 Envoy XL, there is a knocking or tapping sound for 15 to 25 seconds, after starting the engine. It does not happen all the time, but mostly on cold start. This sound is coming from the Passenger side rear of the SUV, behind the 3rd seat and underneath the side window, and behind the plastic panel. I can feel (barely) the knock when touching this area.

Another issue (not sure if related) is that I can practically feel no air (Hot or Cold) coming from the front panel. Defrost works fine, so does the Floor vents. However the front vents do not blow air. A search on the web pointed to a defect with the Actuator or one that needs to be recalibrated after a battery disconnect. I pulled & replaced the HVAC-B fuse (10 amp) and it did the trick but stopped working next time I restarted the truck. BTW: I pulled the fuse while the truck was running.

thanks in advance, sky
 

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I think given the other HVAC actuator confusion, it may be a rear HVAC actuator issue. Time to pull the panel and watch the two actuators back there, and see if they're making the noise.

I'd guess a bad HVAC control unit or class II data bus issue confusing the communication. Try pulling the HVAC-B fuse again with the truck off. Then turn on the ignition only without starting the vehicle or touching the HVAC controls for 2 minutes. See if that recalibration will stick and allow the mode actuator to give you dash vent airflow.

While the recalibration happens, the blower fan should go through all its speeds, and the mode actuator will blow through each of the vents in turn. Then you can try manual mode and see if you can force it to go to the dash vents only.
 

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TO: The Roadie

This is on a 2003 Trailblazer.
Hi, I have the same knocking sound and the rear passenger a/c vents were not blowing. I followed your directions. After taking the hvac 2 fuse out the knocking stoped. At that point I was unable to use any of the a/c or vent controls. Is there any thing else I can try or is it off to the repair shop. Also I was unable to take the interior quarter panel apart at this time, it will be quite a chore. Currently I am on vacation and will have to wait until I return home. If I do manage to take apart the interior, is this fix something I will be able to do myself or do I need a pro?
 

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When you put the fuse back in to force the recalibration, not touching the controls for two minutes, did it not get any better?

It is a trivial job to replace the rear actuator. Tougher to change some of the four in the dash. But the rear is easy. And a lot cheaper than the dealers charge. Let us know when you get home and get the panel off.

Your mode actuator is item #3 in this drawing. GM part# 88892975 available from parts4chevys.com for $48. ACDelco #15-72811 - $33-40 from other online vendors you can Google for.

 

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TO: The Roadie

After I Posted the reply thread I went back and tried a few more times, following your directions. At first, after letting it sit 2 min. after pulling then replacing the fuse there was no knocking, but then as I increased the fan speed it would again start knocking. Then I changed the air circulation before attempting the procedure and I no longer have the knocking and the vents are working. I am not sure how long this will last. Is this just a temp. fix or will the recalibration last? If the problem persists I will let you know how the repairs go. Thanks for the specs.

Bryan
 

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Discussion Starter · #7 ·
To: Roadie

Roadie: you said, "While the recalibration happens, the blower fan should go through all its speeds, and the mode actuator will blow through each of the vents in turn. Then you can try manual mode and see if you can force it to go to the dash vents only."

Clarification: Will this "recalibration and the blower fan & mode actuator blowing thru each vent" happen when I turn the key to ON position or when I Start the Envoy?

I am going outside and following your directions exactly and report back.


BTW: the knocking sound had not appeared again for a while now. The only change I have made is, REPLACING the BATTERY.

thanks
 

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The recalibration and diagnostic is run first time you turn the key ON after you pull the fuse I mentioned and put it back. Or disconnect the battery, which does the same thing as pulling the fuse plus it also erases a lot of learned parameters from the PCM so I don't recommend doing this for simple HVAC problems.

You may have a sticky actuator or a sticky door in the plenum. WOrks sometimes and not others. There's nothing like pulling off the panels and watching the actuators go through their paces.

It will also happen when you start the vehicle if you do that instead of just turning the key to ON, but I recommend just turning the key to ON because you can listen to the activity and sometimes learn things from the noises.
 

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Discussion Starter · #9 ·
Knocking Sound from back of the truck

Well, it's been several weeks and the knocking sound is no longer there. I do get the knocking sound when I pull the HVAC Fuse-B (last any where from 45 seconds to couple of minutes), in an attempt to reset the ACTUATOR for the Front Panel Vent air flow. The knocking sound may occur for a day or so (when first starting the SUV); but finally doesnot happen any more.

Front VENT Air-blowing issue: I have not been successful in getting the actuator to reset by pulling the HVAC Fuse-B (10 amps) and following the procedure as provided in the forum. I think that the actuator may need to be replaced. I have read many many threads on this topic with some general directions... Here is what I think I need to do (at a high level)...
for the 2004 GMC Envoy:
the Actuator is behind the dash on the driver side;
STEP 1: remove the lower panel;
STEP 2: remove the duct
STEP 3: Locate and remove the ACTUATOR with "White Tab"

following directions in the thread below:
http://forums.trailvoy.com/showthread.php?t=10926&highlight=mode+door+actuator&page=4

keeping my fingers crossed and will attempt this procedure and see what comes of it. Will take pictures and post for future reference. At the same time, if anyone has any ideas or recommendations, I am all ears.

jack :crazy: :bonk: :duh: :hissy:
 

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Discussion Starter · #10 ·
Mode Actuator - 2004 Envoy SLT

5.27.2009: Status update...

today, spent some time looking into getting to this mode actuator. (To access the Mode Actuator, you have to get to it from the driver side).

REMOVED the following two items (to date)...

1. SOUND SHIELD: This is the very first cover that should be removed. There are 4 screws... Two that hold the cover to the panel, and two that secure the OBD II interface. There are 2 other components that will have to be removed to completely remove the SOUND SHIELD from the vehicle. One of the component just requires you pinching the connector on one side and it comes out easily. The second component is very easy as long as you take time to understand how it is connected to the SHIELD. Picking on one of the tab will release the component and you will be able to slide it out. This should release the SOUND SHIELD. Pretty simple procedure; NOB's should be able to do this.

2. Cover under Steering Wheel: This cover is pretty straight forward, with screws (four of them) in plain view. This cover comes off easily once four screws are removed.

This is how far I got to today...

I did get some time to look for the ACTUATOR!!!, before calling it a day. I had to get on my back (under the driver side) to see UP on the Left Hand Side. There is a Air VENT (near the accelerator pedal) and the acutator(S) are on behind this vent. The Vent has a sensor attached to it and it also feeds the Air VENT on the left foot area. First (with some force) I was able to release the Air VENT (next to the accelerator) with some pushing and pulling. NOTE where this vent extends to the left foot area.

Once I was able to see the two actuators, I then started the truck and changed the mode setting and actually watched one of the actuator turning the "METAL shaft"...

PROBLEM FACING RIGHT NOW: Could not get the AIR VENT out!!!; it wraps around the accelerator pedal in the back.

I am tempted to spray some wd-40 into the shaft area of the actuator, and see if that helps it turn all the way.

Thoughts/Comments?

TIA... jack

:eek::eek::rolleyes::confused::duh::bonk::bonk:
 
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