Hvac blend door issue, sound behind dash

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Hvac blend door issue, sound behind dash

Postby bigBADbenny » Sat Mar 07, 2020 8:45 am

Hvac blend door actuators.

Symptoms: bubbling, murmuring, buzzing, sound, noise behind dash on startup, after key off, often mistaken for improper coolant system bleed, air pocket in heater core.

The difference being the noise happens on key on, and after key off.

There’s two actuators on the rhs of the passenger footwell, on the hvac airbox which contains the heater core, ac condenser and the blend doors that actuate when the mode button is cycled.

Two potential issues:
The tab on the blend door motor has popped out of the track/cam that moves the blend door: reseat it and test the mode button.

Or the motors position sensing is worn out.

Carefully remove the motor, replace it with a working item new or s/h.

Or open the motors gear case. Slightly bend the actuator position sensing fingers inside to fresh track.

Add some quality, plastic friendly grease, probably silicone grease or food grade grease.

Reseal and reinstall the blend door motor and test using the mode button.

Iirc I tested it before I replaced all of the trim, just to be sure.

Iirc this can be a dash out procedure on LHD cars, but always consult the FSM regardless.

Link from the forum first hit:
https://www.google.com.au/search?biw=49 ... 1vrDUmz7z4


Here’s a pic from when I did mine.
viewtopic.php?f=10&t=20939&p=356707&hilit=fixed#p356707

Here’s the FSM workshop manual link on the forum for procedures:

FAQ: Subaru FSM Service Manual Parts Procedures Diagnostics • club.liberty.asn.au

viewtopic.php?f=6&t=34980

Tools: trim removal tools, a good 1/4” metric ratchet set with hex bits, sockets and extensions, phillips screw drivers in short and long.
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Re: Hvac blend door issue, sound behind dash

Postby Kimmo » Sun Mar 07, 2021 3:11 pm

So yeah, mine was making the stupid noise and pointing the air at random, had a go at this.

Here's a rundown for what trim needs to be removed:

Lift up the front of the back half of the centre console. Pop out the gear lever surround (or whatever for autos), remove the silver trim and non-ashtray, then you can get the left side panel off the front half of the centre console.

Remove the dash side panel (where the passenger door closes up to the dash), and the bit of trim under the glovebox, which can then be removed. Or do this before the other part, doesn't matter.

Now it's all exposed; you just need to unplug the loom and you can get that bastard servo out. I didn't use a ratchet handle, just a stubby screwdriver, and it was doable but no fun. Reassembly even more so.


Anyway, once you've unscrewed the servo, best to plug it back in and test it to make sure its feedback has crapped out - maybe the followers just popped out of the cam. There's a little clip holding it onto the output shaft; careful when you remove it. Here's what you see when you crack it open.
hvac 1.jpg
hvac 1.jpg (118.92 KiB) Viewed 3728 times


My car has 180,000-odd on it, but my tracks were far from worn through. Plenty of spare grease inside the unit too - you can just reposition the old stuff instead of digging up more.
hvac 2.jpg
hvac 2.jpg (146.74 KiB) Viewed 3728 times


I figured the brushes were simply no longer pressing hard enough, so there was no need to bend them off track, just to tweak a bit of extra life back in.
hvac 3.jpg
hvac 3.jpg (119.62 KiB) Viewed 3728 times


Tested it, and sure enough, that did the trick.I got it into my head that the adjacent servo was also playing up (turns out it wasn't; I'd mistaken it for the fresh/recirc servo), and the above pic is from that one - for some reason it has more brushes. Anyway, when I tried to pry the arm off the output shaft, I discovered the hard way it was glued on. Fixed it by sacrificing a 7mm drillbit and slathering it in superglue.
hvac 4.jpg
hvac 4.jpg (135.62 KiB) Viewed 3728 times


Also, bonus discovery - another pair of mystery plugs, this time the same colour (unlike the pair inside the driver's kick panel). Anyone wanna hazard a guess what happens if you plug them together?
I wasn't game in case it initiated the self-destruct procedure or something...
hvac 5.jpg
hvac 5.jpg (150.93 KiB) Viewed 3728 times
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Re: Hvac blend door issue, sound behind dash

Postby bigBADbenny » Sat Mar 13, 2021 7:43 am

Excellent writeup and pics, thanks very much!

Yep it’s simply a second set of diagnostic test mode connectors, the other set is around the cabin fuse box area.
When one pair of connectors is temporarily plugged, eg the radiator fans will cycle etc.
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