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Old 09-07-2019, 08:39 PM   #8
98_Gold_Uranus is a jewel in the rough98_Gold_Uranus is a jewel in the rough98_Gold_Uranus is a jewel in the rough98_Gold_Uranus is a jewel in the rough
98_Gold_Uranus's Avatar
Join Date: Jun 2019
Location: Lake City Florida
Posts: 121

1998 SL2
Default Re: Noise coming from under dash

You are correct sir. I 3D printed it with ABS plastic and a solid infill (I mention that because most things you 3D print are nearly hollow at about 20-50% infill since it takes so much plastic to completely fill up a part; this gear is so small, plus it's a load bearing functional part, figured the solid 100% infill would make it stronger as well).

Installed the motor today and all seems well Works great, and it was definitely the source of the sound I was hearing, as it's completely gone now.

This is the file that I found on Thingiverse

Says it was designed for a Mercedes, but the motor and gear looked identical in the picture, so I took a shot at it, and low and behold it is the exact same gear Even the same gear and motor assembly that's in my 2004 Blazer, that I've replaced the motor in once already for the same problem lol but that was before I had my 3D printer. I printed a second one and put it in my spare parts stash for when the onc in the Blazer inevitably goes out again

I pulled the blower motor off while I was in the area so that I could clean out the evap housing some, and I cycled the recirculation on and off a few times. After observing how the mechanism functions with the door flipping sides as the recirculation is turned on and off, I can see why the noise happens with the failed gear. Whenever recirculation is turned on or off, the motor runs and turns the gears and cycles the mechanism one way or the other. There's a physical stopped that's built into the assembly that stops the final drive gear and output from rotating a full 360 degree turn. The problem is, the motor doesn't actually shut off once it hits that stopped... The stopped stops the rotation, and the motor continues to push against the gearing. So once the gear snaps and looses it's grip on the metal shaft, the gear is free to spin around it. On every rotation, the gear hits a spot where it snags the metal shaft a little, which is what causes the noise to reoccur every 15-20 seconds. Because of the gearing, it's a very SLOW rotation on the final drive gear, that's why there's such a delay in the reoccurring sound. I've noticed that some of the motor assemblies that look nearly identical have an extra 3 pins on the plug and have a blue gear that's attached to a potentiometer in the unit that actually relays it's position back to some on-board computer system that controls the HVAC. I can only assume that with that position information that it actually stops power to the unit and keeps the motor from applying a constant force. Which you would think would prolong the gears life, but apparently not so much, as that link to the gear on Thingiverse, you'll notice that the Mercedes motor assembly has that extra gear, potentiometer, and 3 extra pins for the plug, and apparently it suffers the same fate if people are having to repair them too lol.

98 SL2, 70k miles, kept in the family since new!

Her name is Uranus :)


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