Originally Posted by fingershop

If this repair works as well for you as it did for me, you could save yourself from buying $300 of replacement servomotors!

Background

The Lexus GS A/C system uses two servomotors to continuously adjust the mix of hot and cold air coming into the car, to maintain the temperature(s) you have selected. One is used for the driver-side air temperature, and one for the passenger-side. Interestingly, these servomotors appear to operate even if the A/C is set to OFF mode, perhaps to maintain some air temperature control while the car is running. Other servomotors are used for controlling the A/C system, but the information here is not really about those.

Symptoms

If one or both of these servomotors has a problem then the temperature of the air coming out of your A/C vents will be hotter or colder than it should be. You can use your hand to test the air coming out of the two center dash vents, and because each of these vents is temperature controlled by different servomotors, you can more easily feel any differences in temperature, just by moving your hand back and forth between these two vents.

When one of these air mix servomotors is partially working, it often makes noises that can be heard coming from near the center of the dash. The noises can be described as a soft knocking, a burbling, a rattling, or similar. In this half-working state, the air temperature can be adjusted, somewhat, but it will often stray a number of degrees to the warmer or cooler side of what you've selected, and the temperature you feel can change at random times.

If the servomotor is not working at all then it probably won't make any noise, and you won't be able to change the air temperature on one side of the car, or maybe both sides if both have failed. In this non-working state, it can be stuck at any temperature, anywhere between the hottest and coldest possible temperatures. It may not be possible to repair servomotors that are in this state. The repair mentioned here has only been tested on partially working ones.

Servomotors

The servomotors used for mixing the air temperatures are fairly simple devices. Interestingly, they contain no active electronics of their own. Instead, the A/C computer has all of that. The servomotors contain a small electric motor, some nylon gears, a copper contact, and a potentiometer acting as a positional sensor. The A/C computer reads the position of this sensor, and tells the small motor to spin in one direction or the other. This in turn physically adjusts the air mix damper for controlling the temperature that you feel.

Note: It looks like there are different versions of these servomotors. See post #15 below for a description of two of them. You may have to modify the steps used here if your servo is a different style from mine.

The Problem

You can read about potentiometers on wikipedia.

Here's my best guess as to what causes the servomotors to partially fail.

After years of use, the potentiometer gets some of its resistive material worn off by the copper contact that touches it. This finely eroded material gets mixed with the light grease that coats the potentiometer's surface, which in turn causes the normally non-conductive grease to become somewhat electrically conductive, which makes the potentiometer's signal output vary from what it should actually be. The A/C computer reads this incorrect signal, and tells the servo motor to change its position, in an attempt to reach the correct value, a value that it can't reach due to the incorrect signal. This behavior happens pretty quickly, causing the motor to go back and forth, back and forth, in a jittering motion, which makes the noises you can hear. It also means that the servomotor will randomly drift out of alignment, due to the inaccurate positioning signal, and thus the temperature will be wrong as well.

DIY Repairing Air Mix Servomotors

See the main article on how to remove a servomotor.

Sorry I don't have any pictures of the repair process. When I have time to do my other servomotor then I can add some pictures to this article. Or someone else could post theirs.

In summary, you will open the case, clean some of the disc, clean the copper contact, reapply some grease, position the arm, and close the case. There will also be a few more steps, once you're ready to connect the servomotor back up, so don't miss those.

When you have the servomotor in front of you, you first need to open it. There are 6 tabs holding the servomotor case together. Be careful that you don't bend these tabs too much, or they will break off! And they break off easily! You should only bend them just enough to get the locking bump past the tab. It takes some careful prying of the case to keep the tabs from popping locked again, but its possible in a few minutes.

Once you have the case open, you will see the multi-fingered copper contact, the motor, the small gears, and the largest gear, which has the potentiometer attached, and which looks like a disc with charcoal stripes painted around it. These stripes are the conductive surface that the A/C computer gets its signals from. Also notice the light grease coating on the surface of this disc. If the grease was applied properly by the factory, the quarter of the disc that is actually used will show soft grooves in the grease, where the copper contact touches it. This is the part of the disc that we need to clean. You don't need to clean the other parts of the disc, and we need to use the un-contaminated grease from those parts of the disc later, so don't wipe it all off!

Okay, now for some cleaning. I used a small piece of a sturdy type paper towel. You need to wipe off all of the dirty looking areas of the grease, but leave the clean looking grease where it is. There are two places to wipe off. The quarter of the disc with those grooves in the grease. And the multi-fingered copper contact. Clean the dirty grease off of those areas only. Don't press too hard on the copper contact, if you bend it too much it may no longer press correctly against the disc, and your servomotor would no longer work.

After wiping the grease off the disc and the contact, you should dab some of the clean looking grease from the other part of the disc, and gently wipe it onto the quarter of the disc you had wiped off a moment ago. This will keep the copper contact from wearing into the disc too much.

That's it! Now you're almost ready to close the case back up. First you need to position the biggest gear, the one with the disc on it, and which also has the white arm attached to it on the outside of the case. You need to point this arm away from the electrical connector on the other end of the case. See the picture of the servomotor below for an example. This will position the sensor disc so that the A/C computer can properly initialize the servomotor.

Now, holding the arm so it doesn't move from where its pointing, put the case halves back together, pushing down evenly. Its okay if the arm is off by a slight angle. Verify that all the tabs have snapped back into place.

Next, you need to re-plug the servomotor into your car, but don't screw it back into place yet! First, simply plug it in, position the case so that the white arm is plainly visible, turn your car on enough to get the A/C controls activated, and adjust the temperature control all the way up and down to its highest and lowest settings, observing the servomotor's arm. It might take a few minutes of doing this before the servomotor starts responding in sync with what you're doing with the temperature control, perhaps due to the thin layer of grease. When its working correctly, the servomotor's arm will usually move with each press of the temperature control. Its okay if it doesn't move once you get past a certain temperature setting, since the A/C computer knows its already that temperature inside the car. Now that the servomotor is initialized, you can screw it back into place.

So there you have it! You've removed some contaminated grease, which was confusing the A/C computer, and making your servomotors behave badly.

Originally Posted by firelizard

The words "main article" are a link to said article.

This is the page it leads to https://www.clublexus.com/forums/the...vo-motors.html

Originally Posted by jagui03

i just recently join this platform and for some reason when i click on it, it does not allow me to view it. is there any other way i can see it?

Originally Posted by jagui03

i just recently join this platform and for some reason when i click on it, it does not allow me to view it. is there any other way i can see it?

Originally Posted by firelizard

Users can't move their own threads like that.

I could paste the content (any mod could) into a new thread, but there's obviously beef between the administration and that user and I'm not getting involved. If the admins want to release the DIY to the userbase, I'll leave it up to them.

Originally Posted by Oceans13

I remember not being able to see the DIY when doing my mode door servo. Now i finally know why. Some pics of the reassembly in case anyone needs