I have come to the conclusion that my car's random switch from A/C to heat and back again is likely from the blend door actuator failing. Problem is, I am not exactly sure where it is, what it looks like and how hard it is to get to. Is this something that can be done with a minimum of disassembly, or something requiring the dash to be removed? I am planning a nearly 2000 mile road trip in a week and would love to figure this out before I go.

 

Okay, I found it and have it out. Since I hate reading threads like this that are not complete, it is located behind the glovebox assembly, which is held in place by 5 T20 screws. None of these bolt the hinge to the dash frame and all are accessed by opening the glove box door. One is all the way in the back, two of them bookend the latch and two of them are half way down the sides inside their own little alcoves. You will also need to disconnect the hose to the glovebox cooler and the plug for the light, but that's about it. There are two visible boxes near the middle of the car when you get the glovebox assembly out and you want the uppermost one with the actuator rod that goes to the blend door which is just below the level of the bottom of the glovebox. Also, in a pinch, you can disconnect the rod to the door and just duct tape or zip tie it into A/C or heat, whichever you need. Just don't pull the part the rod attaches to that moves the door out or else you may need to remove the whole dash to get it back in the right position.

Oh, and I found out that it is made by both a French and a German company...in Spain. Nope. Nothing to worry about here!

 

After disassembling the actuator unit, I do not believe it is the real issue. Why? Well, the way the gears are set up a only the motor can change the position of the actuator rod and it cannot simply be pushed or pulled into a new position. It has the motor with a worm gear on it driving a helical gear that is on the same shaft as another worm gear that drives a helical gear. The important thing is the worm gears as they act as a one way clutch. Because of the design of the "monkey in the middle" shaft, the final helical gear that drives the actuator rod will always hold it's position. The only possible exception is that there is a piece with a wiper that shuts down power when it has exceeded it's travel. If that fails then it may cause the issue I am seeing, but only if it is shorting between places. Sadly, this piece is not able to be disassembled and serviced as it looks to have been hot melted together. This puts me back at my electrical square one as it could be this piece or it could be the automatic air con controller. I would take a picture and post it, but my phone died and it needs a charge.

 

Blend door actuator assembly with cover removed

Potentiometer to determine position

Blend door Actuator with cover

Blend door actuator business end


Okay, so here is how the system works. The electric motor is just a standard $0.50 DC motor that runs on 12 volts. It is not a stepper motor as that would have cost real money. However, the system is position sensitive so that the actuator does not go too far in either direction. For this, we have the green switch with two wipers. Once it goes out of range of the wipers, the motor stops. It is not the switch that controls it directly but rather the BCM or maybe the climate control switch. Originally I thought that the three wires meant it was some form of voltage divider and therefore a potentiometer. However, I don't think it is that complicated, although I have not been able to confirm it either since I am all out of working Harbor Freight DVOMs! So it is either a potentiometer with bad carbon tracks or a switch with bad tracks that is giving bad data to the computer that then controls the motor. Either way, that little green module is the culprit. It failed like so many other similar devices in that the thin carbon tracks used on modern circuits just do not hold up to long term use like old fashioned copper ones did.