Limiting Blend Door Actuator Travel to Keep the plastic arm from Breaking

billcu

Head Moderator
Staff member
I read an old post by Jeremi, showing how the blend door actuator goes too far and was trying to reproduce the test to see what temperature I shouldn't exceed to avoid having to fix this blend door problem again. I was testing this in my driveway with temperatures in the 30's (Jeremi was in a heated garage). I saw that when I keyed on, and set the temp to 65 degrees the actuator pegged at full heat even on manual heat mode. I had the black arm lifted up a bit to show how far past the stop, the actuator wants to go. This is what breaks the black plastic arm, no matter what heat you select above 65, when the car is cold, it will peg the actuator to full heat, putting that force on the plastic arm.

To keep this from happening, I disconnected the temperature sensor (a simple thermistor) under the message center keypad, and experimented with different values of resistance. I found a resistor value that lets me limit the temp on the heater control to 79 degrees, which is still full heat, but not fully pegging the actuator and putting the plastic under stress.

Here's a video showing with the heater set to 65 degrees and key on, the actuator arm is pegged to full heat beyond the stop when the temp sensor is connected. I use a heat gun to heat the temperature sensor, and you can see the actuator arm move back away from the stop as it heats up. With my resistor instead of the temp sensor, I can keep this from happening again and still have full manual control of the heat up to the limit of the grey arm blend door (see post below).


This shows how far the actuator wants to go when the thermistor controls it (I've lifted the arm up so it shows how far it wants to go past the stop).Too far with thermistor (Large).JPG

This shows how far the actuator goes now with the resistors and the temp set no higher than 79. It's just a hair from the stop. If you set the temp to 79 with the thermistor in the circuit, the arm will go to full pegged heat when the car is cold. Not anymore.

Limited travel (Large).JPG
 
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billcu

Head Moderator
Staff member
Still chipping away at this. Edit: the limit is the grey arm blend door, the black arm still has a little more to go to fully close I'll update this when I have come up with a fix for that.

While I'm waiting for a new grey arm, I glued up the old one to do some measurements on a spare HVAC. The actuator arm only moves only about 15 degrees (angle) before the grey arm door is open and closed.

I checked, the temperature on my heat control (without thermistor so temps are just relative now) needs to be set to 72 F to stop at the Hot limit set by the closed grey arm blend door. The Cold limit is at 68 F.

No wonder these plastic arms keep breaking, the actuator is designed to go much farther than when the door is fully closed or open.

I'm also testing this in my other gen 2 that I'm driving now. It's easy to see the small limits on the grey arm at the glove box, the heat works great and I know I'm not putting any stress on the plastics. I'll need to see what I can do to get both doors to close all the way to get that little extra heat without putting stress on the plastic.

In the pictures you can see the small angle between the hot and cold limits of the grey arm blend door, the black arm door could go just a little bit more closed if it wasn't limited by the grey arm door.

Hot, grey arm door closed:

IMG_7226Hot.jpg

Cold, grey door open:

IMG_7225cold.jpg
 
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tixer

Lincoln Evangelist
Staff member
Paid Member
You're doing great work on this, Bill. Thanks for all of the detail!

This could have been solved so easily at the beginning with a pair of limit switches. (mechanical, or better yet, optical.) I know it's all engineered to a price, but it seems like such a logical thing to want "feedback" on..
 

billcu

Head Moderator
Staff member
Thanks Mike, limit switches would have had to be part of the system, as it is stock, it is designed to break, with the actuator going too far and the grey door arm not going far enough. My resistors solved the actuator problem, and I just got done slotting the metal arm to handle the grey arm problem. I'll show a video of that soon.
 

billcu

Head Moderator
Staff member
I slotted the metal arm so the grey arm doesn't break when the black arm goes to the limit for full heat. Now with my controllable actuator I should be able to control the heat without breaking the arms anymore.

This video shows how the slotted arm works to let the black arm go to full heat now.

 
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