Jeep Grand Cherokee Dual Blend Door Control Analysis

Jeep Setup:

  • Dual zone control has two motors, one for the driver’s side blend door and one for the passenger side.
  • Driver’s side is easily accessible through the glove box, but passenger motor is located behind the heater box and is inaccessible without removing the heater box.
  • The motors both have a limit pin that constrains movement between two stops molded into the heater box. Force from the motor during calibration is absorbed by the limit pin.
  • The axle connector to the blend door is a 5/16? nylon rod with two grooves cut into it to mesh with the door. The length of the grooves (effective connection contact) is ?”
  • Connection contact is not symmetric and must be aligned for insertion into the door.


  • First experiment was to attach a metal blend door to the axle by inserting the axle connector into a 5/16? ID tube with the door attached and cinching the door to the axle using a set screw. Limit pin was cut from the connector so that full force was applied to the axle and door.
  • Door functioned correctly and was able to withstand the force from driving the motor to a stall point. No lateral movement of the door was observed, but strain was evident at stall point.
  • Door was planned to go through 25 calibration cycles to evaluate reliability of the connection. Incremental slippage was observed and total failure occurred at 21 cycles. Picture shows groove worn in the axle and complete stripping of connection.
  • The nylon axle is not strong enough to avoid deformation from a set screw.

    Nylon Axle
  • Second experiment was to machine a connector that would fit the axle pattern with the full 1/2? possible contact length, and weld the blend door to a tube with this connector on the end. Connector was a tight fit, but would slide on without excessive force required. Again, limit pin was cut.
  • First observation was a lateral movement of the door at the stall point. The nylon axle had a twisting action at the stall point and would force the door away from the motor, causing the door to scrape against the far side of the door tunnel. Experiment was done with the driver’s side door and no passenger side door in place. Passenger side was added to provide resistance to the driver’s side lateral movement. With both doors in place lateral force was observed, but was constrained by the presence of the passenger door. There was still some deflection which reduced the margin between the door and edge of the box, but the door moved freely with no scraping.
  • System was taken through 25 calibration cycles with no failures, but lateral movement appeared to continue and door resting position moved 1/16? away from the motor.
  • Calibration cycling was continued due to small changes observed and the axle totally broke at cycle 31. Picture of the broken axle is shown. Note that the axle is hollow and the break point was into the hollow area just outside the ?” connection area. There didn’t appear to be a gradual failure, it just snapped. Temperature was in the 50?s that day, so the fail point may take longer in warm weather or conversely less time in cold climates.


The limit pin on the axle is the first point of failure. When the pin shears off, the force of stalling the motor is transferred to the blend doors themselves. The second point of failure is the plastic blend door. The plastic breaks where the connection is made to the axle and the doors fall off resulting in a loss of temperature control.

A secure connection of a metal blend door to the plastic axle using a set screw will not withstand long term stresses. The nylon material is too soft for a secure connection and the set screw will strip the axle.

Using a machined connection to attach to the axle has three major problems:

  1. The axle is capable of rotating a full 360 degrees, but the door has movement between the two stall points of 85 degrees. The axle is keyed and must be position within the 85 degree window for the machined connector to be inserted. On a broken door, the axle can stop in any position and the chances are 85/360 (23.6%) that the axle will fail in a position where the connector can be inserted without indexing the motor. On the driver’s side this is easy since the motor is accessible, but indexing the passenger motor is difficult. The passenger side door won’t fit until the axle is indexed with the 85 degrees of door movement.
  2. The nylon axle is flexible and will twist when force is applied. With a smooth fit connection, this action tends to force the door away from the motor. This lateral movement can be constrained against the partner door and into the motor connection on the opposite side. The system appears to be strong enough to sustain this force, but alignment of the doors and force against the opposing motor could be problematic over time.
  3. Complete failure of the nylon axle was observed after 31 cycles. The GC runs the cal cycle every twenty times that the ignition switch is turned on. Complete failure would be expected after ~600 engine starts. Dependent on frequency of use, we would reasonably expect the fix to last 2-18 months. In the picture above, an intact GC axle is shown next to the same axle from a Ford F150. The Ford does not have a limit pin and the system is designed for the blend doors to absorb the force of the calibration (doesn’t work any better than the GC system…the door still breaks). The Ford axle is designed to withstand the force of the motor and is 7/16? diameter with 1? effective contact to the blend door socket and made of a hardened phenolic plastic, compared to a 5/16? diameter 1/2? nylon GC connector. The GC axle is designed to operate with force constrained by a limit pin, not the axle.

From the above evaluations done in 2006, we at HeaterTreater elected not to at tempt to develop a solution using the nylon GC axle to withstand the force of calibration. On the passenger side door, the motor is not accessible without removal of the heater box and the limit pin would be known to be broken if the blend door is broken. With a broken limit pin on the hidden motor, we do not believe that a reliable connection can be made to the axle stub and if(when) the nylon axle breaks, the full system must be disassembled and the back connector replaced.

In our opinion, the only viable fix for the GC is to connect the blend doors together and use the back axle stub as a pivot point for the axle. Once the back stub breaks, it cannot be used as a pivot point and a single control solution is not possible. Our solution solves the blend door problem and cannot create further damage that would result in a very expensive repair. On our Ford products, we do rely on the strength of the axle connection to withstand calibration because it is designed to withstand that force, and we have gone through 100 stress cycles with no evidence of lateral movement or breakage. The GC system was not designed to withstand the same forces and a hollow nylon axle will break over time.

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