Tech Tips
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The HVAC system blend doors.
Blend door is short for Blend Air Door. There are other names such as temperature door, etc. The blend door, blends or mixes the hot air with the cold air. In the 99/04 Jeep Grand Cherokee there are actuator motors that open and close the blend doors a little at a time to adjust how much hot air and cold air to mix for the temperature setting you have chosen.
If your blend door or doors are broken, sometimes you will only get cold air coming thru the vents or maybe just hot air with no air conditioning. The common symptom of the broken blend door is - No heat only cold air from the passenger and or driver's side vents.
What causes the blend door problem?
The problem starts when the actuator motor during the calibration routine, puts excessive force onto the plastic Stop Tab of the drive connector and it eventually breaks off.
The calibration routine happens every twenty ON/OFF ignition key cycles. The computer calibrates the 99/04 Jeep GC HVAC system for a number of reasons.
A rough explanation of the calibration process as it pertains to the blend doors.
The computer turns on the actuator motor and moves the blend door to the upper opening. The computer keeps the motor on until the tension on the motor causes the amps to rise. The computer reads the rise of the amps and now knows where the upper opening is. It repeats the same process for the lower opening. The computer now knows where both the upper and lower openings are.
Along with the sensors and the temperature the system is set at, the computer can adjust the blend doors to let the proper amount of air flow thru the heater core or bypass the heater core to blend the air to the preset temperature.
Depending on how often you use your vehicle, the calibration force could break off the stop tab within a few years. The only thing that breaks on the drive connector is the stop tab that is located outside of the heater box. The shaft of the plastic drive connector does not break. After the stop tab breaks the force of the actuator motor is now transferred to the shaft of the drive connector. As the blend doors do not break at the next calibration point after the stop tab broke. There is an undetermined period of time that could be months or it could be years. During that period of time the actuator motor is putting tension on the drive connector that is in turn putting tension on the blend doors. During these calibration times the full calibration force of the actuator motor is put onto the drive connector shaft and onto the blend door shaft. What ends up happening is the blend door shaft will break.
A typical pair of broken blend doors
The plastic drive connector has withstood the full calibration force of the actuator motor and caused the blend door shaft to break. There will never be anymore force put onto the plastic drive connector then at that period of time after the stop tab broke and before the blend door broke. Which means the plastic drive connector will withstand the force of the actuator motor during the calibration periods without a stop tab. It does not matter how strong the new blend doors are, the plastic drive connector will withstand the maximum calibration tension the actuator motor can put on it.
We have tested the torque strength of the plastic drive connector and have full confidence that the drive connector is strong enough to last indefinitely as a pivot point connection for the Blen Dor Single Control unit and will last indefinitely as a drive connector for the back blend door of the Blen Dor Dual Control unit.
Everyone has concluded with why the blend doors shaft breaks. The stop tab breaks off first from the tension that the actuator motor puts into it during the process of the computer calibrating the HVAC system.
Everyone has come to the conclusion that the blend door shaft is the second and last thing to break. That there is an undetermined period of time before the blend door shaft is broken by the plastic drive connector. That period of time could be months or it could be years.
My Competitor also concluded with the fact that if the drive connector at the back side of the HVAC box is broken. The Competitor’s single control and his dual control repairs will NOT work! (See Competitor's quote below)
As the Competitor’s set screw and steel tubing design is attached to the back drive connector as a pivot point. If the back drive connector breaks! My Competitor’s customers will have to take the loss. OR My Competitor might tell his customer’s that they have to remove the dashboard, and remove the HVAC box and do everything else involved with that process. Possibly telling them to purchase his dash removal product.
An exact quote from the Competitor’s website
The following quote from my competitor came after a page or more of describing how he did all his testing along with all sorts of his numbers, etc. This was taken from the last paragraph of his conclusion.
“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.” (Emphasis added)
Again, that so called engineer is Wrong.
Wait, actually, the guy is right, his products, are attached to the back drive connector as a pivot point. If the back drive connector breaks the competitor has NO backup system to repair the problem.
You can not use the Competitor's Single or Dual control repairs if the back drive connector is broken.
It would not matter when the drive connector broke, before the repair, during the repair, or after his repair has been completed. If (when) the back drive connector breaks, my Competitor’s repairs are useless, no good, throw it away.
With the competitor's repairs you will have to do exactly like the Competitor said, “the full system must be disassembled and the back connector replaced” (Emphasis added)
Now is a good time to add the old sayings that Designers & Engineers keep in mind.
"The more parts there are, the more things can go wrong”
“If something can go wrong, it will”
As I said above, that so called engineer guy is Wrong.
The Blen Dor Store has designed an engineered a very easy No Dash Removal repair for the back drive connector, for the VERY REMOTE chance the back drive connector did fail.
(As any good engineer would have done! "Be prepared and overcome any problems that could arise!")
The Blen Dor’s no dash removal drive connector repairs were designed along with and at the same time the Single Control and the Dual Control repair kits were designed.
The drive connector repair was designed for, if (when) the customer’s back plastic drive connector was broken before or during the repair process, the Blen Dor Store’s blend door kits could still be used.
The drive connector repair can be used before, during and after the Blen Dor repair kits are installed.
The Blen Dor Store has a no dash removal repair for ANY problem that could ever happen to the 99/04 Jeep Grand Cherokee blend doors and their drive connectors.
The Blen Dor Store's customers will not have to resort to removing the HVAC box to repair any problem with the blend doors and their drive connectors.
A few customers have broken the back drive connector attempting to do the repair without the proper instructions!
Maybe they were looking at videos & still pictures! Not the proper instructions, basically correct, but a lot of important information is left out when a video ends by showing the start of a process and then another video starts at the end of that process. A lot of important things happen between the start and finish of a process.
Short videos and a few still pictures are not the proper instructions.
You should not attempt to do the repairs without the proper instructions.
Is your back plastic drive connector broken?
Did it break when installing or after my Competitor’s Repair Kit has been installed?
Are the Competitor’s set screws in a thin wall tubing design giving you problems?
If you answered Yes, to any of the above questions.
Consider one of the Blen Dor repair kits, as the Blen Dor Store’s customers do not worry about those problems.
The Blen Dor products are User Friendly & Worry Free.
- The Diagnosis of a Blend Door Problem
The common symptom for the blend door problem is. The passenger side has lost heat, only cold air comes out of the vents. Than the driver’s side loses heat (If not yet, the driver’s side will eventually lose heat) and you do not have control of the temperatures, it just blows out cold air no matter what the temperature is set at. This is the usual symptoms for the blend door failure. Sometimes the driver’s side will lose heat first. Either way you will not have control of the temperature. There are variations on the symptoms. Basically if you know that the heater core is circulating hot water and you know that the AC is working, but you can not control the temperature. That is the major symptom of the blend door failure.
There are two ways to check for a blend door failure if you have the Grand Cherokee with the Automatic Zone Control system (AZC). The first is you can read the fault codes of the AZC system.
To read the AZC fault codes:
- Turn on the ignition switch.
- Turn on the AZC system. The temperature settings should be displayed for both sides.
- Simultaneously hold down the AC and the Re-Circ buttons.
- Then at the same time, turn the driver’s side temperature (Left) control knob to the right (CW) one click. Release the AC and Re-Circ buttons.
- The code numbers will display one at a time with a short delay in between.
21 or 53 – Left temperature door travel to small.
22 or 54 – Left temperature door travel to large.
23 or 55 – Right temperature door travel to small.
24 or 56 – Right temperature door travel to large.
All of the above codes indicate a blend door problem. 54-56 are the most common.
If you have a 15 or 47 – Left temperature door not responding, chances are the driver side actuator motor has failed. That motor can easily be changed.
If you have a 16 or 48 – Right temperature door not responding, than chances are the passenger side actuator motor has failed. This motor is very hard to replace as it is in back of the heater box and to replace it you have to remove the dashboard and the HVAC box.
If the actuator motor does not rotate when you turn on the system, that does not mean the motor has failed. The computer shuts off the motor if it makes one revolution with no resistance.
If you have the “temperature door not responding” code, the actuator motor could still be in good working order.
The second way to check the blend door is manually.
If you have the Manual Temperature Control (MTC) you will have to check the blend door manually. If you have the Grand Cherokee with (AZC) dual controls checking manually will only check the driver side not the passenger side blend door. You will have to rely on the fault codes for the passenger side.
If you have the Laredo the manual temperature control (MTC) single control heating system, you will check both of the doors manually, as they are jointed together onto one shaft.
To check the blend door manually:
1. Lower the glove box. There are two plastic tab stops sticking up past the top of the back wall of the glove box (Circled in red) that hit against rubber stops. (Circled in green) Fold these rubber stop pieces to the side(They are attached only on one side so as to act as a hinge point)
Below is the rubber stop showing red direction arrow
2. Remove the dampener arm on the right side of the glove box by sliding the end up and out of the key slot. . (Circled in red) The glove box should be hanging down at this point.
3. You should see an actuator motor to the left of the glove box opening. Remove the wire connector at the top of the motor. Remove the two screws holding the actuator motor in place.
4. Turn by hand the white plastic drive connector that the motor was attached to. It should have a little less than a 90 degree turn. If it turns more than 90 degrees, the stop tab and the blend doors are broken. The white plastic drive connector below has had it's stop tab broken off. After the stop tab breaks, there is an undetermined period of time that it takes the white plastic drive connector to break the shaft of the blend door, causing the broken blend door problem.
- Failure of the Stop Limit Point on the Blend Door Housing
The indents in the blend door housing’s upper and lower Stop Limit points, happened during the testing of the narrow steel roll pin.
The decision was made to use a stainless steel roll pin for the new stop limit pin, as it is the easiest, quickest and the least expensive means for attaching a stop limit pin to the new drive connector.
After twenty five mock calibrations – Which would be approximately six months in real time – the testing was stopped.
It was evident that the indent being as deep as it was at the 25th calibration (6 months). In one year the depth would approximately double. The stop limit point on the blen door housing had failed.
The reason the stop limit point failed is that the stop limit pin for the blend door shaft is a narrow round pin, which will cause it to indent easily into the soft plastic 1/8" wide stop limit point.The actuator motor calibration tension will transfer to the blend door shaft as the round stop limit pin indents deeper and deeper.
Eventually the full calibration force of the actuator motor will be transferred to the blend door shaft.
The Stop Limit Pin alone was not a reliable option as the black plastic housing is soft enough to be indented by the stop limit pin.
The decision was made to design the blend doors themselves to be stronger than the force it would take to completely stop the actuator motor, with NO stop limit pin. The stop limit pin would be used in conjunction with the blend doors as a secondary means of absorbing the tension from the actuator motor.
Chrysler designed the stop limit pin (or tab) design, which has failed miserably.
And the so called engineer guy Competitor has copied that same failed stop limit pin design.
The Blen Dor design does NOT rely on the failed stop limit pin idea.
The Blen Dor design does NOT rely on the failed set screw idea.
- The Strength of the White Plastic Drive Connector?
An explanation of the calibration forces as it pertains to the white Plastic Drive Connector that connects the blend door to the actuator motor.
The actuator motor and the drive connector’s torque strength have been tested, the results are.
The torque from the actuator motor to calibrate the system, is 10% less then the torque it takes to completely stop the motor.
The amount of torque it takes to break the drive connector is 25% more then the force it takes to completely stop the actuator motor.
The final results are the white plastic drive connector is 35% stronger then the calibration torque of the actuator motor.
- A few Facts on how the calibration forces are transmitted
What causes the blend door’s shaft to break, is the torque that the white plastic drive connector is putting on the blend door’s shaft. (Fact)
Why this force is on the drive connector shaft, is the stop tab on the drive connector breaks. (Fact)
The force of the actuator motor, instead of being on the stop tab is now being put on the drive connector shaft. (Fact)
The drive connector shaft is transferring that calibration force to the blend door shaft. (Fact)
The blend door shaft is now putting that force onto the edges of the opening in the blend door housing, the computer will then register the rise of the amps from the actuator motor. (Fact)
The blend door shaft does not break the very next calibration time after the drive connector’s stop tab breaks. (Fact)
There are no specific amount of calibration times it takes for the drive connector to break the blend door shaft. (Fact)
It could range from 15 to 75 or more calibrations - 3 to 12 months or more
So that period of time between the drive connector’s stop tab breaking, and the time the drive connector breaks the blend door’s shaft, all the calibration torque from the actuator motor is put onto the plastic drive connector. (Fact)
These facts, plus the torque strength tests done on the motor and drive connector, show that the plastic drive connector has an indefinite live span and is stronger than the calibration force of the actuator motor.
- Short Picture Summary of the Installation Process
Summary of the Installation Process
Lower the Glove Box – See the above description of how to lower the glove box.
Remove the Driver's Side Actucator Motor
Cut the Outer Wall
Cut the Inner Wall
Rotate the drive connector into position
Slide the new Blen Dor Dual Control passenger door in place
Put in place the driver side door and plug in the new drive connector
Replace the Outer Wall piece
Apply aluminum tape and install the actutor motor- The Diagnosis of a Recirculation Door Problem
The two common symptoms are: You feel a draft coming from under the dashboard on the passenger side when the vehicle is moving. The other symptom is, you have the fan turned on HI and there is little or no air blowing thru any vents!
There are two ways to check for a recirculation door failure if you have the Grand Cherokee with the Automatic Zone Control system (AZC). The first is you can read the fault codes of the AZC system.
To read the AZC fault codes: See above description
19 or 51 – AI (Recirc) door travel range too small
20 or 52 – AI (Recirc) door travel range too large
The above codes indicate a recirculation door problem. #52 is the most common.
The 14 & 46 – AI (Recirc) motor was not responding. That is the recirculation door actuator motor failure. To replace this motor you need to remove the dashboard. Or see the no dash removal recirc door fix.
The second way to check the recirculation door is manually.
To check the recirculation door manually:
1. Lower the glove box. (See description above)
Notice the grayish glove box rubber stop in the upper middle edge of the picture below.
2. Look to the top right side of the glove box opening, you should see a grid like plastic area. That is the bottom of the interior vent for the recirculation housing.
3. A flash light would be helpful at this point. If you look thru the grid into the opening you should see the foam covered door. If you can not see the door it most likely has fallen flat onto the bottom of the recirculation housing area and the reason no air is coming thru the vents. The picture below shows the recirculation door housing, with the recirc door in the normal position. The normal postition is blocking the interior cabin air from entering the system and letting the exterior fresh air enter the system.
4. If you do see the door, turn the recirc button on and off and check to see if the door is moving properly. If the door is just hanging there, the shaft of the recirculation door could be broke. Or if the door does not move and you have the 14 or 46 fault code number, the actuator motor could be the problem.
There is also a No Dash Removal fix for the recirc door actuator motor failure.