1. Re-circ door.
a. This door regulates between external air and internal air being pulled into the system. It works by blocking the external intake vent, forcing air to be pulled from the internal intake vent, OR blocking the internal intake vent, forcing air to be pulled from the outside. There is no in between. You can easily check the function by turning the fan on full blast and checking the air flow into the external vent between the hood and windshield. With re-circ off, you should feel a strong air flow into the vent and it should stop when re-circ is on. When the door fails, it hangs in the center and both the external vent and internal vents are open and air will flow into the external vent and out of the internal vent…like having a window rolled down all the time. If you feel air coming in under the passenger side dash, this is most likely the problem.
b. The photo shows the location of the re-circ door behind the glove box opening. The door is covered by a grate and is in the upper right hand corner. You can easily see/feel the door move when you have the system on and switch re-circ on and off. If the door doesn’t move or is missing, add this one to the list.
2. Blend door.
a. The blend door’s function is to divert some, or all, of the air flow through the heater core. The RAM uses an odd, complex method with two blend doors. The heater core is on a horizontal plane and two doors are sandwiched on the top and bottom of the core. In full AC, both doors close against the core(top down, bottom up) and block any air flowing through the core. In full heat, air is diverted through the core with the top door and the bottom door away from the core. This blocks normal air flow in the bottom of the box, forcing more air through the top and down through the heater core. The doors blend the air by partially opening and forcing some air through the core with some bypassing the core through the bottom door. The two doors are linked together and operate in unison to control temperature.
b. The symptoms of blend door failure are an inability to control the temperature of the air flowing through the system. A broken door will generally fall into the down position, blocking the core and having no heat in the system. The box is designed for either single or dual control of temperature on the passenger and driver’s sides. If there is a difference between passenger and driver’s side temperatures, the problem is with the blend door on either a single or dual control system. Of course this test assumes that the heater core is hot. You want to make sure that the radiator is full(radiator, not overflow tank) which you can check by opening the radiator cap on a cold engine. The radiator should be full, air in the system will greatly diminish heater operation and engine cooling. You can also check the heater hoses from a cold startup. The two hoses going into the firewall should warm up at about the same rate and get uncomfortably hot. This is a good indication that coolant is flowing through the heater core.
c. The RAM plenum box is built with dual air flow chambers so that the system can be configured for either single or dual control. On the single control, the blend doors are just connected together and operate as a single unit, but there are still two separate air flow chambers. The single control has a single actuator motor to control both sides of the HVAC system while the dual control has separate motors for either side. Air flows through the AC evaporator core before getting to the dual chambers and if there is a temperature differential across the core, it can show up as a difference in temperature on the two sides. Most auto makers constructed the core with refrigerant flowing from top to bottom on dual systems. With this setup both sides would be the same and a lack of efficiency in the core would be equally spread across both sides. Chrysler moves the refrigerant from right to left, so if there is a temperature gradient across the core, the passenger side is at the back of the core and will see less efficient AC. Charging the system may solve the problem and at least it should be checked. If the charge is too high or too low, it can cause a gradient across the core and it will show symptoms of warmer air on the passenger side. This is a design flaw in the system and there is little margin for less than perfect operation of the AC compressor system. You may have to “tune” the refrigerant charge, monitoring temperature on both sides, to get it perfect. If the refrigerant charge is OK, the next suspect is the blend doors. Dodge has a strange clam shell door arrangement for the doors in the RAM. There are two doors, an upper and lower that move in unison and cover the top and bottom of the heater core which is on a horizontal plane. On a single control system there are four different flaps(two sides/two sets of doors) that have to operate correctly. The plastic components are susceptible to breakage and it is an expensive repair to remove and repair the plenum box when this inevitably happens. The back door can break off exposing the top or bottom of the heater core to the air flow. The system will still operate, but the added radiant heat from the core can significantly affect passenger side cooling. The only way to really diagnose this problem is to cut into the box and observe the operation of the doors. Once you have the box open, you can also check the temperature of both sides of the AC evaporator core to make sure that it is cooling both sides efficiently. It takes a little work to get to the doors and check the operation, but once you understand the root cause of the problem, it can be restored to fully efficient operation.
3. Mode Door 1.
a. This door regulates air flow to either the dash vents or defrost/floor. If you have air flowing only through the vents with no defrost or floor, this door is failing. From our experience, this door is usually the first domino to fail. Not sure why, just is.
4. Mode Door 2.
a. When mode door 1 is set to divert air away from the vents and into the second stage of the system, door #2 chooses whether air goes to the defrost vents to the windshield or out through the floor vents in the passenger and driver’s side footwells. Failure is indicated by an inability to choose between defrost and floor.
b. This door turns out to be the most difficult to replace. In order to replace the door, the dash panel has to be removed. It’s still better than the dealer fix because you don’t have to evacuate the AC system, drain the radiator, and remove the plenum box. But it is more work than the other fixes. When this door breaks, it falls in the down position, blocking the floor vents and diverting all air through the defrost vents. From a safety standpoint, this is good since you’d most likely rather have a clear windshield than warm feet. We suspect that a large percentage of customers will just leave this door unrepaired. You will have heat through the dash vents and to the windshield, just nothing to the floor. It’s inconvenient, but you’ll have to make the determination if warm feet are worth the extra work to fix the door. An extra pair of socks is easier.
