2008 Mercury Grand Marquis, A/C Stuck On Heat

This 2008 Mercury Grand Marquis came in with the complaint that the a/c temperature was stuck on full heat. The complaint could have easily been reversed with no heat present and the system stuck on full cold. It has Automatic Temperature Control (ATC). I used my scan tool and found that there was a code B1249 stored for a fault with the blend door actuator.

This repair will be very similar if not exactly the same for a Ford Crown Victoria or Lincoln Town Car of the same era. There are ways to check the system without a scan tool but some disassembly is needed. Starting at the passenger side of the dash, the center trim panel has to be pulled free of the dash carrier. It is held in place by spring clips.  On this one some extra effort and care was needed for the clips below the HVAC control head. There are also several switch harnesses that have to be disconnected so the trim panel can be placed out of the way.

Next, there are four screws with 7 mm heads that have to be removed to pull the HVAC control head from the dash.

The two motor controls wires need to be back probed and the leads attached to a multimeter set to DC volts. The wire colors are violet and brown/green. There are two wires of each color in this connector and I strongly recommend you obtain a wiring diagram to insure that you are on the correct wires. You may be able to tell by clicking on and enlarging the next picture. The violet wire is in the far left slot in the forward row as shown in the picture below. The brown/green wire should be in the second row, two slots from the far left. It will have an orange/black wire to it’s left side and an empty slot to it’s right.

There should be a nominal 12 volt reading across the two wires as the motor is commanded to move. The polarity will shift as the direction of travel is changed. A few things to keep in mind are that if a hard code is set, this signal may be missing or if the actuator motor is shorted the readings could be substantially lower. You may need to disconnect the harnesses from the HVAC control head assembly for a couple of minutes to clear the codes. You may also have to disconnect the harness connector from the actuator or both.

To test the position feedback circuit of the actuator you must back probe the yellow/green wire in the gray harness connector with all connectors fastened. It is in the forward row and third from the left. A nominal 4.5 volts indicates the full heat position.

While a nominal .5 or 1/2 volt indicates the full cold position. These test results were gotten after the new actuator was installed.

To gain access to the actuator the glove box needs to be removed. There are two 7 mm headed screws located in triangle slots at the glove box hinge. Then the box can be opened and removed from the dash. There are two stop pegs on either side that have to be maneuvered around to freed from the dash opening. With the glove box removed you can look up and to the left of the opening and locate the blend air door actuator on top of the HVAC box. You should also be able to disconnect the harness connector if needed for testing. It takes some finger tip coordination.

Unless your arms are skinny and you are very coordinated , some other parts will need to be removed to replace the actuator. Keep in mind the proper procedure involves dropping the column, removing the dash assembly and loosening the HVAC box. It is a 5.4 hour job if you have experience doing it. What I am going to show you is how it can be done with minimal disassembly in 1.5 hours or less. It does require having a good tool inventory, long fingers and good fingertip coordination.

The passenger side air bag will need to be removed and please follow the manufacturers instructions on how to do this safely. After disarming the system there are two 8 mm headed screws on the underside that have to be removed.

There are two 7 mm headed screws along the lower face of the passenger side air bag assembly.

There is one screw along the lower edge of the far right vent that has to be removed before the vent assembly can be removed.

The vent duct work needs to be removed. There are two screws that hold the duct to the inside of the dash carrier. They both have 7 mm heads.  For some reason the upper one has a zinc chromate finish (yellowish). I guess that was an easy way to distinguish it from the others  since the type of threads are specific to fastening into plastic materials.

The lower one has a black finish and is the same type of threads as the rest of the 7 mm headed screws removed to this point.

Perhaps the most physically difficult part of this job is removing and installing this piece of duct work. I was able to force the duct far enough to the right to be able to slide the left side forward and off of the main trunk. When reinstalling it I had to use a plastic pry bar and multiple screwdrivers to  get the duct in the proper position and then work the outer edges free so that it would slide back over the main duct work trunk. Luckily the plastic on the left edge is reasonably flexible and recovered it’s original shape.

The white flex hose is clipped into the left underside of the above duct. This picture was actually taken during reassembly but you should be able to get the idea.

It took a good bit of effort but the duct was finally free of the dash.

Now there is a lot more room to see and work. One of the front screws is easy to see but the rear screw needs a mirror to get a clear perspective of it’s location.

In the next picture I am showing the tool and the technique for removing the rear screw. Obviously the actuator is removed to give you a clearer picture of the procedure.

This is how it would look with the actuator in place. Even with this tool I was not able to fully remove the screw. I had to finish removing it with a pair of long curved jaw needle nose pliers. Some people advocate simply breaking the actuator at that screw location and then removing the debris and loosening the screw. I had good tools and did not want to take the chance of the wrong plastic pieces breaking.  If you attempt breaking that corner of the actuator, it must be done with the other two mounting a screws removed. Some earlier year models may have a total of four screws. By the way they all have 8 mm heads.

The most physically challenging screw to remove and install (installing is the harder of the two) is the one that is 3.5 inches to the left of the visible forward screw.  Installing the screw required me to use my left hand through the opening behind the radio and my right hand through the glove box opening. I have large hands but you can see in the picture below there is some physical discomfort involved in this job. I also had to use a curved body tool with a piece of vacuum hose attached in my left hand to hold my ratchet and socket in place on the screw head as I used my right hand to work the ratchet.

The squared corner of the duct work is what  makes it difficult. On the next car I may consider cutting it off and sealing whatever hole is left.

In the next picture you can see the white harness connector for the actuator.

Before installing the new actuator I cut a slot for the rear screw. This allowed me to have the screw installed but loose so that the slot would slide around the threaded part of the screw. Once the actuator was properly positioned all of the screws could be tightened down.

Below is an accounting of all of the tools that I used to do this job. Some are very specialized and many mechanics will not have them so it is highly doubtful that most DIYer will have them either.

The old actuator on the left and the new one on the right. Notice that I have not cut the slot for the one screw yet.

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