Having no interior heat on the 1997 to 1998 Lincoln Mark VIII is a common problem. The problem is a part that Ford calls A/C Electric Blend Door Actuator ( F7LZ-19E616-AD) that costs only about $25, but requires over 6 hours of labor to replace. The cost to install this part at a dealer is $600 to $900. It would be far better to make the part so it does not break so easily. The part nearly always breaks at the actuator arm shaft shown below.
Above picture courtesy or Randy "goalie" Hall.
This article describes a simple procedure to reinforce the piece at its weakest point. Apparently the part has been improved by Ford over what was original equipment on the cars. The original part number on the motor assembly was F7LH-19E694-AE. It is now F7LH-19E694-AF. On the part with the AE extension the black arm can be pulled off the shaft, on the newer part with the AF extension don't even try to pry it off. You will break the arm, as LOD member Mark Hannah discovered.
We will reinforce the weak points of the assembly from the outside and the inside as follows:
First wrap the base of the black arm shaft tightly with wire. Twist the ends of the wire and snip off the ends close to the base. Make sure the ends do not protrude outwards as to cause interference. Coat the wire with fresh mixed epoxy to hold it in place.
Choose a nail that can go most of the length of the common hole through the white pinion gear shaft and the the black arm shaft. The common hole is at the bottom of the square hole on the motor pinion shown above. I used a common 2-inch finishing nail.
Coat the nail with fresh epoxy that you mixed up. Do not worry about filling the square hole, as it is not functional in the Mark VIII application. Push the nail into the common hole.
Use another nail to push the head of the epoxied nail into the bottom of the square hole.
If you are curious, these are the internals of Blend Door Actuator Motor:
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