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Hello Bill,
I have a 72 Mark IV. The AC is acting up since I bought it. Sometimes it works great, others it goes on and off like it’s reached temp, then decides it hasn’t. I’m not sure where to start. It appears as thought the compressor and associated units are working properly, I suspect it’s just not getting accurate input from the controller and/or any thermostats that regulate the system. What’s the most common issue and where would I start?
Thanks in advance,
Michael
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Greetings Michael –
The Automatic Climate Control systems of this era are very complicated and do require much patience and a very good understanding of how they operate in order to repair it yourself.
You do not provide very much information (for me to work with) as to what your Mark IV’s ATC is or is not doing. If you have the correct FoMoCo factory manuals you should however begin by reading them carefully and understanding how the system operates. Next you can perform the diagnosis procedures that are shown in the Manual. Many vehicles that we service at Lincoln Land have two or more issues. Since you ask, I will list some of the common Control issues that we see and repair here at our shop. Keep in mind that the refrigeration section is of course separate from the ATC control system and could also be inoperative or weak. Be advised however that there really is no single item that is a more common fail item than the others. When you can provide more information for us we may be able to lead you much closer to the problem area or areas.
ATC Box (internally)
Range Relay
High Blower Relay
Blower Motor
Blower Motor Resistor
Main Control Switch
Sensor String
Any vacuum motor (that operate the mode doors) and the blend door
Poor wire contacts or burnt wires connections at certain circuits
If the ATC system has been tampered with or altered previously, It will need to be restored in order to diagnose properly. We have the ability to supply or service most faulty items that you prove to be faulty. If there are certain parts that you are unable to test, we can offer to test them here if you wish. Good luck to you with the testing and a speedy repair.
Sincerely,
Bill