I have a 1969 Mark III, the air conditioner works okay but when I attempt to change the temperature I get absolutely no heat or temperature change. The system works in defrost also but with no heat at all. The water valve to the heater core stays open when I call for heat. Any ideas?
Assuming that your car has the more popular Automatic Temperature Control option I can tell you that it is a great system but can be complicated to diagnose and repair without the proper Manuals.
The most likely cause of your problem is a failure that is located inside of the ATC box that has caused the system to drive to the full cooling mode and remain in this mode regardless of the position of your control. A less likely failure could be the sensor string (these are sensors that are in a series circuit with the driver’s control that send temperature information to the ATC box so that it can react to deliver the correct air temperature, etc. to the car’s interior as selected). A broken wire in this string will send the system into the full Heat but a shorted wire or sensor will send it into the full Cooling position.
Other easier to check possibilities are as follows….Heater core bypassed due to coolant leak at core, Heater core completely plugged, water valve plugged on the inside or water valve incorrect. The main control switches can also be at fault but their failure is rare. Another thought to consider if you do not have much history on the car is the possibility that a previous owner or his technician may have tried some unknown ill advised repair and has altered the system in an unknown manner. This can and does happen.
It may be of interest to know that when these systems were new it proved to be a challenge for the FoMoCo and the dealer technicians such as myself to learn the diagnostic and repair techniques. Proper diagnosis is half the battle. Now forty years later we are learning something new almost every day at Lincoln Land when it comes to repairing the earlier electronic systems such as this is.
The real good news is that we at Lincoln Land have all of the parts and supplies to repair your ATC as well as the ability to rebuild your ATC box and Servo. If you are doing your own repairs, we can supply you with the very necessary and highly recommended shop manuals for your Mark III. We wish you a quick and easy repair.
Bill and the Lincoln Land staff