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78 Mark Heating Issues

Hello Bill,

I have a 1978 Mark V, and have a problem with the heating system. At first, the system was always heating, regardless of the position of the control knobs (Heating, defrost, temperature knob, fan speed knob). In summer, this was not great...

My mechanics, not very skilled in American electrical systems, just changed a Water valve (located in the engine, just near the air cleaner). After this change, I could close the heating, and I had vents, but I could not have heat! So, in winter time, this is also very great.

So I have two questions :
1 - Is there a simple thing to do with my new Water valve to get hot air ? (It seems to be controlled by a pneumatic wire)

2 - With your experience, do you know if there is a typical disease of the heating system? Should I change the two temperature sensors (inside and outside)? Are there parts of the system that should be automatically changed?

Thanks in advance




Grégoire -

Thanks for your recent inquiry. Your problem is not uncommon to us at Lincoln Land, but in 2009 it certainly is not very common at most AC shops even in North America. In France you are served with more than a triple whammy in this case.

First of all your new water valve may or may not be the correct one for the vehicle and there are several parts failures or combination of failures that can cause your ATC problems. Replacing suspected parts without proving a problem with them is not the way to go and is a costly and time consuming procedure at best. A reasonable knowledge of the system is a definite requirement for anyone attempting a repair. Anything else is pure guess work. The best course for you is to have a set of proper FoMoCo manuals available for your mechanic so that he can see how the system operates and where and how the parts are installed. We have manuals and most of the parts available if and when you may need them.

If you still would like to "try" some popular parts because of your situation and location we would be happy to accommodate you and send to your address the replacements for the "top three" most replaced parts for this condition. Please advise how we may assist you further in this matter.