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Old 07-28-2014, 12:04 PM
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Join Date: Jul 2014
Location: Medford, New York
Posts: 1
Unhappy 1988 560SEC Having Issues with HVAC Looking for Assistance

As stated above. I have a heating problem during the winter months.
If I leave the control on the low fan setting and turn the wheel to max (red) I receive a small amount of heat coming through the system. If I go to Auto or High the air coming out turns cold and during extremely cold weather I don't get heat. I really don't know if the A/C is working well enough but I seem to get AC again using the low setting. I haven't had any reason to go to auto or high on the A/C setting.

I know my way around this car pretty well since I've owned for over 20 years and have done 95% of any service it needed myself. I have the tools and grew up around gas stations when that was the place to have your car serviced and repaired so I'm pretty good with tools.

The car itself is an engineering marvel as compared to American cars and seems to be built to last a life time but you just have to know how to care for it and work on it.

I'm assuming that the Head Control Valve is the cause of this issue. I guess my first question is where is it located? I assume it's under the cowling between the firewall and the wiper system. I haven't had the time as yet to dig into it but am curious if there is someone out there who has experienced this problem and has any suggestions on how to cure the problem since I do use the car during the winter months and would really like to have the heater working properly.
Thank you in advance for any assistance anyone can provide.
Bob Kane

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Old 07-29-2014, 07:10 PM
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Join Date: Jan 2012
Posts: 7,515
I'm not familiar with these cars but, in general from what you describe, if the thermostat is working properly you have a clogged heater core / stuck mostly closed heater valve. For a test, run the engine to temp, heater on hot then feel the inlet and outlet heater hoses, they should be pretty much the same temp if you have good flow. If the outlet is cool, you have a restriction.

Do a search for Mono Valve , some of the older heater valves are rebuildable.

Generally, non cable actuated HVAC systems default to defroster and heat if there is an electrical / vacuum failure. ( I think this is a USA DOT standard )

The hot cold air flap might be stuck on full cold but I'd expect more heat than you describe.
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Old 08-01-2014, 09:21 PM
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Join Date: Feb 2007
Location: los angeles
Posts: 686
The valve is next to the battery, in false firewall area, 4 screws hold plunger with magnetic valve to main body, unscrew 4 flathead screws and lift up valve , inspect plunger , usually rubber seal is torn, you can get the part seperately, called mono valve insert.

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