1986 Mercury Marquis Defroster/Heater Control

TOMICHT

MEMBER
1986 MERCURY MARQUIS
V8
2WD
AUTOMATIC
195,000 MILES

I have lost the ability to switch to “defroster." When I slide the lever to “defrost" the air still comes out the vents and a very little amount of air goes to defrost the windshield. I still can hear little doors opening and closing by vacuum, but it does not sound like it did before.

Just a little history, I too have the heat goes away problem I’ve read about here. When I 1st bought the car, you could roast marshmallows with the heat. Then last year the heat started to fade. I did all the normal to fixes (i.E. Thermostat and insured hot water was flowing to the heater core), but like your site stated it’s the nature of the beast. As long as I could defrost the windshield I was fine, but with winter coming I need to get the windshield defrosted. Also, if this helps, I never had the “floor" option since I bought the car in 2002. When you slide the lever to “floor", all doors close and power is cut to the blower motor. It acts just like sliding it to “off."

What can I do or look for? If I have to, can I make a manual cable to by pass the vacuum? I have a good local junk yard and they have several 1983-1988 Grand Marquis, any parts I should go after or try?

Any help you can give will be appreciated.

Tom

1 Reply

MASTERTECHTIM

MECHANIC
4,749 POSTS

I have had a bunch of these with low heat and I found a blend door vacuum controlled behind the glove box that if disconnected gives full heat so I would unplug it in the winter and connect it in the summer and for the no defrost I would inspect all vacuum controilled doors to make sure the linkages are connected because thats where I think you will find your problem. I usually lie under dash and find all the actuators and then turn heater selecter and make sure all doors move then go from there