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Come along with Eric O. as he does a quick diagnosis and repair of this Mercury Grand Marquis that has a problem with the automatic heat. It seems when the customer goes up hill the heater switches to "defrost" mode. Although a modern car it still uses vacuum to switch modes on the HVAC. Let's see how to determine what is wrong. -Enjoy!
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48 thoughts on “Mercury grand marquis – switches to defrost when going up hill”
  1. Avataaar/Circle Created with python_avatars 06MERC says:

    Thank you!

  2. Avataaar/Circle Created with python_avatars John Lyons says:

    eXACTLY what I needed. Thanks,

  3. Avataaar/Circle Created with python_avatars Rob Mitchell says:

    Thanks for taking the time to post this. Very helpful!

  4. Avataaar/Circle Created with python_avatars Patrick Sterling says:

    Thanks. Mine works fine but good to know what to do just in case.

  5. Avataaar/Circle Created with python_avatars Scott Ramey says:

    Awesome job! Very helpful

  6. Avataaar/Circle Created with python_avatars Dwayne Stimpson says:

    I have yet to figure mine out. Mine nothing changes, it blows out the face vents continuously, a little out the floor and defroster. No matter what mode I select, it never changes. Driving me crazy!

  7. Avataaar/Circle Created with python_avatars MacGyver 2 says:

    Good demonstration. Thanks

  8. Avataaar/Circle Created with python_avatars James McQueen says:

    Great video and a great repairman also. Wish you liver close to me.

  9. Avataaar/Circle Created with python_avatars Ron Peterson says:

    I just replace the O-rings myself and saved $1000

  10. Avataaar/Circle Created with python_avatars Len B says:

    I am a new subscriber and I love your show! Keep up the great work!

  11. Avataaar/Circle Created with python_avatars Donnie Robertson says:

    Great job again like always

  12. Avataaar/Circle Created with python_avatars Kameron Stout says:

    My 94 F150 did the same thing the vacuum hose was broken under the hood I just slipped some hose over it works awesome now

  13. Avataaar/Circle Created with python_avatars AngeredKabar says:

    Why do they use vacuum for actuation?

    It's free real estate

  14. Avataaar/Circle Created with python_avatars Jason Sage says:

    Hey buddy thanks for the solution to fix this on '02 F150 Supercrew. Now maybe there is an easy fix for my temp control issue now, bought brand new OEM blend door actuator and put in a new blend door but I have very limited temp control range such as all the way cold and maybe a few temp degree increases such as 70, 75 and 80 is all the way hot and it should be all the way hot at 90* I tried recalibrating the EATC and didnt seem to do anything and there isnt any codes….

  15. Avataaar/Circle Created with python_avatars Latnlvr says:

    I refurbished my eatc unit myself. Replaced all the o-rings internally and only spent $13 and did it in an hour. DONT GET ROBBED BY A MECHANIC THAT WANTS YOUR MONEY. It wasn’t hard, don’t be afraid.

  16. Avataaar/Circle Created with python_avatars JOHN JORDAN says:

    my air blows out vent and defrost at same time 2005 mercury marquis ls

  17. Avataaar/Circle Created with python_avatars Brian O'C says:

    Didn't read all the comments, but there's gotta be a way to dicker the head unit to fix it's fault! I'd play Frankenstein to try to fix it. You should try it and show it. But the damn thing has quite the core as you said. I hate the fact that parts are designed to fail!

  18. Avataaar/Circle Created with python_avatars Brian Beard says:

    Blend Door Actuator is a friggin nightmare. 35 dollar part that costs 600 to get too. 1000 if you are stupid enough to go to the dealer.

  19. Avataaar/Circle Created with python_avatars Mike Chiodetti says:

    Changed a few of these. Great Video! Installed a few of the round plastic vacuum containers on vehicles that had none. That kept the system working. The check valve and LONG vacuum hose are no substitute for the vacuum canister.

  20. Avataaar/Circle Created with python_avatars TheWicklunds25 says:

    Where can a guy get a vacuum tester like the one in your video?

  21. Avataaar/Circle Created with python_avatars Duncan Mac says:

    Now that is one heck of a core price. Definitely want to turn that in unmolested….Thanks for the video.

  22. Avataaar/Circle Created with python_avatars Donnie Robertson says:

    Another A+

  23. Avataaar/Circle Created with python_avatars Greg Johnson says:

    Have a 98 ford with the same problem I decided i could live with it but after watching this just might drop the car off and tell you to call me when its fixed lol

  24. Avataaar/Circle Created with python_avatars MrSlicer2424 says:

    My 2003 grand maquis A/C switches from vent to floor/defrost under acceleration. Then back to vent. I've seen remanufactured control units on ebay for $120. Should I take the risk?

  25. Avataaar/Circle Created with python_avatars Richard Cowtails says:

    Ohh good…. a DOORMAN……………………………… lol

  26. Avataaar/Circle Created with python_avatars rimmersbryggeri says:

    They use vacuum because they have changed very little since 1953. Do they still operate the windows by vacuum aswell?

  27. Avataaar/Circle Created with python_avatars Ozzstar's Cars says:

    The Grand Marquis is a very popular car at the senior center where I go to play bingo on Tuesday afternoons.

  28. Avataaar/Circle Created with python_avatars InternetDude says:

    If you put in silicone O rings it would be good for the life of the car. Doing it your way means more $$ for you in the long run when the aftermarket one leaks again down the road. The key here is silicone.

  29. Avataaar/Circle Created with python_avatars Ray Milligan says:

    ford Maverick used it, vacuum actuated abortion that lived!

  30. Avataaar/Circle Created with python_avatars jimbola77 says:

    good job partner!!! so the control module was fried???? thank you for sharing God Bless.

  31. Avataaar/Circle Created with python_avatars Steve Turk says:

    Vacuum actuator park brake? Is my hearing faulty? Curious now. 👍👍
    Poops, there it is….”hey dude, there’s your problem..not your hearing but your eyesight!!” LoL 😂

  32. Avataaar/Circle Created with python_avatars TheMadSciencetist says:

    Man, you my favorite digital mechanic. Ever thought of doing an on site series. I have a Titan that needs a blend door actuator replaced. It is a serious pain…anyways carry on and keep fighting the good fight

  33. Avataaar/Circle Created with python_avatars HUBBABUBBA DOOPYDOOP says:

    You make installation of rubber exhaust hangers easier with a spritz of brake parts cleaner… I wonder if it would rejuvenate the diaphragms in the HAVC Programmer Module? My '89 Reatta uses the vacuum system, too. Same color vinyl hoses, the '88-'89 Programmer is a small box, behind the glove box, between the ECM and BCM. The '90-'91 Reatta is in the dash like this Vic. My '77 Cutlass had a similar vacuum system, as many cars did/still do. I think they are pretty reliable, and simple to service; on some models- the actuators are inaccessible unless the dash is removed. Why don't you like the system? I do admit- nothing beats a lever and cable control… the good 'ol days!

  34. Avataaar/Circle Created with python_avatars movieguy73 says:

    i think its the o rings that fail. thats why they want them back.

  35. Avataaar/Circle Created with python_avatars Walter Shulas says:

    Hey Eric great video is that a dorman unit

  36. Avataaar/Circle Created with python_avatars chad harmon says:

    Ford ran those hvac controllers till 2011 especially Ford police cars and grand marquis

  37. Avataaar/Circle Created with python_avatars Daquan brown says:

    Know your glad you ant have to do a heater core on it!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  38. Avataaar/Circle Created with python_avatars throttle bottle says:

    think'd id let the vac hose hang out the dash and connect a small loop for whatever setting i wanted. or use a series of "fish tank" air valves if you want to get silly 🙂

  39. Avataaar/Circle Created with python_avatars Jack Tradesman says:

    I love working on Fords! GM and imports make me money… Ford builds the good stuff. Imho

  40. Avataaar/Circle Created with python_avatars Dalton says:

    Loved the video, the part price is pricey.

  41. Avataaar/Circle Created with python_avatars sledge hammer says:

    Yeah your a fart smeller , I mean a smart feller

  42. Avataaar/Circle Created with python_avatars Windy Abbie says:

    Had the same problem on a 01 Ram Van never did figure it out, wish I saw this video a few years ago

  43. Avataaar/Circle Created with python_avatars Amariah Edwards says:

    Hey, I got a 2005 Grand Marquis and the heater doesnt work in the car. Whenever you switch from cold to hot, the dial just keep spinning as if it not attached from the inside. AC work great jus no heat, any suggestions ???

  44. Avataaar/Circle Created with python_avatars 73 SUPERGLIDE says:

    They hope u 4 get 2 return it

  45. Avataaar/Circle Created with python_avatars amtpdb1 says:

    I have a 04 Lariat and with the air on low sometimes driving down the road the fan goes to high speed. Where would you start?

  46. Avataaar/Circle Created with python_avatars Stephen Cowell says:

    Great video… subbed. They definitely make it so it's not worth your money to tear the unit open… the core price is gonna drive up the junkyard prices too. Don't get me started on '90s Taurus VAPS or blower motor speed control… enough to say that when I sold the Taurus there was a blower speed switch in the glovebox.

  47. Avataaar/Circle Created with python_avatars dassme1 says:

    Hey Eric Dorman sells a rebuild Ford heat/ac controller 😂😂😒