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Thread: Rear defogger mystery.... When does it activate?

  1. #1
    Join Date
    Aug 2014
    Location
    Scituate, MA
    Posts
    70

    Default Rear defogger mystery.... When does it activate?

    In order for the rear window defogger to activate must the temperature be below a certain number? I'm guessing about ten years ago I just couldn't get the small LED to light up after replacing all of the bulbs in the HVAC head. Later, on a cool damp night when I needed the thing to actually work it did and so did the LED on the HVAC control.

    I just finished replacing the bulbs again in that dual HVAC unit and I can't get the defogger LED to light up, but then it's a very nice 70 degrees. I did an extensive internet search on the subject and found a vague recollection like mine about temperature being a factor in order for the thing to work. Am I right about this or should I be shopping for a new head? Any way to test it?

    Yours,

    Bluemill

    2000 coupe, 6 speed, 124,000 miles

  2. #2
    Join Date
    Dec 2011
    Location
    MI
    Posts
    692

    Default

    Mine will activate in all temps but my car is a 1998 with manual HVAC controls.

    To try and get yours to come on, I'd look for the outside air temp sensor, get it cold and then try turning the rear defrost on. That would answer if its temperature controlled.

    1998 Six Speed, Torch Red Coupe

  3. #3
    Join Date
    Aug 2014
    Location
    Scituate, MA
    Posts
    70

    Default

    I ended up checking continuity of the grid, which was fine, then pulled the HVAC head again. I suspected that the LED of the defogger that I had put in 10 years before had a bad connection. Looked at it with a 10x jewelers loop and one lead looked like it had little or no solder on it. Re did it and tried it out, and it lit up great. I had put the non-OEM LED in ten years ago while replacing the unit's conventional bulbs. After soldering in the bulbs I went back with a 9V battery to check the connections. I got mixed up on location and hit the back of the LED and it exploded. I couldn't obtain the original LEDs anywhere so I got some from Radio Shack, 3mm in size. It worked sporadically over the years- sign of a weak connection.

    The rear defroster works great now, and so do the side mirrors, which I never knew heated up too.

    Yours,

    Bluemill

  4. #4
    Join Date
    Dec 2011
    Location
    MI
    Posts
    692

    Default


    1998 Six Speed, Torch Red Coupe

  5. #5
    Join Date
    Nov 2019
    Location
    Cape Coral, Florida
    Posts
    13

    Default

    Rear Defogger is not temp controlled whether manual or auto !!

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