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Thread: 1986 MGM 5.0...defog switch

  1. #1
    I post a lot... Toploader's Avatar
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    Default 1986 MGM 5.0...defog switch

    So nice day today and I put in a new (NOS) rear defog switch in my '86 Tudor.

    Question: Is it normal for the switch to be energized even though the ign key is lol...in my pocket?

    I thought the switch would only be energized with the ign key in the start position? When leaving he car I made double sure that the little tel-tale light is not illuminated.

    Thoughts???

  2. #2
    I'm an air-conditioned gypsy gadget73's Avatar
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    There is a supply of constant power and a supply of key-on power. The key-on pulls a latch that will let the defrost stay on. When the power is cut, the latch releases and the switch isn't supposed to hold. If you hold it in the up position it will come on but it should go out as soon as you let go of the switch.

    one fail mode is they don't release properly and the defrost stays turned on.

    86 Lincoln Town Car (Galactica).
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    I am praying a bad switch was the cause of my constant dead battery.

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    Its easy to verify. unhook the battery neg post and put voltmeter inline from cable to neg post on battery. if you have voltage draw, pull the fuse for the rear window defrog circuit and see if it goes away.

    There is a small voltage drain that is normal, someone smarter than me will post what that is lol.

    Good luck!
    1987 CV LX 5.0

  5. #5
    I'm an air-conditioned gypsy gadget73's Avatar
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    something like 10 milliamps. A stuck defrost switch will kill a battery flat dead in a few hours.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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    On a modern car, 50mA is about the upper limit of acceptable current draw. On one of these cars, Iíd expect to see very little as we donít have a billion modules all communicating over a CAN-BUS waiting to go sleep. A stuck defrost switch should be pulling MULTIPLE Amps, not milli-amps...like Gadget said, a stuck defrost switch will murder a battery in hurry.

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    On modern cars, I've seen 300-500mA initially after power down. After 20-60 minutes (depending on the car) it drops to 100mA or less. 100mA is not uncommon in more electronics heavy vehicles.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
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    ... and it should all work like magic and unicorns and stuff.
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  8. #8
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    Gents today marks the beginning of the third day since I put in the new switch. As of this morning the battery has not gone dead.
    The waiting continues! I think that after five days or so if the battery has not gone dead, then it was indeed a bad defog switch. Speaking of that switch, even Green Sales has not had one in quite some time..........I was fortunate to find just one on e-thief that was NOS....$20.00. On another note.......so many NYC drivers are scum..........came out and found the drivers side Q-panel (where it ends at the door) was pushed in by a vehicle with a high bumper like on an SUV, no the creep did not leave a note.

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    Quote Originally Posted by sly View Post
    On modern cars, I've seen 300-500mA initially after power down. After 20-60 minutes (depending on the car) it drops to 100mA or less. 100mA is not uncommon in more electronics heavy vehicles.
    Sly the thing that puzzled me was the fact that when tested the draw was almost zero (0.000.7) milliamp.

  10. #10
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    Even a digital or analog clock draws more than 7 milliamps (mA)...
    1985 Mercury Grand Marquis LS, "Maisa"
    2008 BMW 530d Touring, "Femma"

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    Yeah, I'm thinking the meter might have a bad fuse in it.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  12. #12
    I'm an air-conditioned gypsy gadget73's Avatar
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    if it wasn't dead in the length of a good dinner, the defrost switch isn't stuck. I can vouch for the dinner timing, old switch in my Towncar was bad, drove to my aunt's for Thanksgiving, it was dead as a stone by the time I was leaving, maybe 4 hours later. As soon as the jumper cables got hooked up I saw the yellow light on the defrost and knew.

    86 Lincoln Town Car (Galactica).
    5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley

    91 Lincoln Mark VII LSC SE, triple black (Timewarp) - poly front bushings, KYB struts and shocks, Holley SystemMax1 lower intake, SilverFox AOD valve body,

    1984 Lincoln Continental TurboDiesel - rolls coal

    Quote Originally Posted by phayzer5 View Post
    I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousers

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    Quote Originally Posted by sly View Post
    Yeah, I'm thinking the meter might have a bad fuse in it.
    Sly two different meters had the same readout.

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    Quote Originally Posted by gadget73 View Post
    if it wasn't dead in the length of a good dinner, the defrost switch isn't stuck. I can vouch for the dinner timing, old switch in my Towncar was bad, drove to my aunt's for Thanksgiving, it was dead as a stone by the time I was leaving, maybe 4 hours later. As soon as the jumper cables got hooked up I saw the yellow light on the defrost and knew.
    Thain the first thing I removed from the car was that switch, with the batt hooked up.........after 5 days I put the new one in, so to me it was the defog switch that was bad.

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    Quote Originally Posted by Toploader View Post
    Sly two different meters had the same readout.
    I've had 5 meters with blown current fuses at one time... so lack of checking the fuse due to multiple meters with the same goofy reading isn't a good excuse. Check the fuse. It's a sanity check. It could be fine and the mystery continues, but it may be the meters.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  16. #16
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    No problem will do, checked this morning battery still good.

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    Battery still good today!

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    Nice.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  19. #19
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    Sly may I "assume" that if after another 3-5 days and the battery still good then it was in fact a bad defog switch?

  20. #20
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    I would suspect that would be the case.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 93 Crown Vic LX (The Red Velvet Cake), 2000 Crown Vic base model (Sandy), 2003 Expedition (the vacation beast)
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

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