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Help! Brake light switch 700 1985

posted by  someone claiming to be Art on  Sun May 1 10:16 CST 2005 [RELATED]

I'm trying to fix my non-operational brake lights. (All 3 are out) On inspecting the switch at the pedal, I see that it has 2 wires connected and a vacuum (?) hose. When the pedal is resting (up) the switch is depressed and the contacts closed. When you put the brake on, they open! Strage, I would have thought the other way round. Also, when resting the switch vents the hose. When the brakes are applied, the hose is stopped up. What is all this stuff for ? Also, there does not appear to be any vacuum coming from the hose either resting or pedal depressed (switcgh removed).

Can anyone explain the workings of this system? Looks like the switch is functional, but is there a relay or something else I need to inspect?
1985 740 GLE







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    Help! Brake light switch 700 1985

    posted by  someone claiming to be ED7 on  Sun May 8 09:04 CST 2005 [RELATED]

    The big orange can at the back of the fuse block may be the problemo.
    The bulb out sensor. Do you have a bulb out light on your dash?
    If not, the sensor has crashed. Yes, I've been there.
    Have fun, ED7








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      Help! Brake light switch 700 1985

      uid 18083's pic
      posted by  lucid on  Sun May 8 10:01 CST 2005
      Author: [PROFILE] [EVERY POST] [THREADS] [BEST POSTS] RSS Feed     [IGNORE ALL POSTS]
      last visit: Fri Oct 24 12:31 CST 2014
      [RELATED]

      "Do you have a bulb out light on your dash?
      If not, the sensor has crashed."

      As others have said, the Bulb Failure Sensor could be stopping the brake light voltage. But with all/both brake lights out, there will be no Bulb Out warning light.

      This is because the BFS requires current flow thru one bulb of a pair in order to set a warning light.
      --
      Bruce Young
      '93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.








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    Help! Brake light switch 700 1985

    uid 27's pic
    posted by  Art Benstein on  Sun May 1 16:10 CST 2005
    Author: [PROFILE] [EVERY POST] [THREADS] [BEST POSTS] RSS Feed     [IGNORE ALL POSTS]
    last visit: Fri Oct 17 20:10 CST 2014
    [RELATED]

    >Can anyone explain the workings of this system?

    I don't have any 700s.

    Electrically, I think it works like the 240s, just the components are in different spots:

    Brake light power is fused battery. It is switched by a simple electrical plunger switch at the pedal. Then it passes to the lamp failure warning sensor, where the power is divided between left and right, so the sensor can compare the current in the two circuits. From the sensor it runs back to the tail light assembly. Cracked soldering in the sensor (big round plastic thing) is a common fault preventing any brake lights.

    Does the '85 have the third high-mounted brake light? I thought that was 86-.

    --
    Art Benstein near Baltimore








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    Help! Brake light switch 700 1985

    posted by  mcduck on  Sun May 1 11:27 CST 2005
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    last visit: Sun Sep 9 11:47 CST 2012
    [RELATED]

    I think you're looking at the cruise control switch.








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      Help! Brake light switch 700 1985

      posted by  someone claiming to be Art on  Sun May 1 13:01 CST 2005 [RELATED]

      So....where is the brake light switch? Is it mechanical or hydraulic ?







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        Help! Brake light switch 700 1985

        posted by  mcduck on  Sun May 1 13:14 CST 2005
        Author: [PROFILE] [EVERY POST] [THREADS] [BEST POSTS] RSS Feed     [IGNORE ALL POSTS]
        last visit: Sun Sep 9 11:47 CST 2012
        [RELATED]

        It's on the pedal, but it doesn't have a vacuum connection.
        The switch itself is classified as normally closed, but the pedal, when not pressed, holds it off.








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    Help! Brake light switch 700 1985

    posted by  someone claiming to be Art on  Sun May 1 10:18 CST 2005 [RELATED]

    Sorry, the switch action is to vent the hose when the pedal is depressed







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