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specialty wiring question: brake pressure switch

Discussion in '40- & 55-Series Tech' started by DSRTRDR, Feb 25, 2007.

  1. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    In the OEM configuration: is the wire to the brake pressure switch at the master cylinder always hot or only hot when the ignition is on? I don't seem to be able to figure it out from my FSM wiring diagram :frown:

    Thanks in advance, Claudia
     
  2. Pin_Head

    Pin_Head

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    In general, the brake lights run off a circuit that is always hot regardless of the key because they are up there in importance with headlights. The empirical way is to just check with the key off.
     
  3. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    brake lights, yes, always on

    but the pressure switches at the master cylinder?
     
  4. Pin_Head

    Pin_Head

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    I haven't seen a '72 schematic that shows the two brake warning pressure switches, but if you have them, they run off the switched ignition side of the fuse block (heater fuse) on later years.
     
  5. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    Thanks, Charlie

    right now they are always hot - I'll have to change to switched wiring then :rolleyes:
     
  6. IDave

    IDave

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    That on my '71 is switched, else the brake light would stay on when I pull the parking brake and leave the vehicle.

    The brake switch gets two hot wires. One is switched, off the same fuse as the heater blowers. The other is nonswitched, and comes off the horn fuse.
     
    Last edited: Feb 25, 2007
  7. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    The brake switch itself works fine; I have brake light on when I operate the brake pedal.

    On the brake pressure switches, I checked again: I have the power to the brake pressure switch when the ignition is on, and no power when ignition is off (as it should). The brake indicator light in the dash works fine for the handbrake, but if I connect wiring to both brake pressure switches on the master cylinder, the brake indicator light stays on (even if I release the handbrake) and only goes out when I operate the brakes.

    It doesn't matter whether I switch the two wires from cylinder to cylinder, and if I connect only of the wires to either master cylinder, the brake indicator light comes on (as it should, I believe).

    Would presence of a proportioning valve in the rear circuit (after conversion from drums to disks all around) lead to different fluid pressures right at the master cylinder and hence brake indicator light coming on? Is there anything in the setting of the brake switch at the brake pedal itself that would influence the circuit for the brake pressure switches?
     
  8. Pin_Head

    Pin_Head

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    My understanding is that the pressure switches sense a pressure loss in front and rear circuits which would occur if there is a leak. These switches are normally closed with no / low pressure in the cylinder. They are both in series with a separate pole of the brake switch, which is normally open. In other words, the switches should only be "hot" when you step on the brake. When you step on the brake, it energizes the pressure switches and if there is no presure in one or both circuits, it completes the circuit and the brake warning light comes on.

    I can't imagine that the proportioning valve would cause a pressure loss in the MC. I think that they may be wired wrong if the warning light comes on w/o the brake pedal being pressed (or the E brake deployed).
     
  9. IDave

    IDave

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    So, if there's power to her brake pressure switches with her foot off the pedal, it implies, at the very least, that the brake pedal switch is not cutting the power off (assuming proper wiring). So, at least, faulty or maladjusted brake pedal switch?
     
  10. Coolerman

    Coolerman SILVER Star

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    The brake pressure switch's should never see +12V. They are wired to switch GROUND to the Brake warning light.

    It works like this: The two pressure switches are wired in parallel with each other and in series with a normally open contact in the brake switch, the brake warning light and on to Fuse A which is hot all the time. The other side of the pressure switches are wired to GROUND.

    The pressure switch's are normally CLOSED switches. When you step on the brake you make the contact in the brake switch which completes the path and the warning light turns on for a brief period until enough pressure builds up in the master cylinder front and rear circuits to cause the switches to OPEN thereby turning off the warning light. Any loss of pressure in the brake system will allow the pressure switch to close again lighting the light. Took me forever to get mine to work properly!

    Hope that clears up the mystery! :cheers:

    Edit: #40 = E-brake switch, #41 = Brake Warning Light, #42 = brake switch, #44 = pressure switches, #43 = to brake lights
    BrakeWarning.jpg
     
    Last edited: Feb 26, 2007
  11. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    maladjustment is entirely possible since I put a new rubber bumper in there ;) so much for wanting to be complete :rolleyes: :D
     
  12. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    I don't remember where, but I remember reading that the brake warning light will also come on if there is a differential between the two pressure switches; that's why I was asking about the prop valve, not that it would cause loss, but build-up of pressure over the front, hence a differential?
     
  13. Pin_Head

    Pin_Head

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    Other types of MCs that use a single switch sense the pressure difference between the chambers. This is cheaper, since you only need 1 switch.
    Proportioning valves don't proportion pressure, but flow volume. They alter how fast the pressure builds in the front vs the rear, but not the ultimate pressure at equilibrium.
     
  14. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    That's what I was thinking, too

    So, I'll fiddle with the brake switch next time in AZ and will report back
     
  15. Coolerman

    Coolerman SILVER Star

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    Adjust your switch so that more threads are showing on the plunger side. This means the pedal will have to go down more allowing more pressure to build in the brake system before the warning light contact is made. That should keep your light off if everything is wired correctly.
     
  16. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    Thanks, I'll try that.
     
  17. kelly148

    kelly148

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    I'm wiring a 79 FJ40 with a Painless harness. The E brake light part works fine. Master cylinder pressure switch part I'm not so sure. When I turn the key on the brake light comes on. If I step on the brake it goes out, is this correct? If so what applies pressure when it's running?
     
  18. DSRTRDR

    DSRTRDR I can mangle anything ... SILVER Star

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    you will need to look at the Toyota wiring scheme for the brake circuit and recreate that - the Painless harness is missing a few connections

    no, it should be off while driving, and only come on if there is a loss of pressure in the brake system (excluding when you have the handbrake set, then it should be on with the key as well)
     
  19. kelly148

    kelly148

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    Right now with the truck just sit/off. The pressure switchs are closed. Is that correct or should they be open?
     
  20. FJforty

    FJforty

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    From Coolermans explanation, they should be closed. When brake pressure is high enough the switches will open. If either brake is low pressure then it's switch will stay closed and the light will stay on when the brakes are applied showing a problem.
    Always wondered what was going on, thanks Coolerman.