1994 CHV no brake lights

Discussion in 'Chevy Truck Forum' started by 94chevyman, Jun 8, 2010.

  1. 94chevyman

    94chevyman Full Member

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    #1 94chevyman, Jun 8, 2010
    Last edited by a moderator: Mar 26, 2011
    new to this site, but i will get to the point.My 94 chevy truck has no brake lights.
    fuses good bulbs good. can anyone help me with this please. my truck has been
    down too long because of this.
    I know it could be brake switch or multi/function switch also,
    dont know how to test either. thanks
     
  2. DaveDanger

    DaveDanger Full Member

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    94chevyman, I can't swear for certain that these wire references are identical to my '96 Silverado, But I *think* they will be.
    To begin with, check and see if turning on the hazard flasher makes the brake lights work. It's the same bulbs, just a slightly different circuit thru part of the system, so that might help pin it down to a specific part of the harness.
    Next, check inside the "underhood relay box". There's probably a relay there that is for the brakelight circuit. Verify it's a good relay. (There's several 4-pin relays there in that box that are in some cases identical). Swap one of them out with your brake light relay and see if that makes a difference.
    Next, the brake light switch is a fairly simple on/off switch mounted right behind the top of the brake pedal. The rod going thru the firewall to the brake master cylinder activates the switch. There should be an orange wire going in to the switch, and a white wire coming out. To my fairly certain knowledge, the brake light circuit is always hot. You should be able to illuminate your brake lights by stepping on the brake pedal ANYTIME, key in or out, turned on or off. Kinda slide in up under the steering wheel and probe those 2 wires with a voltmeter.. with the pedal sitting normally, and then depressed. You should see 12v on the orange wire regardless, and on the white wire when the pedal is depressed. If those checks are OK, you're losing it further "downstream". If those checks aren't good, you're losing it "upstream".
    I have a wiring diagram that shows this wiring circuit pretty well, but I can't seem to get it to the page as a thumbnail... full size pic only. And that pic's huge :confused:
    Msg me if you'd like and I can e-mail you the wiring diagram scan.
     
  3. oneluckypops

    oneluckypops Senior Member

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    dave is this the diagram
     

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  4. crabtruck

    crabtruck Senior Member

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    Welcome to the forum 94chevyman.

    Wiring was "evolving" 94-99, that draw has a dome defeat switch not found in 94.
    Didn't compare farther after that, there are 94 circuits posted in other threads.

    Dave is so smart that even when he is wrong he is still right. :D
    No underhood fuse panel, fuse panel at driver's knee, singles scattered.
    HOWEVER, most of the brake and turn wiring is same, and of course the
    brake circuit was also "always hot"

    Divide and conquer mentality says step through this. (sorry;))

    Check that STOP fuse (not BRAKE fuse) is good and 12v present.
    Check if CHMSL (3rd brake light) comes on with pedal.
    If not, brake pedal switch is bad.
    Be aware brake lights go through turn switch arm, as do HAZ.
    Check (per Dave) if HAZ lights work in rear.

    If HAZ work, wires from turn arm to rear, plus grounds are good.
    If not, remove rear brake/turn/haz bulb and check for voltage with HAZ.
    Voltage may be weird looking but should be noticeable.
    If voltage present, likely bad ground at rear, check connection to frame
    at the driver side rear, will be obvious when you look.
    if you have trailer wires added, check all connections near bumper.

    If HAZ does not work at rear, but 3rd brake light does work,
    have helper step on pedal while you listen and feel with fingertips
    on the brake light relay for click as it turns on and off.
    If no click, unplug brake light relay on firewall above brake booster.
    (Suburban doesn't have that) There are four wires, two are Yellow.
    Both yellow wires should go hot when pedal is pushed.
    If not, you have an open wire between pedal and relay.
    If only one yellow wire is hot, the splice that joins them is failed.
    It is buried in the loom. :(
    If both yellow go hot with pedal, check that the black wire for relay
    is making a good connection to ground by using ohmmeter.
    The actual ground bolt is on the top rear of engine.
    If both yellows hot and ground is good, relay is likely bad.

    Use caution with this step, forcing wire into a socket can hurt it.
    If you have working 3rd brake light and yellows are hot at relay,
    then with relay still removed, connect a jumper from
    either YELLOW to the WHITE wire. This bypasses the relay.
    The white wire is the brake feed to the turn arm.
    If brake lights now work as they should, relay is confirmed bad.

    If brake lights still do not work, but HAZ does, re-install relay.
    Go to steering column in cab and you will see a 10wire connector there.
    The white wire from the relay is the top one.
    Check if that wire is getting hot when pedal is pushed and relay is installed.
    If not, wire is broken between realy and there (not likely)
    If hot and HAZ work then most likely "multi-function switch" (turn arm)
    is defective.

    Hope that helps.

    Pictures and similar problem here:
    http://www.truckforum.org/forums/chevy-truck-forum/24980-ky08362.html#post176765

    Circuit diagram here:
    http://www.truckforum.org/forums/ch...-silverado-brake-light-issues.html#post170879
     
  5. DaveDanger

    DaveDanger Full Member

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    I can see I'm gonna hafta go ahead and finish that electronics degree to be able to carry on an intelligent conversation with crabtruck:) that dood is GOOD!:woot:
    Alright, I think I figured out how to "Manage Attachments" well enough to get the thumbnail on the forum page without overloading the power grid..
    As stated before, this is from a '96 Silverado, so the wiring may not be perfectly identical, but the basics will be the same.
     

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  6. 94chevyman

    94chevyman Full Member

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    #6 94chevyman, Jun 9, 2010
    Last edited: Jun 9, 2010
    no brake lights

    Ok my hazard lights are working but the third brake light is not when hazards on. Brake switch or multi/function switch. Mind you there is a hissing noise coming from the booster i believe, so i was told. Would that make them not work
     
  7. crabtruck

    crabtruck Senior Member

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    Great that the HAZs flash in the rear, never have to look at rear of truck again.
    Hissing sounds like vac leak, also means key is in and truck is running, turn off, take key out.
    No key is needed to hunt this down.

    If noise is there when key is off, that might be your answer.
    That could be the relay buzzing which would give problems.
    Find out if relay or booster making noise, critical answer.

    3rd brake light is not effected by HAZ, does 3rd light work with pedal ?

    Brake sw / multi-func sw / relay is the question YOU must help us answer.
    1) find out what making noise.
    2) tell us if 3rd light comes on with pedal.
    3) tell us if you feel / hear click of brake relay.


    Dave: thanks for the kind words, but if I was that good your fuel pump would work.
    96 starts and ends same but middle is different, ie: note 1 vs 2 flashers.
    I like seeing the schematic, but I'm weird like that,
    some folks get cold sweats from electrical concept chases.
    poor 94chevyman just wants to not hate his truck LOL
    You're more advanced on this stuff than the avg bear, don't duck. :)
    We have to bring him along in easier steps or he'll sell the truck. :eek:
     
  8. 94chevyman

    94chevyman Full Member

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    no brake lights 94 chevy

    It is the relay making the noise not the booster. So does that mean its the relay?
     
  9. crabtruck

    crabtruck Senior Member

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    Huge chance,
    only other reason I could think of for the noise would be poor onnections.

    It should be a cheap swap, might want to go get a new one,
    any parts place should have them.
     
  10. wyochimneysweep

    wyochimneysweep Senior Member

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    could be lower power to the relay, ground to relay bad, or a weak relay
     

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