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  • Front parking lights come on when pressing brake pedal

    Here’s a puzzle...

    ‘71 with mostly stock wiring harness. (I say "mostly" as I have relays installed for the headlights, a set of spot lights, and an electric fan, all of which are fed directly from the positive terminal of the battery)

    Both the turn indicator and hazard flashers are electronic units that have been in the car for a few years.

    I've been running LED bulbs throughout the car with the exception of the front and rear turn indicator bulbs. Last weekend, I installed a set of 1157(front) and/1156(rear) orange bulbs sourced from SuperbrightLEDs.

    Everything works as expected, with one exception...when I press the brake pedal, along with the brake lights, the front parking lights come on!

    By sequentially disconnecting the red wires (parking light circuit) and the green wires (turn indicator circuit) from the front bulbs, then pressing the brake pedal while the ignition is on, I've been able to establish that it's the parking light filaments (red circuit) that are coming on. However, the front side marker lights (also LED), which are on the same red circuit, are not illuminating.

    Anybody got any ideas what could be causing this? Some kind of grounding issue somewhere?

    Click image for larger version

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    Keith, Huntsville AL, 1971 CC66559U
    10.0:1 CR gasflowed head | Weber DCOEs | CP "150hp" Cam | Distibutor by Advanced | Lightened flywheel | Phoenix SS Exhaust System | HVDA 5-Speed | Good Parts suspension and anti-roll bars | Konig Rewinds | Boyd 15 gal tank | Miata Seats and Mr Mikes covers | Carl Visser dash | Mohair hood | Gas-strut bonnet and boot lift kits

  • #2
    Wierd one, Keith.

    I don't have a clue where the fault is, but your instinct on the bad ground is a good one.

    Sometimes when lamps are in parallel, and a ground goes bad, they can end up in series under certain conditions. With incandescent lamps, they will either be dim or not light at all. But because of the way LEDs work, they may both be at full brightless.

    I'd be checking grounds.

    Ed
    For just a little more, you can do it yourself!

    Comment


    • #3
      I don't have a clue either, but a few things I would try in hopes of understanding it better and perhaps eliminating some suspects.

      Note any changes when trying these:
      • Remove the brake lights one at a time.
      • Remove the red fuse.
      • Remove the 1157 front bulbs one at a time.
      • Can you determine if the parking light segment(s) remain illuminated with the turn signal or hazards flashing? Try both.

      R3
      Jim Herter Copperas Cove, TX
      Original Owner ֍ 1970 TR6 ֎ CC 50990 LO
      VDU 506H

      Comment


      • #4
        Thanks Gents.

        Jim - I will certainly try your suggested diagnostics, and will report back. I have a suspicion that the issue somehow originates at the dual-filament rear brake lights. I think when the brake bulb is energized, it's also energizing the rear marker light circuit, causing some current to flow in the red circuit, which might just be enough potential for the front marker lights to illuminate (to Ed's point).

        Think I am going to make a stronger common-ground for the rear tail lights (which I've been meaning to do anyway) and go from there.
        Keith, Huntsville AL, 1971 CC66559U
        10.0:1 CR gasflowed head | Weber DCOEs | CP "150hp" Cam | Distibutor by Advanced | Lightened flywheel | Phoenix SS Exhaust System | HVDA 5-Speed | Good Parts suspension and anti-roll bars | Konig Rewinds | Boyd 15 gal tank | Miata Seats and Mr Mikes covers | Carl Visser dash | Mohair hood | Gas-strut bonnet and boot lift kits

        Comment


        • #5
          Low resistance LED bulbs can sometimes create goofy ground paths not normally encountered with filament bulbs, specially on old oxidized connectors and sockets. Make sure you have good grounds everywhere and you might even have to add some new ones. Maybe you have one of the 1157s in wrong, I’ve had that happen.
          Last edited by lfmTR4; 05-29-2019, 02:09 PM.
          I72 Pimento w/overdrive

          Comment


          • #6
            Originally posted by lfmTR4 View Post
            Low resistance LED bulbs can sometimes create goofy ground paths not normally encountered with filament bulbs, specially on old oxidized connectors and sockets. Make sure you have good grounds everywhere and you might even have to add some new ones. Maybe you have one of the 1157s in wrong, I’ve had that happen.
            Yes, something very similar happened to me when I converted to LED bulbs. I think it’s because some LEDS will start to light at much lower voltages than would an incandescent bulb.

            I fixed it by running dedicated grounds from each rear bulb direct to a good mounting point and otherwise cleaning the remaining grounding points and all’s been good ever since.

            Comment


            • #7
              Here's something you have probably thought of.

              I believe the tail lamps and the front parking lights are all in parallel. Also, the tail lights and the brake lights share bulbs on each side. If there were a short between the brake and tail lamp circuits, maybe in one of the sockets, or even in one of the bulbs, then brake, tail, and front parking lights would all be in parallel. If the tail lights are also coming on with the brake lights, it would support this scenario.

              Putting an 1156 bulb in an 1157 socket might cause this problem.

              Ed
              For just a little more, you can do it yourself!

              Comment


              • #8
                Will be working on this at the weekend, but, while I my gut tells me it is a ground issue at the rear tail lights somewhere, one thing I think I can eliminate is incorrect insertion of the bulbs.

                I've definitely got 1157s where they should be and 1156 where they should be. In fact, due to the staggered bayonets on the 1157s vs equal bayonets on the 1156's, with the sockets I have, I could't actually get the bulbs to locate incorrectly.

                I did establish that the parking lights do still come on when pressing the brake pedal after the red circuit fuse was removed from the fuse box...

                I'll be wiring in a common ground at the tail-light clusters tomorrow, and will see what that brings in combination with Jim's test steps.
                Keith, Huntsville AL, 1971 CC66559U
                10.0:1 CR gasflowed head | Weber DCOEs | CP "150hp" Cam | Distibutor by Advanced | Lightened flywheel | Phoenix SS Exhaust System | HVDA 5-Speed | Good Parts suspension and anti-roll bars | Konig Rewinds | Boyd 15 gal tank | Miata Seats and Mr Mikes covers | Carl Visser dash | Mohair hood | Gas-strut bonnet and boot lift kits

                Comment


                • #9
                  Had the same problem with my XJS, press on the brake pedal and the front (left only in this case) parking light would come on.
                  Problem turned out to be loose ground for the brake and tail lights in the boot. Tightened it up and everything worked normally, Well, the lights anyway.
                  CF1634U+O Pimento/Chestnut
                  2nd owner, since 1975
                  Now in Fair Oaks, CA

                  Comment


                  • #10
                    Well, I figured it out. It's actually not to do with bad grounding or with incorrectly inserted bulbs, the issue is actually with the particular type of 1157 LED brake light bulb I'm using.

                    After going over the whole car with a VOM, I found that when the brake lights are energized (green wires) current "spills over" to the other terminal of the bulb, and backfeeds up the red wires, which flows back up the circuit up to the fuse box, and from there back down through the front parking lights.

                    When I Googled it, there were plenty of hits confirming this. It's because the LED circuits in the bulb are incremental, rather than independent of one another. The parking lights illuminate just a few of the LED sectors, while the brake circuit illuminates all of them. But in doing so, it causes there to be a small voltage on the other terminal of the bulb. Enough to light an LED but not an incandescent

                    My choices were to either insert a pair of diodes on the red wires feeding the rear parking lights to prevent the backfeed, or, replacing the bulbs with others that have independent circuits. I went with the diodes, and all is now working.
                    Last edited by A Brit in Bama; 06-01-2019, 07:47 PM.
                    Keith, Huntsville AL, 1971 CC66559U
                    10.0:1 CR gasflowed head | Weber DCOEs | CP "150hp" Cam | Distibutor by Advanced | Lightened flywheel | Phoenix SS Exhaust System | HVDA 5-Speed | Good Parts suspension and anti-roll bars | Konig Rewinds | Boyd 15 gal tank | Miata Seats and Mr Mikes covers | Carl Visser dash | Mohair hood | Gas-strut bonnet and boot lift kits

                    Comment


                    • #11
                      I love a happy ending.

                      Ed
                      For just a little more, you can do it yourself!

                      Comment


                      • #12
                        Glad you found it.

                        That could have been a tricky one. Good methodical troubleshooting!

                        These dudes have the LED thing down for our cars.

                        https://litezupp.com/epages/ac0e8cab...20Lamps%22/L56
                        Last edited by tr6harris; 06-01-2019, 11:10 PM.
                        '74 TR6 CF13007U aka "Mr. T"
                        Custom Blue (Delft-Like) and New Tan (Formerly Mallard and New Tan)
                        Points, Ballast Bypassed, Bosch Blue Coil, Moss Cobalt Wires, Champion RN12YC plugs.
                        Peaks and tweaks, but the spirit of Original
                        Redlines always.
                        My wife is the Driver, I'm just the Mechanic....

                        Comment


                        • #13
                          If anyone does encounter this issue going forward then Classic Car LEDs in the UK have bulbs where there is no continuity between the two circuits in the 1157. They have two choices available:

                          https://www.classiccarleds.co.uk/col...positive-earth

                          and these (their brightest)

                          https://www.classiccarleds.co.uk/col...-glb380-p21-5w

                          Make sure you order the Negative Earth versions
                          Keith, Huntsville AL, 1971 CC66559U
                          10.0:1 CR gasflowed head | Weber DCOEs | CP "150hp" Cam | Distibutor by Advanced | Lightened flywheel | Phoenix SS Exhaust System | HVDA 5-Speed | Good Parts suspension and anti-roll bars | Konig Rewinds | Boyd 15 gal tank | Miata Seats and Mr Mikes covers | Carl Visser dash | Mohair hood | Gas-strut bonnet and boot lift kits

                          Comment

                          Front parking lights come on when pressing brake pedal

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