Tail Light Troubles - page 1 - Troubleshooting - VWDiesel.net The IDI, TDI, and mTDI source.

• Tail Light Troubles by Kneale Brownson on 16 Jul, 2007 19:24
• I was trying to trace down why my 1994 Jetta TD's brake lights flashed in alternating time with the amber turn single lights when I used either direction of turn indicator, and I was holding the brake pedal with a stick.  Lost all power to the rear lights.  I checked the tail and turn fuses and they're OK.  Is there a relay in the brake light circuit somewhere?
  
  Also, once I discover how to restore power to the rear, what might be causing the crossover between turn and brake lights?  I've checked all the bulb holders and they show no sign of corrosion.  Bulb bases are all clean.
• #1 by Vincent Waldon on 16 Jul, 2007 20:33
• You most likely have a bad ground on that side my friend... since you already checked for corroded bulb and socket then it must be the actual ground connection to the socket itself.. the  the brown wire that eventually makes it's way to a metal screw somewhere.
  
  Because the ground is bad the bulb is completing the circuit thru the other  light bulb's filament.
• #2 by Kneale Brownson on 17 Jul, 2007 03:26
• Thanks, Vince.  Since the alternatating flashing of the brake and amber lights occurred when either side turn signal was used, does that mean there is a bad ground on both?  Do the flashers use a common ground somewhere?
  
  Also, anyone know where there is a common power source for the rear lighting?  A relay maybe?
• #3 by somolovitch3 on 17 Jul, 2007 04:37
• Volksys and their grounds..................
  
  Tail lights and turn signals use the same ground wire to the body screw, since both sides are doing the "flash dance" might even use the same ground for both sides. Find and ensure good ground contact at the body screw(s).
  
  How long did you leave the brake lights on? Could the switch have over heated? Maybe it un-adjusted its self? The wireing and contacts in these cars are maybe a bit on the small side. Quick test is to jump 12v hot to the brake lights, by-passing everything. If the lights come up, the problem lies in the switch, maybe. If just jumping the switch (at the switch terminals) brings them up, then new switch time. Oh, yea..USE a FUSE ! in your jumper wire!!! For brake lights 10 amp should be good.
  
  Let us know how this all works out.
• #4 by Kneale Brownson on 17 Jul, 2007 10:00
• I probably had the brake pedal pressed down no longer than you might sit at a lengthy traffic light waiting to turn left.  I engaged my prop, walked to the back of the car to see the brake lights, walked back to the open driver door to engage the flasher switch, walked back to the rear and the lights all went out.
  
  No joy regarding a "readjustment".  No lights working at the back.  No tails, no brakes, no turns, no hazzard flashers.  Everything works at the front, though.  Has to be more than just the switch on the brake pedal doesn't it?
  
  I'll put together a fused jumper wire and do some testing, probably tomorrow.
  
  Thanks for the suggestions.
• #5 by Jetta Fan on 17 Jul, 2007 11:38
• If you have no lights at the back at all, it sounds like the ground is completely gone. No ground = no lights.
• #6 by rubadubdub on 17 Jul, 2007 14:00
• Quote from: "somolovitch3"

  
  Tail lights and turn signals use the same ground wire to the body screw, since both sides are doing the "flash dance" might even use the same ground for both sides.
  

  
  
  A2 Bentley (i know its not your year but my mk1 and mk2 arent that different) says that both tail light and indicator earth to the 'rear apron', whatever that is? MIght be an american term i dont recognise.
  
  First thing i'd do is check ALL the fuses, i had a radio fuse blow in my Mk1 golf that upset other supposedly unrelated electrics. However if the lights all work as they should up front i'd expect them to be ok.
  
  I'd then be tempted to follow the brown wires coming out the rear light clusters to wherever they go and check they're earthed ok. If they're still not working at all i'd get a multimeter and a wiring diagram and test that power is reaching the clusters for each circuit (hazard, indicator, brake etc) as it will rule out a break before the lights.
  
  

  Quote from: "Kneale Brownson"

  Also, anyone know where there is a common power source for the rear lighting?

  
  The 90-92 jettas have turn signal power coming via hazard switch, rear lights coming via light switch, and the brakes have power via their own power source with switch in between. There will be different coloured wires coming into the rear cluster brackets for each one.
  
  I think all the lights earth via their bulb holders and then a common earth wire screws into the car body 'rear apron'. Maybe check the bulb holder then follow that earth wire as i mentioned above.
  
  Hope that helps.
• #7 by Kneale Brownson on 17 Jul, 2007 14:08
• Most, but not all,  of the brown wires to the various rear fixtures (fender and trunk lid) do not have continuity to a body ground, so their ground has gone to pot somewhere.
  
  The fixture grounds appear to come from the wire harness connectors.  There is no fixed ground wire between the fixtures and the body directly outside the various harnesses.  
  
  Anyone know where the ground connections are for those various harnesses?  Some of the wires disappear into the tops of the fenders and some go under the trunk carpeting.  The MkIII Bentley wiring diagrams don't tell me where the grounds are made.
  
  I don't understand why I have NO lights when some grounds are good and some not.
• #8 by Vincent Waldon on 17 Jul, 2007 16:04
• Usually in this case this is more than one thing happening at the same time.. a bad ground *and* a loose socket or a bad ground *and* a blown fuse.
  
  The current then travels all kinds of weird paths and causes weird symptoms.
  
  Nothing for it but to track things down one by one, starting with fixing all grounds.... then see what's left.
• #9 by Kneale Brownson on 17 Jul, 2007 17:56
• Can I "fix" the grounding issue by tapping into each of the brown grounds and run one heavy ground wire to a good body ground in the trunk?
  
  I've checked the relevant fuses and found none blown.
• #10 by Vincent Waldon on 17 Jul, 2007 18:16
• Can do... it's kinda a shotgun approach.... if you have a multimeter it might make more sense to measure the grounds and discover the bad ones.  Same goes for power to the bulb filaments.
• #11 by Kneale Brownson on 18 Jul, 2007 09:36
• Multimeter shows 12 volts at only one of the connectors for the left tail light when the light switch is on.  None of the other connectors at the back show 12 volts.  
  
  This is driving me crazy.
• #12 by Vincent Waldon on 18 Jul, 2007 19:10
• Yup, now attach the positive of the multimeter to the battery voltage from somewhere and use the negative terminal to check all your grounds.
  
  Bet you find one or more that aren't working.
  
  
  Vince
• #13 by somolovitch3 on 18 Jul, 2007 20:16
• :twisted: rather than tying all grounds together, gather thr ground s from tje right side, tie them to a 14 ga wire and ground that. A ring lug with a #10 sheet metal screw thrue it into the trunk (clean off the "stuff" to bare metal), then repaint to keep off the rust (both sides of the hole).
  
  At this point  STOP!!!
  
  Check to see if any lights work.
  
  UMMMMMM>>>>>>>>>
  
  Could have the rear body harness conecter come loose from the back of the fuse block/relay plate?
  
  Gods and Godesess...check the simple stuff first!!!!
  
  Eeeeek...
  
  You said that there was no 12v at the sockets, yes? Use VOM to check..........what were you using as a ground for the meter? Hell of a long clip lead to the batt Neg? Yes?
  
  ummmmmmmmmmmmmmm.Let us do one thing at a time. Know its a PITA , ......
  
  Okay, who wants to make up a check list , and prioritize it? :roll:  :roll:  :roll:  Hey, where is the :mrgreen:  beer?
• #14 by Kneale Brownson on 19 Jul, 2007 06:32
• Thanks, Som, I've been using a multimeter with the ground clamped onto what looks like a good OEM body ground already in the trunk.
  
  I've gotten power back to the lights somehow, maybe I moved something in a harness when I removed all the trim pieces covering the wiring to the front.  Still have the crossed, alternating flashing of the brake lights with the turn signals when I use the turn signals.  I've noticed a couple of the bulbs aren't OEM, so I'm going to pick up some new bulbs and see what happens.
  
  After that, I'm going to reground each side harness if necessary.  If that doesn't do the trick, I'll just run all new wires from the front because that pathway currently is all exposed.