Brake light problems
12-17-2003, 03:09 AM
#1
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Brake light problems
I have a problem with the warning system in my car alerting me to a brake light failure. Here is what happens.....
with the headlights off:
Application of brake causes the two low level and one high level brake light to operate. Releasing pressure causes the lights to go out - Normal operation.
with the headlights on:
High level brake light off, two low level lights are at "headlights on" reduced brightmess (as normal)
Apply brake pedal
All three lights illuminate to their full brightness.
Release brake pedal - sometimes the highlevel light stays on, but at reduced brightness instead of going off. The auto-check system then reports a bulb failure and bleeps.
Sometimes after repeated brake pressing, on release, the high level brake light will go out and the warning stops......and sometimes it seems to happen when I select drive on the transmision.
I suspect I have a lose wire somewhere but don't know where to look.
Can anyone helpo with the diagnosis of the above or tell me where the switch is (and what it looks like) which detects brake pedal depress or what could possibly be wrong.
When the lights are on (either side, full, dipped, it doesn't matter) I get a warning bleep from the computer. When I depress the pedal the alarm goes away, only to bleep again when I release the pedal.
It is bad first thing after the car hasn't been driven, then after a while it'll stop doing it...... its driving me mad.
many thanks all,
chris
with the headlights off:
Application of brake causes the two low level and one high level brake light to operate. Releasing pressure causes the lights to go out - Normal operation.
with the headlights on:
High level brake light off, two low level lights are at "headlights on" reduced brightmess (as normal)
Apply brake pedal
All three lights illuminate to their full brightness.
Release brake pedal - sometimes the highlevel light stays on, but at reduced brightness instead of going off. The auto-check system then reports a bulb failure and bleeps.
Sometimes after repeated brake pressing, on release, the high level brake light will go out and the warning stops......and sometimes it seems to happen when I select drive on the transmision.
I suspect I have a lose wire somewhere but don't know where to look.
Can anyone helpo with the diagnosis of the above or tell me where the switch is (and what it looks like) which detects brake pedal depress or what could possibly be wrong.
When the lights are on (either side, full, dipped, it doesn't matter) I get a warning bleep from the computer. When I depress the pedal the alarm goes away, only to bleep again when I release the pedal.
It is bad first thing after the car hasn't been driven, then after a while it'll stop doing it...... its driving me mad.
many thanks all,
chris
12-17-2003, 05:43 AM
#2
AudiWorld Super User
Re: Brake light problems
What year car? When you say "high" brake light, do you mean the one in the rear window on the parcel shelf?
It sure sounds like you have a bad ground to me. Will need to know the year though.
pw
It sure sounds like you have a bad ground to me. Will need to know the year though.
pw
12-17-2003, 07:00 AM
#3
AudiWorld Super User
What I learned from ttuling last night helped me figure this one out!
ttuling posted the most likely solution to IndyA8's freaky light problem....a ground connection gone bad. He mentioned the actual ground connections, it never dawned upon me how they were designated on the wiring diagram and how they interact.
Thanks for the tip ttuling!
Your problem is most likely the B104 ground connection. As far as I can tell, it is where the rear brake light ties to ground and the right rear ashtray light ties to ground. I couldn't find any other connections to it.
The L/R brake lights have different ground connections, that's why they are not affected when the lights are on.
Here's some wiring diagrams to illustrate what is happening.
<img src="http://www.audipages.com/Tech_Articles/electrical_equipment/brake2.JPG">
The "high" brake light in the window is powered from J123, lamp control module. It is grounded out through B104 which is the ground connection (rear compartment, right) in wiring harness interior. Where is that? I don't know, but probably under the rear seat would be my guess. Or just follow the wires from the rear brake light to where it is grounded.
What else is connected to B104? The right rear ashtray light as shown in the picture below.
<img src="http://www.audipages.com/Tech_Articles/electrical_equipment/brake1.JPG">
The B104 connection goes bad, especially when it's cold out. As the interior of the car warms up, it gets better. If you put it in drive, it changes some of the circuit most likely, so it provides other parallel paths.
When the brake light does light up dimly, the dimmer power supply is back feed through the B104 connection through the brake light and J123 lamp control module. That's why it stays on sometimes after you take your foot off the brake.
Find that B104 connection, take it apart, clean it, and put it back together again and it should work normally again.
I guess we all learn something every day!
pw
Thanks for the tip ttuling!
Your problem is most likely the B104 ground connection. As far as I can tell, it is where the rear brake light ties to ground and the right rear ashtray light ties to ground. I couldn't find any other connections to it.
The L/R brake lights have different ground connections, that's why they are not affected when the lights are on.
Here's some wiring diagrams to illustrate what is happening.
<img src="http://www.audipages.com/Tech_Articles/electrical_equipment/brake2.JPG">
The "high" brake light in the window is powered from J123, lamp control module. It is grounded out through B104 which is the ground connection (rear compartment, right) in wiring harness interior. Where is that? I don't know, but probably under the rear seat would be my guess. Or just follow the wires from the rear brake light to where it is grounded.
What else is connected to B104? The right rear ashtray light as shown in the picture below.
<img src="http://www.audipages.com/Tech_Articles/electrical_equipment/brake1.JPG">
The B104 connection goes bad, especially when it's cold out. As the interior of the car warms up, it gets better. If you put it in drive, it changes some of the circuit most likely, so it provides other parallel paths.
When the brake light does light up dimly, the dimmer power supply is back feed through the B104 connection through the brake light and J123 lamp control module. That's why it stays on sometimes after you take your foot off the brake.
Find that B104 connection, take it apart, clean it, and put it back together again and it should work normally again.
I guess we all learn something every day!
pw
12-17-2003, 07:10 AM
#4
You'd think they'd learn...I don't know how many B and C chassis cars have odd issues like that
due to the stupid cig. lighters being tied into other circuits totally "unrelated"...Also have issues with rear license plate lights, when both out causing problems in "unrelated" circuits.
Should nickname you PaulW-FastActingFormula!
Should nickname you PaulW-FastActingFormula!
12-17-2003, 09:45 AM
#5
If I recall correctly there is a Tech bulletin out on a similar ground problem.
TSB 97-98-03 describes a possible bad ground in the driver's side footwell. It makes sense that your problem is likely a corroded/loose ground.
12-17-2003, 10:07 AM
#6
Yes, if B104 is supected, also check its chassis connection. (And what year car?)
What year car is this? I'm using 1997 wiring diagrams for now. Ground location details do change in other years.
B104 is an "in wiring harness" ground point. It appears in the second diagram posted by Paul to be connected to 43 "Grouind connection, on right A-pillar, lower part" by a purposefully fat (6 square mm) wire. So a bad connection at 43 could make B104 an bad ground if B104 has no other chassis connection.
It would be helpful if the Bentley manual devoted one wiring diagram section to a "tree" of ground connections, with the chassis connections on the bottom branching up to the various in-harness connections.
Tom
B104 is an "in wiring harness" ground point. It appears in the second diagram posted by Paul to be connected to 43 "Grouind connection, on right A-pillar, lower part" by a purposefully fat (6 square mm) wire. So a bad connection at 43 could make B104 an bad ground if B104 has no other chassis connection.
It would be helpful if the Bentley manual devoted one wiring diagram section to a "tree" of ground connections, with the chassis connections on the bottom branching up to the various in-harness connections.
Tom
12-17-2003, 10:14 AM
#7
AudiWorld Super User
Tell me how'd you figure out it's connected to 43
I too used the 97 wiring diagram, I don't know what year the car is.
How did you figure out is connected to 43 "Grouind connection, on right A-pillar, lower part" by a purposefully fat (6 square mm) wire. I don't see where that is anywhere....so not only do I need to check my ground connections, but if they are in the wiring harness, where those connect to the chasis, correct?
That's what skiracer refered to in his post, the A pillar I believe.
Man, what you can learn here!
pw
How did you figure out is connected to 43 "Grouind connection, on right A-pillar, lower part" by a purposefully fat (6 square mm) wire. I don't see where that is anywhere....so not only do I need to check my ground connections, but if they are in the wiring harness, where those connect to the chasis, correct?
That's what skiracer refered to in his post, the A pillar I believe.
Man, what you can learn here!
pw
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12-17-2003, 10:23 AM
#8
AudiWorld Super User
Here's a JPG of the TSB
ttuling has pointed out that the B104 goes to the 43 (the headbone is connected to the backbone?)...skiracer says there is a TSB on this, it's a simple one pager....here it is if you don't have the Bently.
<img src="http://www.audipages.com/Tech_Articles/electrical_equipment/brake3.JPG">
<img src="http://www.audipages.com/Tech_Articles/electrical_equipment/brake3.JPG">
12-17-2003, 11:02 AM
#10
Other bad ground possibilities
Forgive me Chris for writing "ground" instead of "earth".
It is quite possible, as Paul said, that turning on the headlights gives power to something which pulls up a bad B104 ground, or its chassis connection 43 (if that's the only one), so the high level brake light glows dimly by receiving voltage on its normally grounded side and "grounding" its other wire through the two low level brake light filaments.
There's another way the high level brake light could come on dimly: if it has a good ground but was being fed taillight voltage through one of the low level brake light filaments because one of the daul-filament taillight/brakelight bulbs is poorly grounded.
To test this, turn on the headlights, step on the brakes until the high level brake light is stuck on dimly, then disconnect first one then the other low level brakelight/taillight bulbs. If disconnecting either low level bulb (which will be hot because of the operating taillight filament) turns off the high level brakelight, that bulb has a bad ground, which allows the taillight current to run through the taillight filament, past the missing ground, then on through the brakelight filament on its way to the high level brakelight.
Pay attention to the high level brakelight when working with the wires, in case their movement reconnects a loose ground.
As for why depressing the brake or shifting to drive can make the problem go away, I can only guess the extra current flow of the brake or possibly reverse lights is enough to cause some arcing which temporarily cleans the loose connection.
Some ground paths on the 1997 U.S. model wiring diagram (may be incomplete -- this information is scattered across pages and I skipped those I thought unrelated):
Left brake and tail light: B109 "in wiring harness interior, left, rear", to 59 "near taillight, left"
Right brake and tail light: B108 "in wiring harness interior, right, rear", to 60 "near taillight, right".
All lamps in the trunk lid -- reverse, tail, fog: 98 "in rear lid wiring harness" to B108, then as for right brake light.
Tom
It is quite possible, as Paul said, that turning on the headlights gives power to something which pulls up a bad B104 ground, or its chassis connection 43 (if that's the only one), so the high level brake light glows dimly by receiving voltage on its normally grounded side and "grounding" its other wire through the two low level brake light filaments.
There's another way the high level brake light could come on dimly: if it has a good ground but was being fed taillight voltage through one of the low level brake light filaments because one of the daul-filament taillight/brakelight bulbs is poorly grounded.
To test this, turn on the headlights, step on the brakes until the high level brake light is stuck on dimly, then disconnect first one then the other low level brakelight/taillight bulbs. If disconnecting either low level bulb (which will be hot because of the operating taillight filament) turns off the high level brakelight, that bulb has a bad ground, which allows the taillight current to run through the taillight filament, past the missing ground, then on through the brakelight filament on its way to the high level brakelight.
Pay attention to the high level brakelight when working with the wires, in case their movement reconnects a loose ground.
As for why depressing the brake or shifting to drive can make the problem go away, I can only guess the extra current flow of the brake or possibly reverse lights is enough to cause some arcing which temporarily cleans the loose connection.
Some ground paths on the 1997 U.S. model wiring diagram (may be incomplete -- this information is scattered across pages and I skipped those I thought unrelated):
Left brake and tail light: B109 "in wiring harness interior, left, rear", to 59 "near taillight, left"
Right brake and tail light: B108 "in wiring harness interior, right, rear", to 60 "near taillight, right".
All lamps in the trunk lid -- reverse, tail, fog: 98 "in rear lid wiring harness" to B108, then as for right brake light.
Tom