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Brake lights don't work when headlights are on!

66vetteman

By 66vetteman
in Ask A GM Technician

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66vetteman Enthusiast

66vetteman

Posted
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2000 Chevy Express Van 1500, 5.7L. Brake lights work fine until the headlights are turned on. When the headlights are turned on and the brake applied, the bottom running lights (same bulb as brake lights, different filament) go out and the brake lights do not come on. Third brake light works ok regardless. Any thoughts?

Thanks!

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oldscsc Enthusiast

oldscsc

Posted
Posted (edited)

A bad bulb can actually cause problems like this. The third brake light has it's own circuit, which explains why it still functions. And since the third brake light works, that eliminates the fuse and brake switch as the trouble maker. That means your problem is occurring from the brake light switch to the bulb.

 

You could try swapping out each brake light / tail light bulb with a new one and see if the problem persists. For the price, ya can't compain.

Edited by oldscsc
66vetteman Enthusiast

66vetteman

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I just changed all the bulbs, even the front running/signals. One of the front signal bulbs twisted off in my hand so they needed to be changed anyway. Though it might have been the problem but no luck. Thanks for the info on the brake sw & fuse. Any other thoughts or ideas?

oldscsc Enthusiast

oldscsc

Posted
Posted

See if you can get a helper to keep a foot on the brake (or rig up a broomstick or something similar). Turn the headlights on. Then remove one headlight bulb and see if the brake lights act normally. If there're still acting up, try the other bulb. That's all assuming you didn't put new headlights in it.

 

Let me know if that doesn't fix it and I'll look at the diagram when I can and see what else could be going on.

66vetteman Enthusiast

66vetteman

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Can I disconnect the headlights one at a time, it is easier than removing the bulbs as I would have to remove the battery to get at the pass side one?

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
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You will have either a bad socket, or bad ground. When it is caused by a bulb, it is generally the dual filament bulb having one filament breaking and the loose end connects to the other filament giving a feedback issue.

Try taking out the parking light fuse and see if it works now.

66vetteman Enthusiast

66vetteman

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I will give that a try tomorrow. Getting cold and windy here and have to work outside. Thanks for all the help, I appreciate it!

oldscsc Enthusiast

oldscsc

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I agree with the loose / dirty ground. My computer isn't loading the diagram (or anything else for that matter...) so I can't tell you where it's grounded at.

66vetteman Enthusiast

66vetteman

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I pulled the fuse for the parking lights and the brake lights now WORK with the headlights on. The fuse controls the lic plate, tail, park, fr side markers, glove box & ash tray lights. Bad socket in one of these locations and/or can it be a bad ground as well? I didn't replace the side mkr bulbs but I will. This van has a total of 4 rear lights, two on each side. The top lights are the running/turn (2057), the bottom are the running/brake (1157). All 4 are dual filament.

Edited by 66vetteman
oldscsc Enthusiast

oldscsc

Posted
Posted (edited)

 

A bad socket can cause a bad ground (poor connection). Or your ground could still be loose and / or dirty. Try putting the fuse back in and pull each bulb that is powered by that fuse. Start with the dual filament bulbs (as said above, they are generally the problem). After pulling each one, check to see if the lights function properly and get back to us.

Edited by oldscsc
66vetteman Enthusiast

66vetteman

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Put the fuse back in and pulled all the bulbs, one at a time, replacing the previous one before pulling the next. Brake light did not work when the headlights were turned on.

Doug_Scott Senior Enthusiast

Doug_Scott

Posted
Posted
Put the fuse back in and pulled all the bulbs, one at a time, replacing the previous one before pulling the next. Brake light did not work when the headlights were turned on.

 

That sounds like a bad socket then. It will be one of the rear sockets that does both parking and brake lights.

66vetteman Enthusiast

66vetteman

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I was able to find only one socket so I replaced the one I thought most likely to be the culprit. Now the brake light with the new socket works and the other one doesn't when the headlights are turned on. In fact it is dimmer than before when the brakes are on with the headlights off. Does this make sense? I have another socket on order, be here Monday.

 

Interesting findings when shopping for a tail light socket. GM only services the tailight harness, $111.82 for each side. NAPA had the socket only with wire pigtails for $24.85. They also have the harness assy for $18.52 per side. So the harness assy which has 3 sockets, wire harness and connector to plug into the main harness is LESS than the socket only and over $93 LESS than GM. ??????

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