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Old 06-27-2019, 04:21 PM   #1
Bahn112
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Weird light issue

Hi guys,

I just picked up a '70 LWB and i'm trying to get everything in order so I can drive the thing around.

I'm having a weird issue with my tail lights. Specifically my Drivers side tail light.
When the lights are off the brake light works perfect but as soon as I turn the running/headlight lights on and step on the brake pedal, it won't light up. The passenger side tail light works just fine.

Have any of you guys run into this problem?
Any suggestions?
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Old 06-27-2019, 08:33 PM   #2
franken
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Re: Weird light issue

I'd pull the light assembly out and check the ground. In fact it isn't a bad idea to check/fix grounds all the way back to the battery, since they're usually sketchy.
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Old 06-27-2019, 11:10 PM   #3
Bahn112
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Re: Weird light issue

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Originally Posted by franken View Post
I'd pull the light assembly out and check the ground. In fact it isn't a bad idea to check/fix grounds all the way back to the battery, since they're usually sketchy.
It's my understanding that the light housing itself is the ground so I cleaned up where the assembly bolts into the bed and that didn't do anything.
I thought it was weird that it works perfectly fine as long as the running/head lights weren't on. When the lights are on the parking light will work but as soon as the brake pedal is depressed there is nothing.
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Old 06-28-2019, 02:12 AM   #4
franken
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Re: Weird light issue

K, funny symptoms say there's a ground issue.
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Old 07-01-2019, 11:01 PM   #5
Bonestock3ott
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Re: Weird light issue

Is it possible that the wrong bulb was put in it or maybe someone forced the correct bulb in the wrong way and the brake filament is lighting as the parking light and when the much dimmer parking light filament lights as the brake light you can not see it.
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Old 07-01-2019, 11:03 PM   #6
Bonestock3ott
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Re: Weird light issue

Another possibility is someone spliced a replacement socket pigtail and has the wires reversed
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Old 07-01-2019, 11:13 PM   #7
Bahn112
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Re: Weird light issue

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Originally Posted by Bonestock3ott View Post
Is it possible that the wrong bulb was put in it or maybe someone forced the correct bulb in the wrong way and the brake filament is lighting as the parking light and when the much dimmer parking light filament lights as the brake light you can not see it.
I don't think this is the case as it works just fine as long as the parking light/headlights aren't on.
I did take it all apart and the I am ordering a new socket plug as the one that was currently on the truck looked a little hokey (definitely spliced a time or 2 by the PO).

Think it could be a bad headlight switch? Like it's routing the power incorrectly once the headlights/parking lights are turned on?
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Old 07-01-2019, 11:24 PM   #8
Greenlee
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Re: Weird light issue

I second what others have said. Sounds like a ground issue. I had some weird issues like this after I replaced the wood on my step-side bed. After pulling my hair out trying to figure out what was wrong, I put a ground strap from the bed to the frame and my lights have worked fine ever since then! I guess the new wood somehow interfered with the ground.
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Old 07-01-2019, 11:49 PM   #9
Bahn112
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Re: Weird light issue

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I second what others have said. Sounds like a ground issue. I had some weird issues like this after I replaced the wood on my step-side bed. After pulling my hair out trying to figure out what was wrong, I put a ground strap from the bed to the frame and my lights have worked fine ever since then! I guess the new wood somehow interfered with the ground.
I'm definitely going to get a ground strap kit. I've been reading up on them and they seem like an easy way to make sure that all the circuits have a good ground to run to.

Any suggestions on one kit over another?
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Old 07-05-2019, 10:58 AM   #10
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Thumbs up Re: Weird light issue

I would get the voltmeter out and test it for continuity. See what is going on. I will say it could be in the turn signal switch area. All wires run through it and when the brake pedal is pushed it directs the wiring on what to do out back. If it works with out the lights on but not when on a good chance is the head light switch.
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Old 07-05-2019, 10:51 PM   #11
Bahn112
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Re: Weird light issue

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Originally Posted by Andy4639 View Post
I would get the voltmeter out and test it for continuity. See what is going on. I will say it could be in the turn signal switch area. All wires run through it and when the brake pedal is pushed it directs the wiring on what to do out back. If it works with out the lights on but not when on a good chance is the head light switch.
I'll be ordering a headlight switch when I place my next order with LMC. Hope that fixes it.
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Old 07-07-2019, 06:46 PM   #12
Bahn112
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Re: Weird light issue

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Originally Posted by Bahn112 View Post
I'll be ordering a headlight switch when I place my next order with LMC. Hope that fixes it.
Actually, the more I think about it it probably is not the headlight switch. The left/right split doesn't happen until the harness splits towards the rear of the truck. Since my passenger side works just fine and it's only the drivers side that is having issues I'm circling back to it being a ground problem.
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Old 07-08-2019, 06:37 AM   #13
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Re: Weird light issue

If it's been spliced, try switching the wires. Ask me how I know!

Rick
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Old 07-08-2019, 06:47 AM   #14
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Re: Weird light issue

Sounds like a ground. When the tail lights are off, the power goes from the brake switch, to the brake filament, then it should go to the ground, but since that isn't available, it goes back through the running light filament and grounds through the headlight switch.

When you initially turn on running lights, the running lights are grounding through the brake pedal switch.

When both are providing power, there is no ground available, and the light turns off.

I'd be willing to bet the driver's side tail light is brighter than the passenger's light.
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Old 07-09-2019, 10:39 PM   #15
Bahn112
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Re: Weird light issue

Quote:
Originally Posted by rpmerf View Post
Sounds like a ground. When the tail lights are off, the power goes from the brake switch, to the brake filament, then it should go to the ground, but since that isn't available, it goes back through the running light filament and grounds through the headlight switch.

When you initially turn on running lights, the running lights are grounding through the brake pedal switch.

When both are providing power, there is no ground available, and the light turns off.

I'd be willing to bet the driver's side tail light is brighter than the passenger's light.
In addition to getting some grounding straps, where are the ground locations for the taillights? I want to clean them all up back there to ensure as good a ground as possible.
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Old 07-10-2019, 07:01 AM   #16
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Re: Weird light issue

I think it grounds where the backing plate bolts into the body
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