One brake light not working

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TexRev_50

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Jun 7, 2019
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I?m in a keystone bullet ultra lite. All my running lights work. But my left brake/turn light does not. If I turn my running lights on it will light up but it will not blink or get brighter when  I hit the brakes. I?ve changed the bulb and that did NOT fix the problem. I?m thinking it?s in the wiring of that light since everything else works correctly. Any tips on what it may be or simple ways to trouble shoot while on the road?

Thank you in advance for your help.
 

SpencerPJ

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Midwest
Sometimes you need to wiggle and jiggle the plug in connection.  I know on mine, sometimes I need to go up and down, then leave in down position.  I've cleaned and still mess with it occasionally.
 

Henry J Fate

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Definately varify if the connecter to your tow vehicle is or isn't the culprit. Put the emergency flashers on and have someone stand behind the trailer while you agitate the connecter to the tow vehicle. Look for any signs of the flasher coming on. While you are there, look closely at the connecter pins both male and female.
 

Gary RV_Wizard

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The stop & turn signal is a separate wire and separate filament within the bulb, so you could a problem with the wire, the socket or the bulb itself.  In effect, the running light and stop/turn light are two separate bulbs merged into one package, so you have to diagnose them separately.

For starters, swap the bulb with the one on the other side and see if the problem follows the bulb. If it doesn't, chances are the issue is the socket or the wire to it. A wire problem could be anywhere from the plug on the tow vehicle to the bulb socket connection. My next step would be to test the female plug on the tow vehicle to verify that there is proper voltage on the pin for the left stop/turn circuit. If yes, work your way down the trailer wiring.
 

ArdraF

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We just changed a brake light and I learned something interesting.  We had a bunch of bulbs but Jerry pointed out there were three different kinds.  The bulbs have little "knobs" that fit in place as you install them.  There are ones with "high" knobs, there are one with "low" knobs, and there are bulbs with one high knob and one low knob.  I never realized there was a difference, but our parking light required the "low" knob bulb and the brake light required the one "high" knob and one "low" knob so make sure you replace it with the correct style of bulb.

ArdraF
 

Lonestar Guy

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Gary RV_Wizard said:
The stop & turn signal is a separate wire and separate filament within the bulb, so you could a problem with the wire, the socket or the bulb itself.  In effect, the running light and stop/turn light are two separate bulbs merged into one package, so you have to diagnose them separately.

For starters, swap the bulb with the one on the other side and see if the problem follows the bulb. If it doesn't, chances are the issue is the socket or the wire to it. A wire problem could be anywhere from the plug on the tow vehicle to the bulb socket connection. My next step would be to test the female plug on the tow vehicle to verify that there is proper voltage on the pin for the left stop/turn circuit. If yes, work your way down the trailer wiring.

Gary, I know this is an old post and not sure you will see this but... I have developed the same issue on my Coachmen Freedom Express. I'm electronically and mechanically inclined and will figure it out but to your statement about checking the voltage on the towing vehicle plug... I'm pretty sure mine is sealed/enclosed at the back side of the rubber plug, although it's late and dark and I don't want to go verify! LOL  If so, and once it's plugged in, where would you recommend measuring? I've swapped bulbs and the problem stays with the right side so it probably is a lack of power to the socket, open somewhere along the line. So, there is not a fuse for the light lines?
Thanks!
 

Rene T

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Gary said to check the female plug on the tow vehicle which would be the plug socket like Lou said.
If you have power there, work your way back to the light starting where your cord goes into a junction box.
 

John From Detroit

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ArdraF said:
We just changed a brake light and I learned something interesting.  We had a bunch of bulbs but Jerry pointed out there were three different kinds.  The bulbs have little "knobs" that fit in place as you install them.  There are ones with "high" knobs, there are one with "low" knobs, and there are bulbs with one high knob and one low knob.  I never realized there was a difference, but our parking light required the "low" knob bulb and the brake light required the one "high" knob and one "low" knob so make sure you replace it with the correct style of bulb.

ArdraF

THat is common the reason for the HIgh/Low is so you only insert the bulb one way
If you look at the bottom the High/Low have 2 (really 3) contacts 2 on the bottom and the metal shell i the 3rd

But the hi-hi or lo-lo have only one in the center and the shell so if you put em in 180 rotated. NO differenfe but with the dual filament 3-contact lamps if you put 'em in backwards the BRIGHT element is in the LOW position.
 

John From Detroit

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The standard brake/turn signal wiring (there are two versions)

If you have brake/turn the brake light wire goes to the turn signal switch. there, if not in a "Signal turn" position it goes out to both brake lights.  IS there a fuse in those lines.. Sometimes yes.

1B the signal goes to a relay and that sends power to the lights and in this onfiguration there is a fuse iu[p to 4 of them (one right one left and 2 more for towed/trailer in some cases)

The other standard the turn signals and brake lights are DIFFERENT lamps. This is "Euro Standard" and more or less rare compared to the first here in the US but .. Well I've seen several. heck owned a couple (Cars though) I've seen it on motor homes.

Again there may be independent fuses.

Suspects
The Turn Signal switch
Every connector and every wire from there to teh ground point on the lamp.

Suggest a test light.. Identify the proper wire and probe it starting at the switch and follow it out as best you can. a Horrible Fright cable tracker may help.. I was amazed at how well one performed for me tracking a circuit the other day.. Zeroed right down on the wire and yes it was bundled but if the bundle "Branched" it was very clear which branch was the right one.    I'd not had such good luck in the past.
 

Lou Schneider

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John From Detroit said:
The standard brake/turn signal wiring (there are two versions)

If you have brake/turn the brake light wire goes to the turn signal switch. there, if not in a "Signal turn" position it goes out to both brake lights.  IS there a fuse in those lines.. Sometimes yes.

1B the signal goes to a relay and that sends power to the lights and in this onfiguration there is a fuse iu[p to 4 of them (one right one left and 2 more for towed/trailer in some cases)

The other standard the turn signals and brake lights are DIFFERENT lamps. This is "Euro Standard" and more or less rare compared to the first here in the US but .. Well I've seen several. heck owned a couple (Cars though) I've seen it on motor homes.

Again there may be independent fuses.

Suspects
The Turn Signal switch
Every connector and every wire from there to teh ground point on the lamp.

Suggest a test light.. Identify the proper wire and probe it starting at the switch and follow it out as best you can. a Horrible Fright cable tracker may help.. I was amazed at how well one performed for me tracking a circuit the other day.. Zeroed right down on the wire and yes it was bundled but if the bundle "Branched" it was very clear which branch was the right one.    I'd not had such good luck in the past.

All of the above is unnecessary if the rear brake/turn signal works on the truck.  This verifies everything on the truck works properly except possibly between where the 7 way connector attaches to the truck's wiring and the socket itself.

Testing for voltage at the connector pin (preferably under load from a test light) verifies the connection from the truck's wiring to the 7 way socket is OK.  A corroded connection may pass enough current to pass a voltmeter test but not one using a light bulb load.

I'm not aware of any pickups that use separate stop lights or turn signals, so there won't even be a 3 wire to 2 wire combiner between the truck wiring and the socket.  Just wires and connectors.

Verify you have voltage at the socket pin, then if you don't have voltage on the wires going to the trailer's rear stop/taillight start systematically working rearward from the pigtail on the trailer to the taillight.
 

Lonestar Guy

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Lonestar Guy said:
Gary, I know this is an old post and not sure you will see this but... I have developed the same issue on my Coachmen Freedom Express. I'm electronically and mechanically inclined and will figure it out but to your statement about checking the voltage on the towing vehicle plug... I'm pretty sure mine is sealed/enclosed at the back side of the rubber plug, although it's late and dark and I don't want to go verify! LOL  If so, and once it's plugged in, where would you recommend measuring? I've swapped bulbs and the problem stays with the right side so it probably is a lack of power to the socket, open somewhere along the line. So, there is not a fuse for the light lines?
Thanks!

Well, I found it. Apparently a rodent chewed through a wire... I see one wire at this point... inside the cavity behind the right rear light assembly. I don't know how else it would be cut in half.
 
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