drivers side brake light on trailer doesn't work

[skiprspd3]

Here is my issue. I have an 05 Trailblazer EXT, when I test the trailer plug with a test light the drivers side brake light has no power. If I turn on the drivers side directional it has power. Basically every light on my trailer works except the drivers side brake light, and as I'm sure anyone who will respond to this knows the brake light and directional run off the same wire.
I've also pulled every fuse in both fuse blocks and found none of them to be bad. Any help would be greatly appreciated.

Thanks , Steve

 

[IllogicTC]

What do you mean by "directional?" You mean a turn signal, yes?

Have you checked the wiring on the trailer itself? How about testing for power at the connector?

If it's using a 7-blade plug, you'll need to check the blade on the right with the yellow wire running into it.

If it's using the 4-pin, it'd be the one with the yellow wire running to it. If you're using a 7-to-4 adapter, there's that one pin that has the opposite-style plug than the other 3 (which is the ground). The left turn/brake would be the middle pin on the 3 "normal" ones.

 

[skiprspd3]

Turn signals is correct.

I've checked the blade with the yellow wire, and as I stated there is no power when the brake is applied but there is power when the turn signal is turned on.

 

[IllogicTC]

There are two white wires which run from the stop switch (mounted to the brake pedal). One goes to the rear fuse block to control on-board stop lamps. The other one will run to the turn signal module (listed as "behind lower left of dash), so it shouldn't be too far from the stop lamp switch. Test for power at that wire, on the end of the connector feeding the turn signal module while holding the brake pedal in. If the white wire is receiving power properly, your turn signal module may be going out.

 

[MAY03LT]

Welcome to the site Steve.

The flasher is mounted to the plastic trim under the steering column. There are (2) 7mm screws holding that panel in.

This pic shows the brake input and the drivers side brake/turn output (which then goes to fuse 51 in the underhood fuse block and then to the trailer connectors yellow wire). With the brake pedal pushed, you should have 12v at the yellow wire shown in the pic.

Don't mind the probe that's shown, it was for another project.

 

[skiprspd3]

Thanks, I'll give that a try as soon as I have a free minute. With 3 kids playing sports there really isn't many of those free minutes.

Now if there is no power coming out what does that mean I need to fix?

 

[djthumper]

Does the left brake light on the truck work?

 

[MAY03LT]

Thanks, I'll give that a try as soon as I have a free minute. With 3 kids playing sports there really isn't many of those free minutes.

Now if there is no power coming out what does that mean I need to fix?

Smack the flasher like if it stole something, and recheck.

 

[skiprspd3]

Ok I'll try that.

Yes the brake on the truck works

 

[IllogicTC]

I'm still wondering if the flasher module's logic isn't partially screwed up. That module runs the flashers and hazards onboard, and the flashers/brakes for the trailer harness.

What I'm gathering is that it should be receiving a signal, as you said only the left side the brake lamp doesn't illuminate, which tells me the right side DOES which means it's getting the brake signal from the brake pedal. It's also receiving and operating properly the left turn signal. That says the yellow wire sounds good front-to-back, and that that light's ground must be good as the high-intensity filament (and I assume the low with headlights on) is working properly yes?

 

[jsomething]

I feel like ive seen this before in a diff vehicle. Pretty sure putting in the wrong type of bulb can cause it. (Using a bulb that fits but has a slightly different number) specially since you said it has power when the turn signal is on..

 

[IllogicTC]

I feel like ive seen this before in a diff vehicle. Pretty sure putting in the wrong type of bulb can cause it. (Using a bulb that fits but has a slightly different number) specially since you said it has power when the turn signal is on..

That shouldn't be the case here. ONE wire runs the bright filament (for flashing and for the brakes). There's another wire for the tail lamps (dimmer filament). Then there's the ground, which is the trailer itself. If turn signals are making the bright filament illuminate, feasibly the brake should make it work, as it's one and the same filament and wouldn't require a separate pin as it's on the same wire.

The fact that there is no power on the trailer harness blade for brakes but there IS for turn signals tells me this is likely not a trailer-end issue to begin with.

 

[jsomething]

Forgot to clarify the wrong bulb could be in any of the directionals or brake lights on the truck.

 

[Blckshdw]

Forgot to clarify the wrong bulb could be in any of the directionals or brake lights on the truck.

The OP said the lights on the truck work fine, it's the light on his trailer that is the issue here. I think the issue might be in/near the trailer connector somewhere. Here's the wiring diagrams for reference.

From the second pic, you can see the brake light signal originating from the stop switch, and splitting between the flasher module and the rear fuse block. In the rear block, the signal goes through the brake light fuse, then splits off between both tail lights, and then to the trailer brake wiring (line 12 on the left side of the page) that comes back to the first pic.

It would be convenient if there was another member locally that could do a quick flasher module swap to confirm if that's the problem since that would be quick and easy. If it was found out to not be the issue, then some wiring tracing might be in order. Maybe the connection for the brake wire to the left trailer brake pin has been damaged somehow, and that's why it's not getting it's signal?

 

[SmokeyMcBlazer]

i know you said you checked the fuses. i'm just gonna throw this out there, did you check fuses in the front or rear fuse box? and are both brake lights on the trailer not working or is it just one light on the trailer that is not working?

 

[The_Roadie]

Come on guys. The trailer lamp works like old-fashioned cars. ONE filament is used for both brake and directional. If the directional works, that rules out the lamp, trailer wiring, fuses, brake switch, and vehicle wiring. If one side brake light works, that rules out the vehicle wiring TO the flasher module. Nothing left in the failure flowchart chain but the flasher module.

 

[linneje]

I hate to chime in here, but I have read this problem before and I had it too a number of years ago. If I recall, Roadie is correct. Almost like a "sticky" relay, I could get it to work if I hit the brakes and the 4 ways at the same time, but I believe the problem was the flasher relay under the trim in front of the driver. Sorry if I don't have the technical description right.

 

[Wooluf1952]

I hate to chime in here, but I have read this problem before and I had it too a number of years ago. If I recall, Roadie is correct. Almost like a "sticky" relay, I could get it to work if I hit the brakes and the 4 ways at the same time, but I believe the problem was the flasher relay under the trim in front of the driver. Sorry if I don't have the technical description right.

I believe that's what May03LT was refering to with his technical discription in post #8.

Also,



Welcome

In case you haven't heard:
http://gmtnation.com/forums/topic/218-welcome-to-gmtnationcom-heres-the-story/

 

[IllogicTC]

Come on guys. The trailer lamp works like old-fashioned cars. ONE filament is used for both brake and directional. If the directional works, that rules out the lamp, trailer wiring, fuses, brake switch, and vehicle wiring. If one side brake light works, that rules out the vehicle wiring TO the flasher module. Nothing left in the failure flowchart chain but the flasher module.

I initially pointed toward that in post #4. The more info and the more I looked at it, that's actually been my final answer, just never came outright and said it.

 

[The_Roadie]

Meanwhile, back at the old site where nobody's home, the only reply in 30 hours has been from the incompetent wannabe-diagnostician Loverotties, who was (as usual) far off the mark. Someday we should start a thread about his goofs.

 

[djthumper]

I have been waiting to see the flasher results...

 

[Ekim Eromdirp]

I am currently having this same issue, my view on this is that if I have right turn signal, shouldn't be a wiring problem since it is the same wire (right turn/brake). Since the above mentioned relay is a "trailering" relay, I could assume that the trailer relay, relays all the "commands" to the trailer connection. Since I detect turn signal at the connector on the truck, wouldn't you think we could rule out wiring or harness? I just need to know with a certain amount of certainty that this is a relay issue. My mind just sees the name "flasher" relay and I ask how can a "flasher" relay have anything to do with a brake light?

 

[The_Roadie]

Welcome! "Flasher" relay is just short hand for "combining brake signal with turn signal and four-way flasher signal which are the same as the turn signals all into one master signal that turns on the trailer light that you want" relay.

If you have a brake light, the wiring and lamp is OK. If you have a turn signal, the lamp and wiring is also OK. The flasher relay is what combines the two functions into one lamp filament on the trailer.

You could easily combine the signals with diodes so the trailer lamp would light up if EITHER the turn signal or the brake signal was sent to it. But that would have a undesirable side effect of the brake light would OVERRIDE the turn signal and if you had the turn signal on at the same time as the brakes, the lamp would not flash. It would just stay on steady because you were on the brakes. The flasher relay makes sure that the turn signal gets a chance to flash the lights EVEN IF you're on the brakes. That's the critical detail.

 

[Ekim Eromdirp]

Thanks for the Welcome The_Roadie.....LOL ......love the long version of "flasher"........So you would be confident in saying my relay COULD cause just the right brake to fail when everything else is working? I am confident in knowing that the Right signal works at the tail of the truck. The left signal works at the tail of the truck. The left brake works at the tail of the truck. The right brake does not. I am just trying to assure myself the relay can cause this failure before I invest in a relay.

 

[The_Roadie]

When you say "at the tail of the truck" are you talking about at the trailer connector at the back of the truck? The flasher relay combines signals going to this connector only. If by "tail of the truck" you mean the tail light assembly on the truck, the flasher relay isn't involved.

Relays aren't that expensive, after all......$20-25 on Amazon.

 

[Ekim Eromdirp]

Yeah at the trailer connector.

 

[Ekim Eromdirp]

Relay ordered..................will report here for results when relay is received/installed.

 

[Mounce]

I'm assuming that you looked up the part number and ordered the right one, right? There are many different ones but the ones for our platform are part number 141339259944 (OEM)

 

[Ekim Eromdirp]

I ordered GM OEM number 15235201

 

[The_Roadie]

I ordered GM OEM number 15235201

You got the right part number. "141339259944" is an Ebay item number for one listing of the right part. 1 demerit for Mounce.

 

[Mounce]

Ah crap! Demerit respectively taken, I just saw the p/n and went to item specifics (on my phone, couldn't copy it from the title) and copied that number and didn't look at it even after I posted.

 

[The_Roadie]

No sweat. Your recruiting on the OS leaves you with a net positive for the year.

Phone interaction with the site is not optimum, but it's better than asking anybody left at trailvoy for help.

 

[Mounce]

I might not be able to help you higher-ups here work on the site but I'll put in my time by being a thorn in the side to the guys that stuck around over there and try to be their own heroes on trailvoy by leading everyone here. I do it for spite. If it weren't for the guys like you, Roadie, those guys wouldn't know what they do now about our rides.

 

[DocBrown]

Good call on the flasher relay. A couple of years ago I spent 2 hours, delayed leaving for a camping trip, and replaced the entire plug on the truck which still didn't fix the issue. When I returned I bought the relay and problem solved.

 

[Ekim Eromdirp]

OKAY BOYS!!! We have liftoff!! Ordered a relay on the 16th and got it yesterday the 28th.....long story and it took waaaaaay too long for my taste. BUT, installed tonight and that was not hard at all. Checked right at my plug on truck and had all the lights there and hooked up to my trailer and I am very pleased to say that was my problem!! Thank you The_Roadie very much for sharing your knowledge and insight. And all you others who pitched in to help me sort through this issue. You guys rock!! Thanks!!

 

[The_Roadie]

Thanks for the kind words.....it's the precise reason we started the site. To pass it on.