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Third Brake Light (CHMSL) Problem

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Old 12-06-2005, 12:23 AM   #1
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Third Brake Light (CHMSL) Problem

I have a 2000 Blazer.

I was wondering if anyone had suggestions on troubleshooting a third brake light that doesn't work.

I used a multimeter and getting no power to the led itself and don't think it is a fuse as the fuse box under the hood has no fuses in the spot labeled CHSML.

Anyway if anyone knows the path of the wires from the light to the switch to the battery that would help too.

All other lights seem to be working.

Thanks,
Rick
Old 12-18-2005, 06:45 PM   #2
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Re: Third Brake Light (CHMSL) Problem

I have the same problem. I have no idea what it is. I know there is a relay on the 97 and lower model years but the 98+ is diff. Anyone?
Old 01-13-2006, 06:14 PM   #3
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Re: Third Brake Light (CHMSL) Problem

i have the same problem someone please help
Old 01-13-2006, 07:05 PM   #4
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Re: Third Brake Light (CHMSL) Problem

You should have two fuses labeled TR CHMSL (10A) and VEH CHMSL (10A) in the underhood fuseblock (top of left front fender). When you step on the brake, you should have battery voltage on the power side of each fuse.

The third brake gets power from the VEH CHMSL fuse and is continously grounded.
Old 01-15-2006, 10:21 PM   #5
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Re: Third Brake Light (CHMSL) Problem

what about on the 94-97 s10's that dont have the fuse box on top of the fender under the hood? where is the fuse or relay for those?
Old 01-16-2006, 12:16 AM   #6
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Re: Third Brake Light (CHMSL) Problem

i had this problem on my 94, it was the stupid ground in the light its self, just lost contact some how
Old 01-18-2006, 08:39 PM   #7
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Re: Third Brake Light (CHMSL) Problem

Thats funny I just found out 3 days ago my 3rd brake light it not working anymore. i cant figure out why, I looked at the fuses on the side of the dash (where all the fuses are) but the one for it is also the same fuse for like 10-12 other things and they all work so I was at a lose. where does the light plug in at? I took it off the truck but it goes inside under the headliner.....


I have more time this weekend when im off work to fool with it( I hope)

Oh yeah I am driving a 1996 S-10 Blazer

Last edited by Chris's_96Blazer; 01-18-2006 at 08:41 PM.
Old 01-22-2006, 10:05 PM   #8
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Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by lifted91dime
what about on the 94-97 s10's that dont have the fuse box on top of the fender under the hood? where is the fuse or relay for those?
Check the 3rd relay to the right if you are looking at the truck. Its on the passenger side of the truck, there should be 4 relays there.
Old 02-24-2006, 11:52 AM   #9
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Re: Third Brake Light (CHMSL) Problem

I have been having this problem for a long time now. I subscribed to ALLDATA.COM and found some better wiring schematics. It shows there to be a "cruise control cell" right before it branches off into the third brake light.

So we have these possibilities:
1) A fuse somewhere
2) The brake switch on the pedal
3) This cruise control cell
4) Bad wiring

Here is the schematic that I stiched together. All involved - please look at this and see what you think. I called up the GM dealership asking them about this cruise control cell, and they said that it isn't a part they can find in their database to replace.

http://www.xourque.net/thirdbrakelight.png

Maybe we can all figure this out if we put our heads together!
Old 02-24-2006, 11:57 AM   #10
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Re: Third Brake Light (CHMSL) Problem

Just for info CHMSL stands for
Center
High
Mount
Stop
Light

the TR SHMSL fuse is for TRailer, or for a cap third light,, the VEH SHMSL would be for your truck!

If you've tried all and still can't find the problem you can do this:

go to a shop that sells truck caps. Get a device called a Logix Box, what this little thing does is you feed a wire from both left and right brake lights to it, and it has an output for your third light.

It makes it so when you put your blinker on, it pulls power from the other side to light the brake.
I used to work at Van & Truck world in Ottawa, we sold them for $20.00 canadian.

This will solve your probelm %100 if you can't find out what is actually causing the problem.
Old 02-24-2006, 12:58 PM   #11
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Re: Third Brake Light (CHMSL) Problem

Looking at the scat, two possibilities jump out at me. the first, if the 510 Ohm resistor is open, the bulbs would fry immediately, and if it were shorted you'd blow a fuse. After re-reading your post, however, you're not getting power to the lamp assembly, so the theory about the resistor being open is shot, but it could still be shorted and you just haven't located the fuse.

the other possibility is the High Mounted Stoplamp Relay. If it's shot, the power will never get sent up the line to the lamp, however I can't help you on where it's physically located.

If the stop switch were bad, you'd get no break lights at all, so that eliminates the brake pedal.

I don't think the cruise control would affect this, but if you even have CC, does it work? If the cruise circuitry were a dead short to ground, the CHMSL wouldn't work, but then again, you'd blow that 10A fuse... If CC works, the problem is isolated to the CHMSL circuit which leaves you with the relay or a malfunction inside the multifunction switch.

-sloe
(elctronics technician, US Navy)
Old 02-24-2006, 01:05 PM   #12
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Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by sloe
the other possibility is the High Mounted Stoplamp Relay. If it's shot, the power will never get sent up the line to the lamp, however I can't help you on where it's physically located.
In that schematic, it appears as though that there is only a high mounted stoplamp relay for the PICKUPs, not the utilities. Am I reading it wrong, or is there definitely a relay?

With my Jimmy:
- The two side brake lights work.
- The cruise control works perfectly (I use it all the time)
- All fuses (engine compartment, and the box next to the drivers side area) are OK. I checked them all.
- There is NO VOLTAGE coming through the lines at the light.
- The LED strip lights up when direct voltage is applied from an outside source.

I REALLY hope that it isn't the wiring itself. But since there are others reporting the same issue, I would think that it was some fuse or bad relay.. Hmmm...
Old 02-24-2006, 01:11 PM   #13
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Re: Third Brake Light (CHMSL) Problem

Well according to RockAuto.com, there is a relay available for my specific vehicle:
http://www.rockauto.com/catalog/cata...&parttype=3452

I am going to obtain one of these and install it this weekend to see if it fixed it. I will definitely reply to those post with my results! I will also take pictures.

Thanks!!
Old 02-24-2006, 01:16 PM   #14
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Re: Third Brake Light (CHMSL) Problem

ya'know, I totally missed the "pickup" and "utility" tags in that. now I feel stupid

Looking closer, looks like either connection from stop switch, or broke wire somewhere. start chasin'

-sloe
Old 02-24-2006, 01:21 PM   #15
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Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by sloe
ya'know, I totally missed the "pickup" and "utility" tags in that. now I feel stupid

Looking closer, looks like either connection from stop switch, or broke wire somewhere. start chasin'

-sloe
LOL - don't worry... It's not a very good schematic. I called a GM dealership and they told me that there is for sure a high mount brake light relay for the Blazer/Jimmy. I have them holding it and will replace it tomorrow! Hopefuly it will fix the damn thing.

THANKS AGAIN!!
Old 02-25-2006, 04:43 PM   #16
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Re: Third Brake Light (CHMSL) Problem

I SOLVED IT!!

It was not a relay. The utility vehicles DO NOT have a relay for the center high mount stop light. ONLY THE TRUCKS DO.

I discovered that in my fuse box, I have been looking at the layout all wrong. There are two fuses (TRCHMSL and VECHMSL). Both of which I thought had fuses in them. As it turns out, both of those fuses were completely GONE. I have no idea what happened to those two fuses. I certainly did not take them out. All I did was put a new 10amp fuse in the VECHMSL and now the third brake light works!

Now the question is, where did those fuses go?

TO THOSE WHO ARE HAVING THE SAME PROBLEM:
Look in your fuse box that is in the engine compartment. The fuse box cover has a diagram of the layout for all the fuses. The VECHMSL fuse is in the upper right hand area. Is yours missing? Or is it there and blown out?
Old 02-26-2006, 12:59 PM   #17
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Re: Third Brake Light (CHMSL) Problem

This is odd mine was missing too and I put one in there and now my 3rd brake light works. I dont know how or why it wasnt there but it is now and working good. I have been messing with this damn thing for a while now and I am glad it is working. Thanks alot for info. This is a really common problem I think it would be interesting to see why there was no fuse there.
Old 02-26-2006, 05:03 PM   #18
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Re: Third Brake Light (CHMSL) Problem

That is very very strange. Have you ever seen your third brake light work in the first place? Have you ever taken the thing to a dealership, or any other repair shop?

Anyone else have this fuse turn up missing?
Old 02-27-2006, 12:56 PM   #19
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Re: Third Brake Light (CHMSL) Problem

Mine didnt work when I got it. So I can't really suspect any reason why the fuse was missing. I noticed there was a blown 10A put in the extra fuse slot inside the door. So maybe it kept blowing fuses, although I drove mine around yesterday and I checked when I got home and it was still working. I dont know if this is just concidence or something maybe dealers were doing?
Old 02-27-2006, 01:27 PM   #20
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Re: Third Brake Light (CHMSL) Problem

Yeah, I just drove between Portland and Seattle this weekend and was in a lot of traffic (so lots of brake lighting) and it did not blow. Crazy stuff! I should call a GM dealership and maybe they will be honest and give me some sort of clue as to why the fuse was gone..
Old 03-02-2006, 05:49 PM   #21
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Re: Third Brake Light (CHMSL) Problem

I get fed up with dealerships. I went in one time to see how much the ac delco plugs were they were going to charge me 55$ for 6 of them. I went and walked out. Went to Advance auto and got the origional Ac delco platnums for 2$ a piece.
Old 10-18-2008, 08:54 PM   #22
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Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by Xourque
I SOLVED IT!!

It was not a relay. The utility vehicles DO NOT have a relay for the center high mount stop light. ONLY THE TRUCKS DO.

I discovered that in my fuse box, I have been looking at the layout all wrong. There are two fuses (TRCHMSL and VECHMSL). Both of which I thought had fuses in them. As it turns out, both of those fuses were completely GONE. I have no idea what happened to those two fuses. I certainly did not take them out. All I did was put a new 10amp fuse in the VECHMSL and now the third brake light works!

Now the question is, where did those fuses go?

TO THOSE WHO ARE HAVING THE SAME PROBLEM:
Look in your fuse box that is in the engine compartment. The fuse box cover has a diagram of the layout for all the fuses. The VECHMSL fuse is in the upper right hand area. Is yours missing? Or is it there and blown out?

K Sorry to bring this back. But, I just replaced my blown fuse and the 3rd brake light still does not work. Any other suggestions?
Old 10-18-2008, 09:27 PM   #23
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Re: Third Brake Light (CHMSL) Problem

Did you check the fuse after you pushed the brake pedal? If so, was it blown? If it blew again, you've got a short somewhere and you get the fun part of finding out where. If it's not blown, then...
Old 10-18-2008, 11:06 PM   #24
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Re: Third Brake Light (CHMSL) Problem

Yes checked the fuse and it is not blown again. I checked for voltage at the connector and got 12 volts. So it must be the lights! I was already at the salvage yard today and got a 3rd brake light. But apparently it is no good either!
Old 10-19-2008, 02:39 PM   #25
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Re: Third Brake Light (CHMSL) Problem

Actually found a working light! Back in business!
Old 10-20-2008, 05:55 PM   #26
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Re: Third Brake Light (CHMSL) Problem

on the CHMSL the volts when the light is on without braking is 12-14.4v
when you are braking it increases to 20-30+volts. thats why leds dont work in there all the time
Old 01-10-2009, 12:27 AM   #27
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Re: Third Brake Light (CHMSL) Problem

so on a 96 blazer is there a fuse or relay for this third light and where are they located.. mine doesnt work either.

Ray
Old 07-13-2009, 09:16 PM   #28
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Re: Third Brake Light (CHMSL) Problem

mine doesnt work because of the flat metal circuit behind the bulbs were coroded and broke.
Old 10-25-2010, 10:43 PM   #29
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Re: Third Brake Light (CHMSL) Problem

Check the brake light switch on the brake pedal. It may be out of adjustment or just faulty.



__________________________________________________ _________________________
PUTCO REPLACEMENT LED THIRD BRAKE LIGHT, SMOKED -- Manufactured through an exclusive i-on chrome coating technology applied to the interior lens for greater reflection; Features super bright and innovative designs that create a unique vehicle appearanc
Old 10-26-2010, 11:39 PM   #30
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Re: Third Brake Light (CHMSL) Problem

I recently bought a 2000 S10, and my over head brake light wasn't working, so I replaced the blown bulbs. When I did that, NONE of the brake lights worked. Took the bulbs out, and the two back ones began working again.

My thoughts is some sort of wiring problem, but far all I know it's as simple as a blown fuse somewhere. Any suggestions?
Old 03-16-2011, 09:35 AM   #31
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Re: Third Brake Light (CHMSL) Problem

Im haveing the same problum.. its not a fuse and its not the bulbs what else could it be ?
Old 03-19-2011, 12:32 PM   #32
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Re: Third Brake Light (CHMSL) Problem

I cant even find my fusebox under the hood 96 noma?
Old 03-19-2011, 04:45 PM   #33
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Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by raven2510
on the CHMSL the volts when the light is on without braking is 12-14.4v
when you are braking it increases to 20-30+volts. thats why leds dont work in there all the time

I am but a simple backyard mechanic for 40 yrs and that just does not make sense to me. These electrical systems do not have a provision for increasing the voltage over the battery voltage to the lights. They are only 12V lights. Did you actually have a volt meter hooked up to the light and check it?
Old 06-25-2011, 12:25 AM   #34
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Lightbulb Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by chevys104zlife
Im haveing the same problum.. its not a fuse and its not the bulbs what else could it be ?
I just fixed mine ('94 S-10 LS) the other day. I removed the brake light fixture from inside the truck behind the dome light and pulled the whole fixture out from the outside of the cab. Sprayed the wires and contacts with electrical cleaner and taped up the ground wire so it would not short out on any metal. Put everything back together and it works fine. Make sure your ground or power wires are not touching any other metal.
Old 07-23-2011, 02:45 AM   #35
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Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by STEVEB82
I just fixed mine ('94 S-10 LS) the other day. I removed the brake light fixture from inside the truck behind the dome light and pulled the whole fixture out from the outside of the cab. Sprayed the wires and contacts with electrical cleaner and taped up the ground wire so it would not short out on any metal. Put everything back together and it works fine. Make sure your ground or power wires are not touching any other metal.
I have the same yr as yours. Did your light not work prior to doing this? I can't find the fuse for this light anywhere, just paid $6 for two new bulbs and the light still won't come on.
Old 08-03-2011, 10:52 PM   #36
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Re: Third Brake Light (CHMSL) Problem

Hoping for insight
Old 08-30-2011, 11:27 AM   #37
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Re: Third Brake Light (CHMSL) Problem

I just fixed the CHMSL light on my 2001 which had been inoperative for a long time. Besides replacing the fuse I had to repair/splice both the power and ground wires of the pigtail from the light bar to the first connector. They were both broken at the grommet leading into the body under the rear window. It appeared that the wire length from the grommet up to the light bar was a bit short for the full travel of window and over the years of stretching they finally broke. The wires were not free to slide in the grommet. In order to access the wires, I had to cut the grommet away from around the wires to break them free of the grommet. (note: the wire insulation was still intact, found the breaks by doing a continuity check between the connector and the solder end at the light bar).
Thanks to the Forum for getting me started in the right direction in fixing this problem.
Old 09-25-2011, 03:43 PM   #38
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Re: Third Brake Light (CHMSL) Problem

my 2000 Sonoma chmsl hasn't worked for some time and I now have at least one answer - heat. I needed help to fix a problem with the tail/brake lights so took the truck to my favorite auto electric experts so asked them to check the CHMSL.

They did all the basic checks and finally determined that one of the rivets holding the sockets and wiring together had melted the plastic and juice was no longer getting to the bulbs. I thought it was the AZ sun but he said it's more common in vehicles seeing a lot of city driving - the bulbs are on a lot and they get very hot.

The fix if this is the problem is to replace the entire assembly. The manuals - and he didn't have a part to look - aren't clear but my tech thinks the harness is attached and you have to remove some of the interior trim. A PITA and a very bad design. Thanks, GM.
Old 11-15-2011, 09:30 PM   #39
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Re: Third Brake Light (CHMSL) Problem

My high mount wasnt working either so I pulled the plastic panel in the back where the interior light is and found the wires for my high mount brake light. Checked the power at the connector and had nearly no voltage.....So I cut the connector off and checked and it had almost 11 volts so I soldered the wires directly together and now my light works fine.....I know this post is old but I hope this helps.
Old 12-06-2011, 01:02 PM   #40
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Re: Third Brake Light (CHMSL) Problem

Realized the other day my CHMSL on my '98 LS (ext cab) wasn't working, so I popped off the cover. Both bulbs were black from use, no doubt original, so I replaced them. No good. Did a search on here and saw this and other threads. I was scared to death it had something to do with the trailer wiring I added a month or so ago. It is a nightmare under the dashes on these!

No voltage at the bulb bases on the outside, or on the rivets that bring power in to the metal strips in the light base. Popped off the dome light lens on the inside; getting power there, on the wire crimps, but NOT on the rivets they were attached to. Based on comments here about heat affecting the plastic, I used a screwdriver to spin the crimped lug about 45 degrees on the rivets -- BINGO! Everything is working. Put it back together, dead again. Opened it up again and the wires had pushed the terminals back to their original positions (I think the base of the dome light forces them out of the position I put them in). Moved the terminals further and held the wires in position while installing the dome light, and everything is working fine.

In better weather, I'll probably solder the terminals to the rivets, on both sides.
Old 12-06-2011, 04:08 PM   #41
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Re: Third Brake Light (CHMSL) Problem

I don't know if this will help anyone, but when i replaced the dome, map lights, and 3rd brake light to LEDs the festoons bulbs have a + and - you will notice a dot of blue or green paint on on end of the glass.
Old 12-07-2011, 04:29 PM   #42
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Re: Third Brake Light (CHMSL) Problem

holy hell! glad i found this thread, my third light hasnt worked since I bought this last truck of mine. After checking every existing fuse, testing the wiring with a multimeter, cleaning the light connections, and replacing the bulbs i gave up on it.


Turns out i had the same issue... only a missing VEHSHMSL fues in the under hood fuse block. Replaced the fuse and now the relay clicks and the light works! Wonder why the fuse was missing in the first place?
Old 10-14-2013, 12:33 AM   #43
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Re: Third Brake Light (CHMSL) Problem

I have a similar problem. It's a 2001 Tahoe, CHMSL not working. It used to, but not any longer. The fuse in the engine compartment is good, but I replaced anyway, just to check... no luck. I checked voltage at the connector at lamp, and there was almost no current. I pulled the fuse in the engine compartment and checked voltage, almost no current.

So, am I looking at a problem between the fuse box and brake pedal? Thanks in advance.
Old 10-14-2013, 12:35 AM   #44
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Re: Third Brake Light (CHMSL) Problem

Another, possibly related, problem is also going on. I hooked up my trailer (first time with this vehicle) and the running lights, blinkers, etc. all work, but the brake lights do not work. Could this be related to the CHMSL not working?
Old 10-17-2013, 01:12 AM   #45
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Re: Third Brake Light (CHMSL) Problem

Quote: Originally Posted by ckutin
Another, possibly related, problem is also going on. I hooked up my trailer (first time with this vehicle) and the running lights, blinkers, etc. all work, but the brake lights do not work. Could this be related to the CHMSL not working?
When it comes to any wiring problem its always best to look through wiring diagrams to trace what circuits share grounds, fuses, relays....
Different years have different circuits. For instance my year of vehicle has a relay on the firewall for the CHMSL. No mention of it on the site. Found it by looking though a wiring schematic.
Old 10-27-2013, 12:37 AM   #46
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Re: Third Brake Light (CHMSL) Problem

I have been trying to figure this dang thing out for a while now off and on finally got some time to work on it. Checking and shooting all wires, fuses, and bulbs.

Then Bingo turned the wire connector on back of the light a little bit and the damn thing now works. Wow do I feel dumb but at least I learned some while trouble shooting it.

Thanks ROSS98
Old 11-27-2013, 02:44 PM   #47
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Re: Third Brake Light (CHMSL) Problem

Thanks the fuse in the hood fixed my dads s10.
Old 12-27-2013, 02:59 PM   #48
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Re: Third Brake Light (CHMSL) Problem

Looking at many other sites. this appears to be a common problem.. I'm here looking because last weekend my daughters 2002 Blazer had the Center Brake working and on Christmas I noticed it wasn't. It was way cold to spend much time outside. It seems that broken wires right at the Grommet is very common on the Blazers since the Brake light moves with the rear hatch. I opened the hatch and did a quick wiggle on the wire while the Brake pedal was depressed.. NG. The Schematic for these cars show a white wire going to the Flasher for the lower Brakelights and a separate one running direct from Brakepedal to High Mount Brake light. So there's not much there.
I'll pull and reinstall the connector at the Brakepedal first and hope for a bad connection. then I will pull the inside dome light fixture and buzz it out from there.
You guys with missing fuses... can you be sure you had fuses in there and a wiorking light or was the Brake light not working since you bought the car?
Old 01-18-2014, 06:15 PM   #49
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Re: Third Brake Light (CHMSL) Problem

I actually had plastic melt enough to cover a ground contact. Took inside lens cover and plate off and cleaned off excess. Bout drove me nuts as fuses were all good and multimeter checked out all wires good till I got to ground side of lite contacts.
Old 03-14-2014, 12:15 PM   #50
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Re: Third Brake Light (CHMSL) Problem

There are about four minor problems with every single S10 I've ever seen. This is one of them. I replaced the bulbs on mine, and it worked brilliantly for about a week.
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