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Go Back   Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum > 2002 - 2009 TrailBlazer/Envoy Tech > 02-09 Vortec 4.2L I-6 Engine and Drivetrain > OEM Issues

OEM Issues Original Equipment Repairs and/or Problems

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  #1  
Old 03-03-2009, 02:09 PM
miller4456 miller4456 is offline
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2004 Chevy TrailBlazer LS
Sand Beige Metallic 4.2L I6 4X4
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fog light wiring

i want to add fog lights to my 04 tb ls 4x4. i understand the part about having to make brackets and what not. i want this to be a oem upgrade but i know that there isnt any kits that you can buy to do this. i want the oem style fog lamp push button switch. i have already purchased a new headlamp switch assembly with the fog lamp button but without washer button. what is the correct way to wire this up using the original relay and fuse in the fuse box. i dont want to go splicing a crap load of wires to make this a quick fix, but i want it to work just like it would if i were to have purchased the tb with factory installed fog lights. o, and is there a way to keep the fog lamps on when high beams are on. thanks, this is my first post on this forum and i am hoping to gain alot of knowledge from it as well.
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  #2  
Old 03-03-2009, 03:32 PM
the roadie's Avatar
the roadie the roadie is offline
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The OEM fog lamps aren't directly controlled by the switch, but by the BCM. The wires may or may not be in the harness anyway, depending on how GM decided to run the assembly line. Check the back of the headlight switch. There should be a 12-pin and an 8-pin connector. On the 8-pin connector, see if there's an orange wire in pin A. If there is, you might be lucky.

The fuse (under hood fuse block) #11 and relay 39 are likely not present if you didn't get the option, but you can put them in.

The BCM isn't guaranteed to control the fog lights if you didn't buy the option, but it might. Install the fuse and relay, and the relay might click when you turn on the switch. Oh, but then the relay wiring OUT of the fuse block to the non-existent lamps probably won't be in the vehicle's harness, so you'd have to disassemble the fuse block to put it in.

Probably easier to run your own relay not in the fuse block, and just connect the control line to the headlight switch pin A, which goes to ground when you turn on the switch.
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  #3  
Old 03-03-2009, 03:52 PM
miller4456 miller4456 is offline
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2004 Chevy TrailBlazer LS
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my truck has the orange wire on the 8-pin connector. so this is the fog lamp lead wire. whats all into taking the bcm apart to attach the fog lamp wire.
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  #4  
Old 03-03-2009, 04:13 PM
trailrez trailrez is offline
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Far as I know , is the orange wire is always hot- key on or not. So keep in mind when wiring things up.
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  #5  
Old 03-03-2009, 04:31 PM
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the roadie the roadie is offline
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Quote:
Originally Posted by trailrez View Post
Far as I know , is the orange wire is always hot- key on or not. So keep in mind when wiring things up.
Sorry. Not in this case. I have the shop manual, and am an engineer, so I tend to read the schematics when answering. GM doesn't always follow their own traditional color code.

Quote:
Originally Posted by miller4456 View Post
my truck has the orange wire on the 8-pin connector. so this is the fog lamp lead wire. whats all into taking the bcm apart to attach the fog lamp wire.
No need to touch the BCM, which is electronic anyway.

That orange wire is grounded by the new switch in your headlamp control assy. It's usable as a relay CONTROL wire. If you install a relay and fuse in the fuse block, you save yourself a tiny bit of wiring time, but then you have to tap the relay output leg to wire up to the lamps.

I think the real reason GM ran the fog lamp control through the BCM was to make sure they could turn if off when the ignition switch was off, without going to the risk of running the fog lamp power through the already overloaded ignition switch.

I would recommend just adding a relay of your own to the circuit. Fuse the 12V supply to the contact side of the relay, and take the other end of the contact to the fog lights. On the control side, find some source of switched power (only hot in RUN/START modes) under the hood, such as at fuse 27 for the backup lights. Hook that supply to one side of the COIL of the relay, the other end of the relay COIL to the orange wire at the back of your new headlight switch.

Do you want me to post a copy of the fog light schematics? Are you familiar with this sort of stuff?
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  #6  
Old 03-04-2009, 09:01 AM
07TB 07TB is offline
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2007 Chevy TrailBlazer LS
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To ad to what The Roadie said. I added foglights to my 07 LS using the stock foglight switch. The wiring for the foglight switch and the wiring to the ubec were present. As were the fuse and relay. I had to add the wires from the ubec to the foglight sockets and a ground wire also. I had GM to change the bcm program and installed the new program with a Tech 2.

You will need to get access to connctor C4 of the ubec. This is the gray 32 cavity connector and install the foglight power wires into cavity F6 using terminal p/n 12110844. I sort of wrote up a procedure showing this, but haven't quite finished it yet. I went the whole route and bought a new LT bumper cover and SS grilles and trim. I can send you it and it will get you through the wiring part. You'll just have to figure out the foglamp mounting if you're keeping the LS bumper cover.

Mark
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  #7  
Old 03-04-2009, 05:56 PM
miller4456 miller4456 is offline
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roadie, a schematic would be great. i am very capable of figuring out wiring and such as i am going to school to be an engineer and i have dealt with electrical stuff my whole life. the fuse and relay are in the underhood fuse block, so i would like to use them if possible. and 07tb, if you could send me that procedure explaining the fuse block part, it would be great as well. i appreciate all the help.
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  #8  
Old 03-04-2009, 06:52 PM
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the roadie the roadie is offline
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Quote:
Originally Posted by miller4456 View Post
... i am very capable of figuring out wiring and such as i am going to school to be an engineer and i have dealt with electrical stuff my whole life....
Comrade! Nice to find somebody who's an impedance match around here! Fewer reflections at the interface and lost data that way. Can't tell you how many times I answer an electrical question for somebody who turns out not to know how to use a meter. Sigh....

(However - Recently laid off after 35 years in semiconductor test systems. Career advice for anybody in college now: don't do anything that can be outsourced to China or India. That wasn't what got me laid off due to low semiconductor business right now, but doing something that MUST be hands on is my best long-term advice anyway.)

Anyway:

http://www.roadie.org/schematics-FOGS.pdf
http://www.roadie.org/schematics-FRONTFUSEBLOCK.pdf
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  #9  
Old 03-05-2009, 11:37 PM
miller4456 miller4456 is offline
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2004 Chevy TrailBlazer LS
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roadie, the two schematics are awsome. they explain it perfectly. only one more question, if i use the stock fog lamp switch and and the relay and fuse in the underhood fuse block, does the little orange light come on when the fog lamps are on. do you have to run another wire for them to work.
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  #10  
Old 03-06-2009, 02:53 PM
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the roadie the roadie is offline
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The wire is probably there in the harness, but it looks like the light comes on from feedback all the way from the fuse block. See it in the headlight switch schematic? The reason the fog light control goes through the BCM is so it can be suppressed when the ignition is off, and also for another reason I recently discovered hidden in the engine section of the shop manual. In times of a failing alternator, where the battery alone is running the vehicle and the voltage is steadily dropping, the BCM is involved in a load-shedding algorithm. First the entertainment systems, then other unnecessary systems like HVAC, then things like the fog lights, can be turned off as the voltage droops, to extend the life of the battery running the engine. Very subtle emergency feature that's not mentioned AFAIK in the Owner's Manual. Or in the lighting section of the shop manual.
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