Mystery fog relay in rear fuse box

madklicker2

Original poster
Member
Mar 2, 2012
9
I was poking around yesterday to get more familiar with my truck's electrical system, and found what I thought might be a missing relay. Before I continue, I must say that I don't know of any electrical malfunctions at this point. Everything seems to be working correctly, including my fog lights.

In the inner lid of the rear fuse box (the one under the driver's side second row passenger seat), there is a hole and raised lettering that indicate a place for a "fog relay". Here's what I mean:

View attachment 22663

If you remove the inner lid and refer to the fuse box diagram on the outer lid, it seems to indicate that I have a missing relay. The arrows indicate where the diagram says there is a relay #45, called "RR FOG LP", and where the relay would be plugged in:

View attachment 22664

I've searched and found posts about the relay in the under-hood fuse box labeled "45", but not this one in the rear fuse box. Anyone know what this is, and if this relay is needed? :confused: I didn't know these trucks had "rear" fog lamps. Sure, they have large-ish reverse lamps, but not fog lamps.

My truck is a 2003 Envoy SLT.

Thanks!
 

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McGMT

Member
Jun 17, 2012
621
madklicker2 said:
I was poking around yesterday to get more familiar with my truck's electrical system, and found what I thought might be a missing relay. Before I continue, I must say that I don't know of any electrical malfunctions at this point. Everything seems to be working correctly, including my fog lights.

In the inner lid of the rear fuse box (the one under the driver's side second row passenger seat), there is a hole and raised lettering that indicate a place for a "fog relay". Here's what I mean:

View attachment 9069

If you remove the inner lid and refer to the fuse box diagram on the outer lid, it seems to indicate that I have a missing relay. The arrows indicate where the diagram says there is a relay #45, called "RR FOG LP", and where the relay would be plugged in:

View attachment 9070

I've searched and found posts about the relay in the under-hood fuse box labeled "45", but not this one in the rear fuse box. Anyone know what this is, and if this relay is needed? :confused: I didn't know these trucks had "rear" fog lamps. Sure, they have large-ish reverse lamps, but not fog lamps.

My truck is a 2003 Envoy SLT.

Thanks!

Some euro cars have a different tail lamp setup than others and that is what that is fer.... THere was a thread showing the different lights somewhere search around, I think it was in Saudi Arabia...
 

cbwilsha

Member
Dec 4, 2011
33
I have the exact same vehicle and can confirm the same exists with me. Some years back, I believe someone asked this question on the OS and the answer was that certain countries (e.g. UK) do, in fact, require Rear fog lamps. In any event, there's nothing for you to worry about.

C.B.
 

madklicker2

Original poster
Member
Mar 2, 2012
9
I appreciate the quick responses! I thought it would be something like that, but I wanted to be sure. I guess rear fog lamps in an area like the UK would be a good idea. I'll search again to find those threads you guys are talking about.

Thanks again!
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
It was an electrical mystery, and those upset me until I solve them, so I dug into the shop manual and posted years ago on trailvoy: rear fuse block fog relay - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

I actually bought a headlight switch that supported that function, but then realized it's the kind of feature that evil GM would have suppressed in the BCM if the vehicle was for US domestic use. But I do have a neat extra button. :wink:

What STILL bothers me is why they went to the trouble of making a more complex mold for the fuse block cover, SIMPLY to make it easier to add the relay at the end of the production line, after the cover is on, in case the vehicle gets targeted for an export market? That, to me is nuts, and coddles the production floor workers. 15 extra seconds to remove the cover to instal the relay is worth the design time to put a labeled hole in the cover? That can let junk fall in and thwart the intended purpose for the cover?!?!? :hissyfit:

I'm not OCD about most things, but not understanding what went on in fellow designer's minds - THAT bothers me. :crazy:
 

madklicker2

Original poster
Member
Mar 2, 2012
9
the roadie said:
It was an electrical mystery, and those upset me until I solve them, so I dug into the shop manual and posted years ago on trailvoy: rear fuse block fog relay - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

I actually bought a headlight switch that supported that function, but then realized it's the kind of feature that evil GM would have suppressed in the BCM if the vehicle was for US domestic use. But I do have a neat extra button. :wink:

Wow, Roadie! Thanks for the link! I've just spent an hour digging through the OS looking at that and several other threads that talked about that relay and the rear fog lamp. All good info. Wish I had searched more thoroughly earlier.

I'm a bit embarassed, actually. The reason I have such a low post count at this point is that I usually find the answers to my questions through diligent searching, and I don't usually have much (if anything) to contribute. I guess I just got lazy this time.

Oh, and another thing. I had downloaded the shop manuals from here a while ago, but never got around to looking at them. Well, your response was incentive enough to take a look. Wow (again). :biggrin: This is the sort of thing I remember from the Navy - technical manuals with everything you ever wanted to know about whatever it is you're working on. The schematics alone are lifting the veil of mystery already. I have already seen what you and others have said about the European headlight switch, body control module, relays and other components.

It raises a question: Is all the wiring still there in North American vehicles to support European parts? If so, could something be done to use that wiring to make a particular mod work? Or, is the lack of BCM programming support pretty much the death knell for any mod using those circuits? Seems from the schematics that might be the case. Although, with enough wires, fuses and relays, I suppose anything is possible. :wootwoot: Food for thought, if nothing else....
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
In my short :wink: time on the two forums, I've seen many, but not all, optional functions be locked out by BCM configuration. The one glaring omission, that GM probably forgot they could charge money for a post-purchase upgrade, was the cruise control. If you have (mostly) a rental fleet vehicle or LS (stands for "less stuff") trim level truck without cruise control, you'll be glad to hear that adding it is as easy as slapping in a new stalk (multifunction switch assy.)

Headlight aiming from the driver's seat and power folding mirrors are options that would be nice to add, but GM *did* lock those out with reduction in wiring and components.

On the manual issue, I buy the factory shop manual as soon as I buy any vehicle. In the case of the Envoy, it was over a thousand pages of bible-thin paper. But I was obsessed with getting into the minds of the designers and understand the systems, so I sat down for over a month, every evening, and read it cover to cover. I'm sort of like the Enterprise's Scotty that way. Reading tech manuals is recreational FUN. :crazy:
 

McGMT

Member
Jun 17, 2012
621
the roadie said:
In my short :wink: time on the two forums, I've seen many, but not all, optional functions be locked out by BCM configuration. The one glaring omission, that GM probably forgot they could charge money for a post-purchase upgrade, was the cruise control. If you have (mostly) a rental fleet vehicle or LS (stands for "less stuff") trim level truck without cruise control, you'll be glad to hear that adding it is as easy as slapping in a new stalk (multifunction switch assy.)

Headlight aiming from the driver's seat and power folding mirrors are options that would be nice to add, but GM *did* lock those out with reduction in wiring and components.

On the manual issue, I buy the factory shop manual as soon as I buy any vehicle. In the case of the Envoy, it was over a thousand pages of bible-thin paper. But I was obsessed with getting into the minds of the designers and understand the systems, so I sat down for over a month, every evening, and read it cover to cover. I'm sort of like the Enterprise's Scotty that way. Reading tech manuals is recreational FUN. :crazy:

Ya know, I keep hearing about the FLT and LS stripped down stuff but it is funny as mine was ordered as a Fleet vehicle LS and I have cruise and power seats and such... It was a rental car for 5k and that's it according to carfox....
 

madklicker2

Original poster
Member
Mar 2, 2012
9
the roadie said:
In my short :wink: time on the two forums, I've seen many, but not all, optional functions be locked out by BCM configuration. The one glaring omission, that GM probably forgot they could charge money for a post-purchase upgrade, was the cruise control. If you have (mostly) a rental fleet vehicle or LS (stands for "less stuff") trim level truck without cruise control, you'll be glad to hear that adding it is as easy as slapping in a new stalk (multifunction switch assy.)

Headlight aiming from the driver's seat and power folding mirrors are options that would be nice to add, but GM *did* lock those out with reduction in wiring and components.

Well, I guess that answers my question. The days of simple electrical systems in cars are pretty much gone, I guess.

the roadie said:
On the manual issue, I buy the factory shop manual as soon as I buy any vehicle. In the case of the Envoy, it was over a thousand pages of bible-thin paper. But I was obsessed with getting into the minds of the designers and understand the systems, so I sat down for over a month, every evening, and read it cover to cover. I'm sort of like the Enterprise's Scotty that way. Reading tech manuals is recreational FUN. :crazy:

It's interesting you say that. This is part of my bookshelf, showing some goodies I've collected over the years:

View attachment 22674

And yes, I've read all of them, cover to cover (many of them more than once). I think I was the only electrician in my shop on the ship that actually liked reading tech manuals. The other guys only did it out of necessity. Oh, and on the day I brought my Envoy home, the first thing I did was sit down with the Owner's Manual and start reading. I was done with it in a couple days.
 

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Voymom

Member
Feb 3, 2012
2,523
Wow...all this time and I never realized that Phantom had the same thing. I have poked around that fuse box a lot in the last 8 months, and I had to go back outside just now because I didn't remember seeing it before, but yup I have it too lol
 

Phantom

Member
Jun 17, 2012
277
the roadie said:
It was an electrical mystery, and those upset me until I solve them, so I dug into the shop manual and posted years ago on trailvoy: rear fuse block fog relay - Chevy TrailBlazer, TrailBlazer SS and GMC Envoy Forum

I actually bought a headlight switch that supported that function, but then realized it's the kind of feature that evil GM would have suppressed in the BCM if the vehicle was for US domestic use. But I do have a neat extra button. :wink:

What STILL bothers me is why they went to the trouble of making a more complex mold for the fuse block cover, SIMPLY to make it easier to add the relay at the end of the production line, after the cover is on, in case the vehicle gets targeted for an export market? That, to me is nuts, and coddles the production floor workers. 15 extra seconds to remove the cover to instal the relay is worth the design time to put a labeled hole in the cover? That can let junk fall in and thwart the intended purpose for the cover?!?!? :hissyfit:

I'm not OCD about most things, but not understanding what went on in fellow designer's minds - THAT bothers me. :crazy:

Now Roadie, is that something that can be bypassed, with say a jumper wire and toggle switch? Cause I would soo put spotlights mounted on the bumper and wire up a toggle switch for all the people who like to ride close enough to where you can't see their headlights.
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Phantom said:
Now Roadie, is that something that can be bypassed, with say a jumper wire and toggle switch?
You can put anything in the back with a switch and independent wiring. I don't think there's any benefit to try to save 10% of the complexity of a simple project by trying to locate hidden factory harness wires that may not be there.

For tailgaters, I think a manual activation of backup lights might be just as effective in inducing panic.
 

Matt

Member
Dec 2, 2011
4,033
You could always just get a Wombar (TM) and brake check them. :yes:

2012-04-20_16-41-04_32.jpg
[/IMG]
 

Voymom

Member
Feb 3, 2012
2,523
Matt said:
You could always just get a Wombar (TM) and brake check them. :yes:

2012-04-20_16-41-04_32.jpg
[/IMG]

Matt OMG I love that idea!!!! I would paint mine with a giant bullseye on it :biggrin:
 

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