Sunroof - please explain hot fuse without feature

[Elizabetty]

This sunroof fuse was mentioned in passing in another thread but I knew a lot less than so I'm hoping this will help me get a clearer picture. I don't necessarily know a lot more now but I feel like I can ask my questions in a way that helps me get an answer I can understand better.

In the photo where my finger is pointing I have an empty fuse slot. My assumption is it is fuse 28 from the diagram attached. My lid doesn't show this fuse would exist but it is in my fuse box and empty.

I have a fuse in the sunroof, slot 20 spot. It is live and receiving voltage. I do not have a sunroof.

Out of curiosity, I put a fuse in the assumed slot 28 and it receives voltage also.

When going through diagrams I don't see anything else that the sunroof fuse in my fuse box would be connected to. I also don't see what the assumed fuse 28 would be connected to elsewhere.

In regards to the sunroof fuse which has been in the box, if it's receiving voltage, does that mean something is wired to go somewhere and doesn't since I don't have the relay for it or the switch to activate the sunroof? Or is the power simply there but there wouldn't be any wiring?

For the empty slot that does receive voltage when tested with a fuse, but I took it back out, I have the same question as above. Is there any difference in the answer based on if the fuse is in it or not. I understand the function of the fuse isn't to actually supply the power. I'm just curious what the voltage is doing that is available.

I understand vehicles can be set up for features they don't have. What I don't understand is from an electrical standpoint if there are no parts or switches for the wires to go, what is happening at the fuse block where they are receiving voltage then? It was explained to me when posting about some blunt cut wires behind the dash that they were for a brake to be wired in and they posed no threat especially since they come that way from the factory so I don't worry about them. However, this is different from simply wiring in a brake controller versus hacking in a sunroof and then running the entire wire harness because nothing exists, just the power.

Obviously, if in the wiring schematics I saw these fuses were somehow supplying power or interacting in a way with other items in the vehicle, I wouldn't be as curious because they are necessary but I don't see anything in any diagrams so I'm thinking they are standalone items.

Oh and how would that affect RAP if at all?

 

[mrrsm]

Best Practices? Locate and Compare the Two Different Vehicle Wiring Diagrams (Side By Side) and Check their OEM Harness Part Numbers for DIFFERENCES between the two relating to "De Luxe" Models (Envoys) having Harness Connectors and Fuse Boxes with subtle differences. Sometimes, they all come with the SAME Harnessing... but NOT always.

 

[TJBaker57]

Before I even get into these questions,,,,

There is so much difference between our 2002 trucks I am wondering if yours is a much later build date than mine.. On the drivers door by the latch there should be a sticker. At the top right is the build month and year. What do you have there??

 

[Elizabetty]

Before I even get into these questions,,,,

There is so much difference between our 2002 trucks I am wondering if yours is a much later build date than mine.. On the drivers door by the latch there should be a sticker. At the top right is the build month and year. What do you have there??

View attachment 107246

Now that you mentioned it, I remember you saying something before about the fact they were both 2002 but seem to have some differences going on. My build date is 05/02.

 

[TJBaker57]

My assumption is it is fuse 28 from the diagram attached.

If this diagram is the wiring diagram you posted.... That is for the TrailBlazer EXT, the long wheelbase version. Not the short wheelbase that I think you have.

And while on that subject I believe the wiring diagrams indicate there is NO RAP relay in the short wheelbase versions only the long wheelbase EXT versions.

I'm in the middle of some oil changes here so My replies will be piecemeal in relation to your post.

 

[Elizabetty]

Yes. I've been thinking for some time now that I have an EXT box in the rear and I had that confusion in one of my other threads but it didn't really seem to stand out as being a problem as far as any comments go but it makes some things frustrating when trying to understand my vehicle because not only does the cover not match the box underneath it, the box doesn't seem to match the 2002 wiring diagrams or my user manual. I ran into this issue with the 2002 wiring diagrams for the radio also not matching and my fuse box having OnStar fuses in it but I don't have OnStar. However, it appears my radio is wired for onstar. Things just don't line up and I don't know what matters and what doesn't.

 

[TJBaker57]

while on that subject I believe the wiring diagrams indicate there is NO RAP relay in the short wheelbase versions only

I want to clarify just a little here. This means there is no RELAY, but there is still RAP (retained accesory power), it is just handled by the BCM directly without an external relay.

 

[TJBaker57]

Yes. I've been thinking for some time now that I have an EXT box in the rear and I had that confusion in one of my other threads but it didn't really seem to stand out as being a problem as far as any comments go but it makes some things frustrating when trying to understand my vehicle because not only does the cover not match the box underneath it, the box doesn't seem to match the 2002 wiring diagrams or my user manual. I ran into this issue with the 2002 wiring diagrams for the radio also not matching and my fuse box having OnStar fuses in it but I don't have OnStar. However, it appears my radio is wired for onstar. Things just don't line up and I don't know what matters and what doesn't.

These sorts of things make me wonder if the rear fuseblock was swapped out by a previous owner.

 

[TJBaker57]

I have a fuse in the sunroof, slot 20 spot. It is live and receiving voltage. I do not have a sunroof.

Out of curiosity, I put a fuse in the assumed slot 28 and it receives voltage also.

I looked at the part number of your fuseblock label and see that one superseded the one I have.


I too have the empty slot in the 28 position as well as an empty spot for a standard relay just below the sunroof fuse (I have a sunroof) and the slot for rear fog lamps which I do not have.

My take on this is as a manufacturer GM likely chose to reduce inventory by not making a dozen or so different fuseblocks for every possible combination of options. Instead they use maybe the same fuseblock and just don't install fuses and related external wiring for options not present in that truck.

Using the same fuseblock would of course result in the fuse sockets having power to them, even though there is maybe no wiring leading from those fuseblock connectors out to things like sunroofs that are not there.

A previous owner may have put a fuse in your sunroof fuse socket?

 

[TJBaker57]

regards to the sunroof fuse which has been in the box, if it's receiving voltage, does that mean something is wired to go somewhere and doesn't since I don't have the relay for it or the switch to activate the sunroof? Or is the power simply there but there wouldn't be any wiring?

For the empty slot that does receive voltage when tested with a fuse, but I took it back out, I have the same question as above. Is there any difference in the answer based on if the fuse is in it or not. I understand the function of the fuse isn't to actually supply the power. I'm just curious what the voltage is doing that is available.

My guess here is that all the 'wiring' that is internal to the fuseblock is there...

But the harness that connects to the fuseblock from underneath it likely lacks the wires that would otherwise go to those missing options.

And it would make no difference at all if there was or was not a fuse in there, except that perhaps the place where the fuseblock to body harnesses hook up to underneath the fuseblock might then have pins with power that have no mating terminal in the harness?

A guess on my part as I have never really looked specifically for this sort of thing that I can remember.

 

[budwich]

So looking at the gm si, it is interesting that they do not provide any difference connectors / part numbers specific to the different box. To me, that means that it does not matter what box is in the truck as the wiring leaving the box is the same. Plugging in a fuse according to your fuse box layout will only "connect" those circuits as identified by the position in the box NOT the label on the cover as any cover can be put on any box. The box and ultimately the connector is the governing entity. The box "layers" can not be interchanged if I understand the pictures so you can't have an upper layer / fuse receiver plugged onto a lower layer / tracking.... assuming there are the "regular box layering" in the system.

EDIT: my mistake, any fuse layer can be bolted on the underlying track base layer.

 

[TJBaker57]

And I should point out that GM can make errors in putting these things and documentation together.

Case in point: My 2002 has the rear wiper. My fuse box label has NO item labeled for rear wiper. Not a mention. The online pdf of the Owners Manual likewise has no rear wiper breaker or fuse labeled. Same for the printed factory manual.

Do they expect me to believe my rear wiper is unprotected by fuse or breaker?

Elizabetty posted a picture of her fuse box label back in December. Her label supersedes mine and shows a 15 amp circuit breaker for the rear wiper in slot 9. That was I think the circuit breaker that kept coming loose? My fusebox currently has nothing in that slot..I cannot honestly say I remember my rear wiper working. Just can't remember.

 

[budwich]

that is true.... but I am not sure that the labeling will indicate ALL "subtending circuits / elements" on a given fuse. That is why they issue schematics. Anyway, tjbaker57, I do see that the SI shows body options against the rear wiper for the 2002 year. Depending on the option, the fuse is "contained" within the "domain" of the LGM "label". The "other" body type has the breaker instead.

 

[TJBaker57]

So now I need to explore my rear wiper circuit. I simply cannot remember if it ever worked since I have owned it these past 12 years!!

I will begin with the assumption that the label Elizabetty posted is correct and my label just forgot to add that component and I need to insert something where her labels calls for a 15A rear wiper breaker.

Naturally I will test the terminals first for power one one side and some unknown resistance indicative of a load on the other.

I surely don't expect the motor to turn at this point having been stationary for more that a decade.

Back on topic here......

I understand the function of the fuse isn't to actually supply the power. I'm just curious what the voltage is doing that is available.

I would answer that the voltage is not doing anything. Like a battery sitting on a workbench that is not wired to anything. You can test the voltage and see the value, but it is a 'potential' that is not going anywhere. Not being used.

Wire that battery positive to one side of a fuse and measure between the other side of that fuse and the negative terminal of the battery and you will see the same value on your meter but it still isn't "doing" anything (other than registering on the meter).

Such it is with the vehicle fuseblock. Where the fuse is connected through the harness to the battery positive but the other end of the fuse is wired to nothing then there is nothing being done with or without a fuse in the slot.