Dash cluster light - Electrical Circuit Number 8 question

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Mark80K15

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Hi, I have a 1980 GMC K15. My dashboard gauge cluster lights don't come on when I turn on the headlights. I've replaced the printed circuit on the back of the cluster plate but that didn't help. I did that because the current one looked pretty ratty. Then I found this thread that had excellent advice from @chengny https://www.gmsquarebody.com/threads/dash-cluster-light-issue.22455/#post-434590

That thread confirms that the plug for the instrument cluster lighting circuit must be correctly seated in the fuse block column labeled LPS. My LPS column is in a different place than the 1987 truck in that thread, but no matter. Like that thread, I have no wire/plug seated in the LPS column. So @chengny suggested looking for a grey wire hanging down somewhere.

The beginning of that thread shows the cluster plate wiring harness connection and points out the grey wire (in slot #2). The thread suggests that grey wire is for the gauge cluster lights, and that the other end of that grey wire must be plugged in to the LPS column.

In my case I can't find where it is hanging down. I did find the grey wire in the cluster plate wiring harness connection and it looks intact there. I used a web PDF version of the "1980 Truck Wiring Diagrams" manual (I think) to try to trace that grey wire from the back of the cluster plate. That manual shows a grey wire exiting the wiring harness connection of the cluster plate and proceeding on "Circuit Number 8, Instrument and Panel Lights". In particular, the grey wire is labelled in the diagram as ".5 GRAY-8B". It goes to "SPLICE 8", and one wire that comes out of that splice is ".5 GRAY-8A". This wire terminates in the diagram at LPS in the FUSE BLOCK. This is the wire I can't find under my dash.

So, my question is: where is the splice? I need to find it to confirm if there is a ".5 GRAY-8A" wire coming out of it, and where that wire is hanging, so I can plug it in. Is it just in the wiring harness somewhere? Do I need to pull the harness apart to get at it? Thanks! Mark
 

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AuroraGirl

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1980 gmc k25 fuse box, probably the same as yours. see how i have an orange tap going to that?

Look for a harness with a 1212 sticker like mine in the photo

my backlighting works (except for behind the speedo in one spot, the contact on the prnted board was ripped off)
 

Mark80K15

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Hi @AuroraGirl , thanks a lot for posting that. I attached a pic of my fuse box ("fuse_block_picture.JPG"). It has a few wires removed for clarity. I left one orange wire which is for the courtesy light. There was never anything plugged into my LPS terminal. As you say you have a 1212 orange wire there. I don't see an orange wire with 1212, but I do have a weird double tap (with 2 orange wires) plugged in to the terminal below the one for the courtesy light. See attached pic "double_tap.JPG". In the picture, the double tap is hanging in mid air for clarity.

I suspect the orange wire coming out the far end of the double tap is the 1212 you're talking about going to the cluster plate. I'll confirm that once I have the cluster plate back in place. In the meantime, do you know what the other orange wire is? It is in the double tap closer to the fuse block.

In the attached "mystery_orange_wire.JPG" you can see the wire I'm asking about terminating into something at the upper right of the fuse block. That is not visible in your picture. Anyone know what that is?

One more thing, I have the fuse block schematic from the GM 1980 GMC Service Manual (attached "fuse_block_diagram.JPG"). Does anyone know what the codes are for the various receptacles? I circled an example. In that example, do &T63, UF2, and U35 correspond to 3 different circuits? Or one circuit? Which one(s)?
 

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AuroraGirl

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Hi @AuroraGirl , thanks a lot for posting that. I attached a pic of my fuse box ("fuse_block_picture.JPG"). It has a few wires removed for clarity. I left one orange wire which is for the courtesy light. There was never anything plugged into my LPS terminal. As you say you have a 1212 orange wire there. I don't see an orange wire with 1212, but I do have a weird double tap (with 2 orange wires) plugged in to the terminal below the one for the courtesy light. See attached pic "double_tap.JPG". In the picture, the double tap is hanging in mid air for clarity.

I suspect the orange wire coming out the far end of the double tap is the 1212 you're talking about going to the cluster plate. I'll confirm that once I have the cluster plate back in place. In the meantime, do you know what the other orange wire is? It is in the double tap closer to the fuse block.

In the attached "mystery_orange_wire.JPG" you can see the wire I'm asking about terminating into something at the upper right of the fuse block. That is not visible in your picture. Anyone know what that is?

One more thing, I have the fuse block schematic from the GM 1980 GMC Service Manual (attached "fuse_block_diagram.JPG"). Does anyone know what the codes are for the various receptacles? I circled an example. In that example, do &T63, UF2, and U35 correspond to 3 different circuits? Or one circuit? Which one(s)?
You dont appear to have roof lamps, correct?
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I found an old thread of mine, it looks like i was right, the orange taps are all for feeding options primarily. I have roof lamps, one of my orange taps will lead to the roof markers. The Diagram I posted wshows which fuseed outputs are connected to what legs of what parts of the box here
 

AuroraGirl

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Hi @AuroraGirl , thanks a lot for posting that. I attached a pic of my fuse box ("fuse_block_picture.JPG"). It has a few wires removed for clarity. I left one orange wire which is for the courtesy light. There was never anything plugged into my LPS terminal. As you say you have a 1212 orange wire there. I don't see an orange wire with 1212, but I do have a weird double tap (with 2 orange wires) plugged in to the terminal below the one for the courtesy light. See attached pic "double_tap.JPG". In the picture, the double tap is hanging in mid air for clarity.

I suspect the orange wire coming out the far end of the double tap is the 1212 you're talking about going to the cluster plate. I'll confirm that once I have the cluster plate back in place. In the meantime, do you know what the other orange wire is? It is in the double tap closer to the fuse block.

In the attached "mystery_orange_wire.JPG" you can see the wire I'm asking about terminating into something at the upper right of the fuse block. That is not visible in your picture. Anyone know what that is?

One more thing, I have the fuse block schematic from the GM 1980 GMC Service Manual (attached "fuse_block_diagram.JPG"). Does anyone know what the codes are for the various receptacles? I circled an example. In that example, do &T63, UF2, and U35 correspond to 3 different circuits? Or one circuit? Which one(s)?
Those 3 alphanumeric codes are RPO codes. RPO are Regular Production Codes and belong on the SPID label (Service Parts Identification Label) that originally sat in your glovebox. The U35, for example, is the cargo area lamp. if you dont have a lamp facing your bed on the cab, you wont have any wiring for it. U35 is electric clock (mounted in the extra gauge pod on the dash) and T63 is headlamp warning buzzzer which I frankly know very little about but your might have it. I only have the seatbelt one.
 

AuroraGirl

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Ok, thanks a lot @AuroraGirl . To answer your question, no, I don't have roof lamps.
Then there is a chance my "LPS" tap is the roof lamps
but maybe not. MARK I may have something that helps you!
I have head lights, but no tail lights.

Brake lights? The above makes sense because (even though the power to each circuit passes through the headlight switch) the H/L circuit is segregated from the parking lamp circuit. The H/L's are protected by a fusible link on the firewall. The tail lights/front parking (and front side markers)/courtesy lighting are protected by a fuse in the main fuse block (TL/CTSY).


Here, let's do it this way:

Check for voltage at the fuse socket labeled INST LPS (top fuse, 2nd column from right). The line side should be the left clip - but check both sides.

If you have no voltage there, read this:


The reason the fuse socket is dead for the panel lighting is because there is no power coming back from the rheostat in the HL switch.

The flow goes like this:

The red 3.0 wire from the firewall fusible link is tied into the line side of the far left column (looking forward). The actual connection is in the lower half, there is a shunt about halfway up that supplies the sockets in the upper half. These sockets are for various options (clock, cargo light, etc) that require constant battery power.

Power comes out of the TL/CTSY fuse and up to the HL switch on the .8 orange lead. The voltage is reduced as needed by the rheostat, and is lead out of the HL switch at pin 44. It goes back to the fuse block (from the HL switch) on the 1.0 green lead. The tail light power does not pass through the rheostat (just goes through a switch).

That 1.0 green lead is tied into the fuse block at the hot side of the 5A Inst Pnl fuse. The load side of the fuse is connected to the panel illumination lighting via a .5 gray wire.

Probably the easiest place you can start ringing this out at the HL switch:

Power in to the rheostat on the .8 orange at pin 40 but no power out on the 1.0 dk green at pin 44? Bad rheostat.

Power at pin 44 but not at the line side of the instrument light fuse? The green 44 wire is broken/disco'd.

If there is no power into the rheostat on the orange wire, check the load side of the TL/CTSY fuse. Power there? Broken orange lead from the fuse block to the HL switch.



Here is the dwg that goes with the description of the circuit (above):


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found on this thread: https://www.gmsquarebody.com/threads/park-side-marker-lamp-problem.13662/

so based on more info I found, my roof lamps are NOT one of the orange wire leads to the fuse box. so its possible the harness I mentioned is the one youre looking for maybe
 

AuroraGirl

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Here is my trucks SPID label if you wanted to see an example Mark : )
 

Mark80K15

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Wow @AuroraGirl , that's very helpful! I had no idea the codes on the SPID label are the ones found on the GM 1980 GMC Service Manual (attached "fuse_block_diagram.JPG"). Thanks!

And thanks for the @chengny post, it helps. I still need to know which orange wire that is that goes to the terminal on the upper right of the fuse block, as illustrated in the attached "mystery_orange_wire.JPG".
 

AuroraGirl

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Wow @AuroraGirl , that's very helpful! I had no idea the codes on the SPID label are the ones found on the GM 1980 GMC Service Manual (attached "fuse_block_diagram.JPG"). Thanks!

And thanks for the @chengny post, it helps. I still need to know which orange wire that is that goes to the terminal on the upper right of the fuse block, as illustrated in the attached "mystery_orange_wire.JPG".
yeah... lol
 

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