Instrument Lights and Daytime Running Lights Not Working

[Double-A]

The red wire with the yellow terminal doesn't connect to a bulkhead connector, it goes straight through the firewall (there's a hole with a bunch of other wires running through it). So do I need to replace the little junction box or find a fusible link?

As for the starter area, I will have to go get a mirror tomorrow or something. And I saw a heat shield on the exhaust manifold by the starter so hopefully that's a good thing.

[bd]

Before I can help you with what to repair, we need to clearly establish what you presently have.  It seems the supply wiring has been altered from the factory configuration.  We need to determine to what extent, so you can safely restore power to the fuse box and headlamp switch.

I want to begin by determining where the various wires connected to the firewall junction block run - where they go.  (If the blue wire at the junction block doesn't connect to anything, it can be eliminated).  I also want to verify how battery power is routed into the cab and to what extent the factory circuit protection has been circumvented.  You may need to fabricate and install some fusible links.  The process isn't difficult, but follows an exact formula.  I will inform you when you get to that point.  The firewall junction block should clean up with some careful scrubbing with a compact stainless steel wire brush and some WD40. 

But first, post a pic of the bulkhead connector that clearly shows the wire colors entering the engine compartment side of the connector and the relative positions of the wires with respect to the connector cavities.  Are there two 12-gauge red wires entering the bulkhead connector?  Use your voltmeter to determine whether both red wires are constant 12 volts.  Assuming the 6.2 diesel, the opposite ends of the red wires should connect to the firewall junction block through a fusible link, and the alternator 2-cavity regulator connector, respectively.  Do they?

The 2-cavity regulator connector also should have a 6" fusible link that jumps between the terminal shared by the 12-gauge red wire and the 1/4" output stud of the alternator.  This fusible link is the circuit protection for the headlamp switch.  Give it a tug to see if it stretches or separates or if the terminals appear burned. 

Determine where the remaining wires attached to the junction block go.  One of the 12-gauge red wires should run down to the starter and share the 3/8" solenoid post with the battery cable.  Does your truck have a trailer brake?

[Double-A]

The only question I can answer tonight is the trailer brake. It does have a trailer brake, it was installed by a PO, however it seems to work. I can't test that though cause I don't have a trailer.

I will answer the rest of the questions tomorrow or later in the week.

I'd like to thank you for all your help so far too.

[Double-A]

Ok. I took a picture of the firewall connector that I think you spoke of ( http://imgur.com/a/0323V ). I took a few pictures to try and get as many details as possible. Most of the wire's shielding is faded and I cant really tell what color they are (sorry). The connector also looks awful, like its been melted, and I don't know why...

It does look like that connector has 2 12 gauge wires but I am not sure if one goes to the alternator. The wires connecting to the alternator are a larger (12 gauge I think) yellow wire, and 2 smaller red and white wires that are in a plastic connector. I am not sure if they have fusible links.

 One of the wires on the firewall connector looks like it could go down to the starter. I didn't have time to see where the wires on the firewall connector went.

I hope this info helps.

[bd]

Thus far, I've assumed and based my discussion on your 1986 K20 with 6.2L diesel.  Am I correct in this assumption?

FYI - The black-brown "GOOP" that fills the bulkhead connector cavities is a thick, anti-oxidant/sealant used to protect the electrical connections from moisture and other contamination.  It can create the appearance of being burned.

Looking at the image, below, the upper leftmost 12-gauge red wire (circled) should connect to the alternator regulator through the two-cavity (3-wire) plug.  The 12-gauge red wire in the center of the image (circled) should run to the firewall junction block.  Do your best to verify the wire routing.  Post a pic of the wire connections to the alternator.

With your voltmeter clipped to a good clean ground, back-probe the two red wires (circled) in the bulkhead connector using your voltmeter.  Both should measure constant battery voltage with the ignition OFF.  Do they?

The firewall junction block should receive battery power through a single 12-gauge red wire connected to the starter solenoid battery cable stud.

I suspect that the red wire with yellow terminal attached to the firewall junction block that runs into the cab through the grommet in the firewall, connects to the trailer brake control.  Please take a moment to verify this if you can see the wire inside the cab.

[Double-A]

You are correct about assuming my truck is a 1986 K20 with 6.2 Diesel.
How can I probe the connections you want me to probe at the firewall connector? I don't see a way  of disconnecting it..
The alternator on my truck doesn't have a 2 cavity 3 wire plug. It has a 2 cavity 2 wire plug (they are white and red wires) and a lug with a 12 gauge yellow wire.

I'll do the rest of the tests tomorrow and see about that wire on the junction block being the trailer brake control.

[bd]

How can I probe the connections you want me to probe at the firewall connector? I don't see a way  of disconnecting it..

Shove the positive probe of the meter through the 'goop' into the cavity occupied by the wire you are probing until it touches the terminal (roughly 3/8" give or take).

The alternator on my truck doesn't have a 2 cavity 3 wire plug. It has a 2 cavity 2 wire plug (they are white and red wires) and a lug with a 12 gauge yellow wire.

Post a pic of the wire connections to the back of the alternator.  To what does the opposite end of the 12-ga 'yellow' wire connect?

[Double-A]

Ok. The wire you circled in my picture and labeled as "Alternator" has no voltage. The other wire you circled does though.
The wire you suspected as the trailer brake control is indeed one of its wires.

The yellow wire on the alternator goes to the glow plug controller ( http://imgur.com/DO9Zyyu its the wire on the top lug that's yellow )

Here is the back of the alternator ( http://imgur.com/Pe7Lv0R sorry its hard to see but there are a bunch of things in the way.) The stud has the yellow wire I described that connects to the glow plug controller.

I didn't test the junction block cables to find the power for it. I will do that tomorrow if its still important after this info.

[bd]

You're making good progress and have narrowed it down, significantly.  Now, you need to focus on the 12-ga red wire at the bulkhead connector, labeled "TO ALTERNATOR" in the previous image, AND on all of the wires connecting to the alternator.  I'm guessing that someone replaced the alternator and regulator pigtail, and in the process neglected to reconnect the 12-ga red wire that runs to the bulkhead connector. 

Look closely behind and around the alternator for the "abandoned" red wire from the bulkhead connector.  It may be bare or it could have been taped up in the loom out of sight.  Hopefully, it wasn't removed. 

Move some of the things that are shrouding the back of the alternator and post a few more pics of the alternator wiring, especially the wires attached to the output post and where/how the red & white regulator pigtail is wired in.  Remain diligent.  You are almost there.

[Double-A]

You are correct about the alternator being replaced. A PO replaced it. But all these wiring problems were done before the PO who replaced the alternator.
I will go take those pictures, search for the "To Alternator" wire, and find out more about the red/white pair.

[Double-A]

Ok.. I took some more pictures ( http://imgur.com/a/xKiNi Picture 1 is the alternator, picture 2 is showing what the yellow wire connecting to the alternator lug is.)
The red wire that connects to the 2 cavity 2 wire (red/white) is the 12 gauge wire I think.
I believe the wire that connects to the white wire is brown but it could be black.

 Again sorry its hard to see some of the stuff.

[bd]

Pick up a 20-amp, auto-reset circuit breaker...

     

          ...and attach a couple of 12" 16-gauge leads with insulated alligator clips.  This will make a handy jumper for testing.

Use the described jumper to temporarily connect between the alternator output post and the "red wire" running to the regulator.  Do your running lights work with the headlamp switch, now?

[Double-A]

The output post is the one with the yellow wire correct?

[bd]

The output post protrudes out of the back of the alternator from a red plastic insulator.  I can barely make it out in your photo, but it looks like the yellow wire is attached to it.  Is there also a heavy gauge red wire connected to that post?

[Double-A]

The only wire attached to that lug is the yellow one. It's the red wire that is labeled "to alternator" in the other picture is spliced into the red wire that's next to the white wire.

Here is a picture with labels of what I mean ( http://imgur.com/oLM0kr2 )