Yes, I tested the bulbs for continuity. As well as the PCB circuit that flows through the bulb. And then replaced them with LEDs and tested again. And then swapped the PCB and tested again. I'm fairly confident that neither the bulbs nor PCB are the issue.

Bulb power is PIN 10, and gauge power is PIN 14 which leads to the IVR. Both don't work on the dash... Although I do have power to 10, and not to 14. Lights have a fuse (INST PNL) but gauges don't appear to. All my fuses tested good and have been re-seated. At this point I'm tempted to just run new wires to the cluster and be done with it, but I have to imagine the fix is near, since I've eliminated so many things...

 

Did you test each bulb, installed in a holder, as shown in a picture I posted above? I test each bulb, out of the holder, then install them in a holder & test again. The contacts inside the holder often need to be cleaned. Sometimes the contacts are broken off inside the holder. The contacts on the outside of the holder, where they touch the PC need to be cleaned, & bent out a little so they make good contact with the PC. Clean ALL the contact area on the PC. Install the bulb & holder in the cluster after they test good. Clean the contacts on the PC where the plug goes!! I can tell from your cluster pictures that the contacts need cleaned on the PC. I use fine steel wool. I clean all the contacts with the PC removed from the cluster.

Then, with the cluster out of the truck, test the lights while installed in the cluster. Best way is to get the harness plug out of the van you got the cluster out of. There is a picture of the plug I use to test clusters posted above. Using this makes things a lot easier. Cut it out with 6" of wire or so. Strip the end of the wires coming out of 10 & 9. Hook a battery charger to the wires, ground to 9, positive to 10. Plug the charger in, see if you have 2 dash lights. You may have to turn the lights out in the room to see them, especially if you still have the blue covers over the bulbs. You may have to wiggle or turn the holders to get the lights to come on, even with cleaned or new components.

If you don't have the plug, you can use alligator clips on the PC plug location, if you are careful. Hook the clips to where 9 & 10 are, & test.

I have found that you have to test each & every component, down to the smallest part, & clean everything, to eliminate problems. I'll repeat it again: Clean everything, every last part, every last contact. Test everything, every step of the way. They install & reassemble.

Also, LED's are polarity sensitive, you you may have to spin the holder 180 degrees to get them to work.

 

Not sure why you don't have power to #14 for the IVR. It is a resistance wire, a single strand under the insulation. It may be possible that it is broken somewhere. I'll have to check my manuals & see if I can figure out where it runs from.

 

Test the gauges if you haven't already:

 

Well, I looked at a coupla PCBs/cluster/gauges....sorry to say I'm of no help. Like I said before, yours is different than mine...not by much but still different. I couldn't trace Pin 14 back more than a foot......no help. I'll take another peek at an older one tomorrow. Funny, I use to know these things...hehe

I'm wonderin' if your new ICVR is touching anything it shouldn't.....on the bottom of it.....mebbe kiltered down a bit? Just a thought.

 

I forgot about the LEDs being polarized. After I swapped to the LEDs, the same lights were working that had been working before, so I didn't even think about it... What are the chances that the same blinkers and fasten belt light that were working before the swap, were the exact bulbs that I happened to put in with correct polar orientation?

All the lights do work now. So I assume the old crumbling backing had weaker connection on half the lights, because I had tried to swap them out with other incandescent bulbs a few days before. But I just put the plastic backing from the van on last night, with LEDs in it from the start.

Anyway, now that the lights are all working the only remaining issue is to get power to PIN 14 for the IVR.

 

I had cleaned everything pretty well while I was combining the two clusters. That was a pic I took before I started. I had been afraid to clean the old one very much, as it was crumbling to the touch...

I was thinking about how nice it would be to have taken that pigtail from the van while I was out there, after seeing the pic of your tester above... But I didn't have anything sharp enough to cut it out with. Just a screwdriver and a socket set with me at the time.

 

Excellent!!

One down.......

 

I could only trace pin #14 about a foot as well... It seems to just go directly into the main harness and disappear. Does it run through the ignition tumbler switch to get its switched power I wonder? If so, that's probably a point of failure.

I will have to take another look at my ICVR, but I don't believe it's touching anything. Though I don't get power to pin 14 even with the cluster unplugged.

 

I was looking at some wiring diagrams earlier today and the '76 - '78 diagrams show power coming from a BLUE/r wire, into a splice (can't remember the color) and then into the ignition switch. I could be way off here cause my eyeball ain't as good as it use to be.....and I REFUSE to wear a monocle....hahaha

Of course, nothing on the 18 pin connector was marked in these schematics, just going from memory....something in the harness is bothering me....

But you're onto something here...

 

This may help hopefully, from a '78: Circuit-297 BLACK/gr goes from ignition switch to S-303 where the circuit is changed to #30 BLACK/gr where it goes through a 8 - 8.5 Ohm resistor, to the ICVR.

...And : Circuit-30 BLACK/gr powers the fuel, temp and oil pressure gauges.

Check continuity on the ignition switch...wait...check for BATT voltage on the ignition switch at the BLACK/gr wire....Key ON...engine OFF

Do you by chance have a radio receiver suppression choke on your rig?

 

The service manual shows two different things when it comes to wire 14. In one diagram, it shows it going to the ACC terminal on the ignition switch. In that diagram it shows the 8.5 ohm resistor, then a radio choke, then on to the IVR. In a different spot in the manual it says the IVR is powered by a 15 amp fused circuit coming from the fuse box.

I suspect they leave the fuse box out in the one diagram. Power would go from the ignition switch ACC terminal to the fuse box, then to the resistor wire, then to location 14. So, I would double check the ACC fuse, probably replace it even if it looks good. Clean the fuse holders too. Maybe check the connections on the ignition switch as well.

I can't see anything else that is powered by the ACC fuse, other than the gauges. Everything else seems to have its own fuse or circuit breaker. But, I don't have a wiring diagram to be able to tell for sure.

 

I checked the ignition switch. I have 12v on the yellow wire, but no other wires have power. I only have wires to 5 of the pin slots, one slot is empty. Is it missing, or should that be empty? The back of the ignition tumbler has 6 pins...

I have PPL/WHT (brake), RED/BLU (neutral), RED/GRN (starter relay) - actually two RED/GRN wires that go to the same slot, YEL (fuse box? has 12v), and one BLK/GRN. The diagram shows two BLK/GRN, one goes to the fuse box, and one goes to the gauges. Am I missing a second black/green wire? Or do they both go through this slot and it splits off later, and there's supposed to be an empty slot?

I'm not sure, what is a radio receiver suppression choke?

 

ACC fuse tested good, and had 12v on both sides of the wire. But I replaced it anyway, and cleaned up the fuse holders some more. Still 12v on both sides with the new fuse and clean connectors now, so probably safe there. Good to know though, I didn't realize it went through the fuse box.

So this means the problem should lie between the fuse box and the instrument cluster plug...

 

I agree the diagram makes it look like the splice is at the switch, but according to this, it's where the resistor is:
I'm familiar with the radio suppression choke being mounted near the voltage regulator, but the diagram makes it look like it's behind the dash...