I'm working on my 87 Firebird LG4 automatic, unfortunately, the Factory service manual I have is for a 90 model, and the wiring diagrams aren't quite the same, so far, I have the following diagrams to work off of.

What light on the dash works? just the choke light that I can see. I'm not sure whether the speedometer works or not, I haven't tried driving the car.

The following indicators are not working
-Hi beam
-turn
-seat belt
-oil pressure low
-temp high(should illuminate while cranking right?)
-charge light
-SES

Additionally, the Fuel gauge appears to no longer be functioning. I just put a new tank and sending unit in the car, and while the tank was out, I cycled the new sender to make sure it works like it should, and it did!? now it seems stuck at 1/4 tank, with about 8-10 gallons in it, which I would expect it to read near full. have there been issues with floats and aftermarket tanks not playing nice together? the only other things that have changed since are the engine accessory drive (had a bad water pump so I put a serpentine belt setup on it) and the battery cables, which were atrocious, so I replaced, and significantly upsized them.

Things I have checked or tested
-resistance between the negative post on the battery and the engine block, the engine block and the chassis, and the chassis to the negative post, are all less than my meter leads (meter read 0.3 ohms)
-power at every fuse when there should be.
-exterior lights are functional, and react as expected, hazards flash, blinkers flash, high beams change and headlights come on and off ect.
-Alternator is charging, 12.8 volts engine off, 14+ running.

I think my next step is to take the cluster out and start checking bulbs, does anyone have any advice or go to things that they have found in a similar situation? I have a hard time believing almost every bulb is dead, but I also don't see anything otherwise.

 

I noticed in one of the drawings, G200 has 2 wires going to it, and on my car, I can only see one wire there, looking at ground distribution, it shows one of those is from S205, as above, and the other coming from the radio, I'll take a closer look tomorrow sometime.

 

Pull the cluster and check the ribbon sheet on the back where all the screw contacts are. They may be corroded. I had a wonky fuel gauge, volts that were reading 8, and a temp that was maxed. All due to corroded contacts on the back of the cluster.

 

I'm kinda expecting a problem like that, I just don't want to take the cash apart yet...

 

so, I haven't really taken anything apart yet, but I did find one thing of moderate significance, I started the car the other day, drove it around the block and the SES light came on, which if nothing else, tells me 2 things, the ECM is working (CC Qjet) and there's power to the lights and fuel level gauge, so my issue is wither connections to the cluster, or related to grounding.