I saw this thread a couple days ago and it looked like you all had it figured out with the bad grounds so I didn't pay much attention to it until today. Some good advice given by all and it's hard to make a diagnosis until the tests get done.
It would appear that bad grounds are not the main problem from what I gather by reading some threads like this one.
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i have 2 extra complete gauge clusters that i have plugged in and tested to see if it was something with the actual cluster. all 3 have exhibited the same results. this tells me that it is not something with the cluster.
since i have multiple clusters i can test it with it in and with it out. no difference either way.
all gauges work when lights are off. turn the lights on the and fuel gauge goes all the way down and the temp gauge goes all the way up.
the dome light does work and i turn the knob to the dome light position. as i turn the switch it also changes how bright or dim the turn indicators glow.
i am going to grab some 1157 bulbs on the way home to test the LED locations.
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This could be caused by a voltage drop on the power wire to the gauge cluster which feeds the fuel and temp gauges. These gauges have a resistor in the positive to ground circuits and it is balanced by the positive and negative sides of the circuits. IOW if you have more ground on one side of the resistor it will read empty on the fuel gauge and hot on the temp gauge. Conversely a voltage drop on the positive side of the gauge will cause a higher ground reading and give the symptoms that are seen in the bolded sentence above.
Another clue is the bolded sentence in this post.
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Ok, so I am back to fully functional. I have no idea what it was, but I do know that only my 68 gauge cluster works, but my 2 other cluster, both from 72 trucks, still have the same issues. All I can say is I disconnected the main harness under the hood, blew it out with compressed air, put it back together and the stock gauges are working fine.
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All the power to the cab goes through this firewall block via a 12 gauge red wire that is tied into the battery and the alternator on the engine side of the truck and after it passes through the firewall block it tees off again to feed the ignition switch, the headlight switch, the horn relay and the fuse panel.
The following prints show these tees and the path of the positive wiring through the firewall block.
Follow the large red wire to the firewall block.
This is the inside cab print again follow the large red wire. Ignore the arrows they are for the accessory feed from the key switch.
The horn relay is on the left center and the fuse panel is bottom right. The inside of the firewall block is on the left lower. Notice the two pink wires going into the fuse block. The vertical one is from the key switch and feeds the fuse panel with key on power. The horizontal one goes over to the dash cluster and feeds the no. 3 terminal on the cluster plug. The next print will show this as well as the headlight switch.
This diagram is mainly for the idiot light cluster but the gauge cluster plug connections are on the top of the diagram. Your plug should compare with that one. Also notice the gray wires off the fuse panel are for the dash cluster lights which are fed from the headlight switch via the dark green wire.
A poor connection at the firewall block will affect all the positive feeds inside the cab and they may be intermittent as well which, is very frustrating. Even if there is a connection to the light switch, if it is not a good one will result in high resistance to current flow to ground through any loads. The voltage and current will follow the least path of resistance to ground which will leave less power for the higher resistances to ground. I hope this makes sense.
What you need to do is test for battery voltage at The firewall block on the red wire and at the horn relay or the key switch inside the cab with the key off and then test the pink wire at the key switch or the ignition unfused terminal on the fuse panel and at the number 3 pin on the cluster plug.
I have to leave it here for now, The boss wants to go to the store so check the voltages and maybe pull the fire wall block apart and clean it thoroughly and see if things don't improve.