When you are checking do you have the gauge cluster grounded properly???
Aren't electrical issues just the best? .
Grounding the cluster is required but clips may not be enough. Good 12 gauge wire with round terminals and nuts and bolts will ensure a good connection. The ammeter may be leaking voltage or there is a intermittent break in a wire behind the dash.
The cluster needs to be bench tested with extended wire run from the senders and power and ground. If it works on the bench but not in the dash then you start looking for a bad connection somewhere in the car. It might be one of the gauges is bad or intermittent so they should be dismantled and cleaned.
The good part is that when you become proficient at fixing the problem on the bronco you will be in a position to fix just about anything electrical! It's important to know what to look for and to qualify circuits in a logical order so that you progress toward the solution.
You are bouncing back and forth and may accidently get everything to work but without positively identifying the cause of the fault or faults it's likely to return to haunt you.
I found a wiring diagram and from the IVR there are three voltage drops in order, oil, temp, and fuel. The ground for the gauge circuit is the cluster case. The gauges are attached to the case by screws and since the voltage and current is very low
then if there's weak ground then the first drop which is the oil gauge could work and not the other two. In addition, how the gauge electrically works and where the ground point is located on the gauge case would be useful to know. In any event, 40 to 50 years old gauges need to be dismantled and cleaned at minimum to achieve any kind of reliability.
What time are you going to do it?
Okay, can you make 4 gauge cables or do you buy them? I'll measure my 72 and provide instructions and pictures to bullet proof the grounding system which is where we start. Do you have a amp clip?