Over time, the ammeter used in '73-'85 Full-Sized Jeeps tend to increase in resistance, and eventually overheat to the point where the wires running to and from it overheat, melt the insulation and either take out adjacent wires or short out entirely. This is especially a problem because, unlike other vehicles, the ammeter in the dash reads the battery-charge DIRECTLY, without a shunt. That means that the battery is being charged by the alternator EXCLUSIVELY through a 10-gauge wire that runs from the alternator, to the firewall bulkhead, to the ammeter, back to the bulkhead connector, then over to the (+) stud on the starter relay on the passenger-side firewall. Over twelve feet of 20 or 30-year old 10-gauge wiring, with an ammeter in-line, is often strained by charging the battery.
That is why, if the battery in your FSJ is run down, DO NOT JUMP-START IT AND TRY CHARGING IT OFF THE ALTERNATOR!!! Only charge it on an external battery-charger. This is the most common cause of dashboard wiring fires known to FSJ owners.
The best solution is to add an additional charging circuit, with a fusible-link, directly from the alternator to the (+) stud on the starter relay (the one that the (+) battery-cable runs to). Use at least a 10-gauge wire, with a 14-gauge fusible link, and make sure to route it away from the exhaust manifold or any sharp edges. When doing this, it is also necessary, and VERY IMPORTANT, to reposition the original red charging wire from the charging stud on the back of the alternator to the (+) stud on the starter relay. You may also need to crimp and solder a larger ring-terminal onto that original red wire to fit it on the larger stud.With that original wire connected to the starter relay rather than the alternator itself, the alternator will cease trying to charge the battery if the new charging wire burns out.
With that additional charging circuit, the battery will now be charged completely through that wire, and the original wires will only carry (+) voltage to the dashboard. This will also cut the electrical-load of those original wires to the ammeter in half..
An immediate, short-term (and free) solution to reduce the risk of a dash-fire is to bypass the ammeter. This is as simple as relocating the ring-terminal from one of the studs on the back of the gauge to the other stud, and tightening it down. You can see both studs circled in the picture below:
You can access them from under the dashboard (no need to pull the dash-panel), just to the right of the steering-column. DISCONNECT THE BATTERY FIRST, and remove the left A/C ductwork. Then stick your head under the dash, and reach up there with a 3/8" wrench to loosen one nut. Pull the ring-terminal off that stud, put it on the otehr, and tighten the nut down. You don't even have to remove the nut or terminal from the other stud.