don't despair -the 1985 XJ is easy tofix but here's some questions:

1) did ALL the gages stop ? (6 of them?)
No Tachometer, No Temperature, No Oil Pressure, No Battery Voltage, No Fuel reading in either Left or Right tanks? When you drive the car, does the speedometer work?

2) do you mean the gage lights are still working when you turn the Headlights Switch to ON ? ("backlights").

3) What Fuses did you have to replace?

4) what do you mean by the "bottom half still works"?

 

first things first when troubleshooting:

1) remove the Tachometer (RPM gauge), by pushing it in (towards the front of the car), and at the same time, turning it (counter-clockwise) to the "10 pm"position until you feel it "release" from the notches where it is held.

>>> pushin by the black edge, do not push on the glass itself !<<<

2) once it releases, you can carefully and gently, pull itout and lay it over the side complete with all its wires still connected. But be very careful while pulling. Put a small towel over the outside area to protect everything from scratches.

3) once the Tachometer is out, you will carefully get your hand through the opening, and you will be feeling your way to the LEFT, and above the Oil Pressure Gauge, looking for a Ground Wire Loom (a bunch of Grround Wires connected to One Terminal/Connector).

this Ground loom is attached to the body with one screw. You want to make sure it is still attached to the body of the car, and that it is TIGHT, in other words, that you cannot move it. If loose, proceed to find a way to tighten it.

Once you go through this step, and assuming you found the Ground Wire Loom to be fine, then you will put the Tachometer back in place, in the reverse of how you took it out.

let me know what you find before going to the next step.

 

yeahh I know it can be frustrating, that's why Ididn't tell you about the other half of the job until you did the first half !!

1) now you need to repeat the job but this timeyou will remove the Speedometer, (same way as the Tachometer), and find the other Ground loom to the RIGHT of the Speedometer opening, above the Fuel gauge. Verify it is tight.

2) since you are already familiar with the Transmission, verify that the Ground Strap from the Transmission to the Chassis (passenger side of tranny) is making good clean contact. (I completely removed mine, cleaned the contact points with a brass brush, then brushed the chassis contact point, then the transmission contact point,the two bolts and washers, and reassembled, you will notice a difference, it "revives" the electronics).

3) make sure the Battery isfully charged. Weak batteries make the ECU (Engine Control Unit) go berserk, creating all kinds of symptoms.

4) disconnect the Battery cables, wait 15 minutes and reconnect, (do not cross the wires!) then start the car to see if the Trip Computer resets. (again, assuming the battery is fully charged).

5) on the Intake Manifold side of the engine, (passenger side), nearest the firewall, right below the valve cover, at therear of engine, there is another Ground Loom, this time a number of ground cables are held by one bolt, make sure those Grounds are not cracked and that the terminals are clean and tight.
These are the Grounds for the Coolant Temperature Gauge, the Oil Pressure Gauge, etc.

6) check ablack rubber Plug, near the above Groundsthat connects all Engine Sensors and Fuel Injector Harness to the the main wiring Harness. You want to "twist" each half of the Plug while pushing them together, (use both hands) to make sure good contact is being made. The contactscan corrode inside.

7) On each side of the engine, at the Firewall, there are two Rubber Boots coveringtwo through-the-firewall "pivots" on which many Brown Positive Wires are connected to;Undo the nuts after pushing back the rubber boots, and use a brass brush on each ring terminal to make them shiny clean, same with the "pivot" bolt.Again, this "revives" the electronics.

Let me know what happens.

 

And: replace Fuse Number 4 with a fresh fuse.