Some excerpts from the Capri-List archives about testing gauges. These tips were originally written by Ted Szypulski and William Morrison:
Testing temp gauge:
Since the temperature sender at the engine simply connects the gauge to ground with a resistance that varies with coolant temperature, one very easy test to isolate the problem is to remove the wire from the sender and with a piece of wire (or the test leads from your meter if they unplug from the meter) just connect the wire to ground. Ground is most easily accessed at the negative battery terminal. If the gauge moves to the hot range, then the gauge and wiring is good and the sender is bad. Try that first, and just to be safe, don't leave it connected too long. The gauge takes some time to respond, so be sure to hold it on ground for at least 15 seconds. Make sure the key is on the RUN position and the battery is good when doing this test.
Testing fuel gauge:
Grounding the wire attached to the sender should should make your gauge show full, if your instrument cluster voltage regulator is working correctly as well.
Testing Ammeter gauge:
In my MKII, the ammeter is a voltmeter (about 1.5V will deflect it to full 45 amps, reverse the battery for a full -45 amps). The voltmeter measures the voltage drop across the main +12 V buss wire that runs from the alternator output to the positive battery terminal. Therefore, one side of the ammeter is connected via a wire to the positive battery terminal, and it's the small wire on the terminal. The other side of the ammeter goes to the wiring harness (loom) and is soldered to the big red wire coming out of the alternator. Note that the starter motor current does NOT flow in the +12 V buss wire, it has it's own mondo wire from the battery. So, the ammeter will read negative current if the battery is helping the alternator with the load, and positive if the alternator is handling the load and charging the battery.
Well, that's for the MKII. I know you have two MKI's, so your mileage may vary. Batteries not included.
As a bonus, my ammeter now works! It had NEVER moved in 26 years of owning the car, so I guess I repaired a factory mistake in this process. I paid special attention to the wiring of the ammeter while re-wrapping the harness. As many know, Ford uses the mondo cable from alternator to battery as a shunt for the dash mounted meter. The meter essentially measures the voltage drop, or rise across this cable as current flows from or to the battery. Contrary to my wiring diagram, there is a fusible link that attaches directly to the battery positive terminal. This is the very flexible black wire on a ring lug. There is a splice about six inches from the terminal, at the other end of the fusible link. It splices the link, the mondo 12 buss cable (red wire), and one side of the ammeter (yellow wire). I had thought the second ring lug was the ammeter wire, as shown on the wiring diagram. Instead this other wire is a 12 volt feed to the horn relay. Again, the wiring diagram is wrong. Here is another unfused wire from Ford. Interestingly, the horn button supplies switched and fused power to the relay, which then supplies raw 12V from the battery to the horn(s). While I had the harness apart at the two large round connectors that go through the firewall, I tested the ammeter circuit. Both my new and old harnesses were wired correctly and were continuous. I then tested the ammeter interior wiring by pulling the gray connector up through the hole and putting an ohmmeter on the two wires (red and yellow) and I got continuity. So, the interior wiring and ammeter at least weren't open. Next, I simulated a 1.5 volt drop across the now missing shunt (mondo wire) by connecting a 1.5 V battery across the interior wires, and the ammeter indicated +/- 45 amps, depending on the polarity. So, my meter even works! I didn't find anything amiss, so I put the car back together and expected the meter to work. As I said above, it now does. My guess as to the original problem - connectoritis where the gray connector goes through the firewall.