I'm mainly posting this for my own reference.
We're currently tootling around in an Ford EL Falcon that's got a laundry list of issues wrong with it, but not the least of which is (well, was, so I thought) lack of a temperature gauge. It turns out the temperature gauge works just fine, it just looks like the motor's always cold. That probably explains the steep fuel usage too.
Ford's instrument clusters are pretty sweet, one of the better features being the in-dash diagnostics mode. I'm re-posting the instructions here for my own reference, as the site I found them on has a tendency to change (ie, break) their URI scheme:
The diagnostic mode can be entered by using the following method
- With the ignition key in the off position, press and hold the Odometer reset knob for 10 seconds.
- whilst holding the button in, switch the ignition to the on position. (Note : It is not necessary to start the car).
- With the ignition on, press the odometer button 3 times in quick succession.
- On the third press, hold the odometer button down, until the Seatbelt lamp is no longer illuminated.
- The Odometer should blank out. After a period of 10 seconds, the display will flash FORD
- Press the Odometer button again to go to the first Diagnostic mode
To view the next diagnostic mode, press the Odometer button for about 1 second
To view the previous diagnostic modes, press and hold the Odometer button for about 2 second
I'm almost certain the thermostat's stuck open - the temperature will climb to quite normal ranges, before the fans kick on, when the vehicle is sitting still, when it's travelling at highway speed the motor is always dead cold.