Nope, there isnt any idiot light, just the gauge, driven by the variable resistance in that 'one wire' sensor. Its a lot easier to reach once you undo the clamps and pull off the large rubber inlet air duct. (But then the engine wont run very well if at all.) Folks have have problems with it for several reasons:
1. Normal aging of the sensor or crud in the tube where the sensor screws in.
2. The connector gets weak and doesnt make good contact with the sensor.
3. The tube the sensor mounts into should be on a bracket that bolts to the transmission bell housing. If someone replaced the starter, and didnt put that bracket back under the bolt - your temp gauge wont tell you much.
4.. I had a gauge that would read too high - if the engine was running - but would read correctly the instant I stopped the motor, and turned the ignition back on. This was due to a bad ground wire at the negative terminal of the battery. I took all of the wires coming to that weird thing on the negative post of the battery, and soldere them together, and ran a wire to the bolt where the ground wire is clamped, next to the battery. Bingo - the gauge read correctly.
As a test: With the ignition key on, if you ground the wire that connects to that sensor - the gauge should shoot all the way to Hot. When its disconnected from the sensor, it should read all the way 'Cold'.
I think there are a couple of different sensors with different resistance values used in the various Escorts, so if you replace it, be sure to get the right one for your year of car.
By the way; earlier 2ng gen. Escorts with automatic transmissions had a 3rd sensor on that piece of tubing - it was used as input for the transmission 'torque converter lockup' function.