Originally Posted by 88cougargt
They most definately are polarized. The gauge has a fixed resistance that is in series with the sender resistance. The sender resistance varies causing the current through the gauge to vary. This causes the voltage drop across the fixed resistance to change. All resistance gauges are actually voltage measurements. Even your DMM makes a small precision voltage for measurement.
If you have an old cluster laying around, briefly touch a 9v battery across the terminals. The gauge will start moving. Flipping the battery over will drive it back the other way.
Inside the meter is an adjustable resistor for trimming the fixed value. A length of wire is wound around a core and connected to the adjustable resistor. The needle is attached to the movement of the core. As current flows through the coil it generates a.south pole on one side and a north pole on the other. Reversing the direction will swap the poles. Fixed magnets on either side attract or repel the coil porportional to the current.
Might be worth checking.
If the gauge in question is an 1968 model gauge then I have to disagree. The '60s and early '70s models are extremely simple and have no adjustable resistor, no magnets, no poles.
The 'movement' consists of a bimetal strip that is heated by 5VDC passing through a wire wrapped around it and going to ground through the sending unit. The sending unit varies the resistance, thus regulating the flow through the wire and the heat produced. The metal strip bows and flattens depending on the heat generated, and moves the needle. The gauge can be connected either way and will still move in the correct direction.
There are high and low range adjustments, but they are physical adjustments that effect the shape of the bow, not electrical adjustments that effect resistance.
The needle movement is fairly slow compared to the newer gauges that are 'driven' by the electrical current and thus respond much faster than the old gauges that need to heat and cool to move the needle.
As a sanity check I did connect both a fuel and temperature gauge from an old ('67) Mustang cluster - they work normally when power and ground are 'reversed'.