Mechanical Temperature Gauge

Our mechanical temperature gauge used what’s called a Bourdon movement, which consists of a C-shaped, flattened brass tube connected to an indicator needle on one end and a length of tubing with a bulb that fits into the cooling system of the engine on the other. Inside the bulb and tubing is a liquid that expands—as it gets warm that expansion causes the tube to move (like one of those curled-up party favors you blow on New Year’s Eve, only not to that extreme), as the tube moves a link transfers that motion to the pointer indicating the engine’s temperature.

1941 Ford Instrument Panel

All electric instruments work on a similar principal—voltage is applied to the movement of the gauge, from there the electricity flows to a what’s usually called a sender that varies the current flow through the gauge depending on the engine’s temperature, oil pressure, the level in the gas tank or whatever else is being measured. The increase or decrease in current flow causes the needle on the gauge to move.

There are several types of electrical instruments; those in the Ford’s panel were the thermostatic design, a simple, inexpensive style used in OEM gauges even today. Electricity flows through a special wire wrapped around a bi-metal strip in the gauge to a sender of some sort. As the bi-metal strip heats up and cools down based on the resistance in the sender, the bi-metal strip deflects, moving the needle on the gauge accordingly. Cheap to manufacture, accuracy is not great, one of the reasons we elected to install modern movements rather than simply convert the original gauges to 12V operation.

Another type of electric gauge is the balancing coil type. These use a pair of electro magnetic coils on either side of what’s called an armature that the indicator needle attaches to. Again, the current flow through the coils is determined by the sender. That in turn changes the strength of the electro magnets and their ability to make the armature rotate, which controls how far the needle moves. While these gauges are accurate, their shortcoming is their limited range of movement.

A huge improvement in electric gauges was the development of the air core movement. As John McLeod explains, these gauges use a magnet on the rotor assembly that turns in a chamber filled with silicone fluid. The silicone functions as a shock absorber for the rotor assembly. The air core coils are wound at right angles to each other around the rotor assembly whose magnet is free to rotate under the influence of the fields developed by the coils. The variable resistance generated by sending units (a volt gauge uses varying voltages in the charging system), alters the current from the battery that flows through the coils, which in turn affects the magnetic field, generated by the coils. By varying the current going to each coil, the magnetic fields change and cause the pointer attached to the rotor assembly to move. Using an air core driver circuit, these magnetic fields can also be reversed, which makes it possible to obtain more than a 90-degree deflection of the pointer.

Retrofitting

Updating early instrument clusters or an individual gauge is much more complicated than stuffing a new movement into an old housing. Just fitting of the new movements can be a challenge. In the case of our instrument cluster adapter plates were necessary to position the new gauges properly and the temperature gauge required a spacer to provide the room necessary.

New gauge faces are often produced; they may be printed by various procedures or laser etched on the lenses, but the big difficulty is often reusing the pointer. In some cases new hubs that fit the shafts of the new movements are put into the original. If necessary new brass pointers that duplicate the old are carved out with a water jet or laser cutter. In any case, the balance of the new pointers is critical.