11 October 2009: The electric clocks in 1950s cars are very ingenious little devices. Before the days of quartz crystal movements and semiconductor controls, clockmakers resorted to clever electro-mechanical methods to achieve a continuously running clock that didn't need winding. Basically these 12V clocks are self-winding mechanical clocks. They are mechanical in that they have a spring and an escapement mechanism which regulates the speed of the clock and keeps accurate time. The clever part of the clock is is the self-winding mechanism. This consists of two moving electrical contacts. The first electrical contact is attached to the spring winder. As the escapement mechanism slowly allows the spring to unwind, the electrical contact attached to the spring winder approaches and eventually makes contact with the second electrical contact. This second electrical contact is attached to the moveable arm of an electric solenoid. While the spring is unwinding the circuit through the solenoid is open because the two electrical contacts are not touching. When the spring unwinds to the point where the contacts touch, the circuit through the solenoid is complete and the solenoid is energised and the moveable arm with the second electrical contact jerks forward. This throws the first electrical contact attached to the winding spring away from the second electrical contact, breaking the circuit. The solenoid and its moveable arm revert to the unenergised position. The second electrical contact attached to the winding spring is able to move against the direction of operation of the clock because it is connected to the clock drive mechanism by a ratchet and pawl. When it is thrown forward by the solenoid, the pawl locks the spring in the forward position and the clock spring is rewound. The whole process is then repeated as the escapement mechanism slowly allows the spring to unwind until the two electrical contacts meet again.
Once running these clocks will keep on running and keep very good time. It is soothing to hear the ticking of the escapement mechanism and the occasional doonk as the solenoid rewinds the clock. However, they have one major flaw and that is trying to restart the clock if the battery has been disconnected or allowed to run down. These clocks rely on the jolt of the solenoid to start the esapement mechanism. If the power to the clock is removed the clock will continue running until the two electrical contacts meet. If there is no power the clock stops as the tension in the spring drops away. When power is reconnected, the solenoid is energised and the contacts are thrown and the clock is rewound. Oftentimes, however, the jolt of the solenoid is insufficient to make the escapement mechanism start working. This is especially so if the clock has been sitting for some time and all the journals become stiff. With the spring in the wound position and the escapement mechanism stationary, the clock is unable to operate and cannot be restarted.
The clock in my Mercury was in exactly this state when I removed it from the car. I have already fixed four of these clocks for different vehicles and so I have a standard procedure. First is to lubricate the clock with WD-40. I only apply the tiniest amount to all the axle journals and to the concentric shafts that the hands attach to. It is important not to get the escapement mechanism wet with lubricant because it is so light and delicate that the lubricant drag will stop it from working. At this point the clock should run if wound by hand. Second is to clean the points of the two electrical contacts with a points file. Years of arcing normally creates a build-up of soot and erodes the surface. At this point the clock can be connected to a 12V source and it will normally run continuously. Sometimes the escapement mechanism needs to be gently coaxed to start working. Next I solder a 450V 0.05uF quenching capacitor across the points of the clock. In operation, when the electrical points separate, the collapsing magnetic field in the solenoid causes a high voltage spike which generates an arc across the points. This arc erodes the point surface and leaves soot which reduces the ability of the points to conduct electricity, leading to even more arcing. The quenching capacitor absorbs the voltage spike and helps to extinguish the arc, prolonging the life of the points. Finally, and I admit this is a bit corny but it does save heartache, I arrange a cotton thread through the clock and out of the casing so that if the clock does not start, a gentle tug on the cord under the dash coaxes the escapement mechanism into action. When the clock gets running, the escapement mechanism pushes the cotton thread clear and it operates unhindered by it. Just to finish it off John repainted the hour and minute hands in orange fluorescent paint and it looks a real treat.
Many websites and parts suppliers insist that these old clock movements should be thrown in the bin and replaced by "much more reliable quartz movements." This is simply not true and with a bit of understanding as to how these clocks work and how to maintain them, there is no reason why they won't continue to tick for years to come. Hearing one of these beautifully crafted machines ticking away and winding itself automatically gives a lovely feeling of satisfaction that just isn't achieved by scrapping it and installing a boring quartz movement.
