Recently a dash clock installed originally in a 1948 Cadillac came into the shop for restoration. The clock was manufactured by the DELCO Appliance Division of General Motors and is perhaps one of the more difficult auto clocks to restore in terms of adjustment. Additionally, a few parts are very fragile in terms of physical handling and electrical overload. Proper fusing according to the manufacturer specification is advisable when performing maintenance on any piece of vintage vehicle equipment.
Picture no. 3 shows the rear of the movement with the back cover removed. It is fairly simple in design. The 6 volt connection is made to one side of the solenoid (approx dime size at the top of the movement. The other side of the solenoid 40 ohm coil is at ground potential or chassis ground. When energized, the coil draws about 160 milliamps. In series with the 6 volt input is a tiny reed switch consisting of "always" closed contacts. A 22 microfarad 12 volt electrolytic buffer capacitor connected one side to the 6 volt input and the other side to ground (small grey component near bottom of movement) protects the reed switch from arcing and thus becoming destroyed when making contact with the contact point on the balance wheel.
The bottom pic shows the balance wheel which is the "heart" of the movement. It has two basic functions. Located on the lower portion of the balance staff is a contact which "brushes" the reed switch (discussed above). The balance staff is at ground potential thus when contact is made with the reed switch, the solenoid coil is energized producing about 7 milliamps of current. When energized, a "plunger" is drawn inward and. (The plunger can be seen in pic no. 3 along the right-hand side of the solenoid coil). The recoil motion of the balance wheel spring rotates the balance wheel in the opposite direction, braking switch contact and the coil is DE-energized. The plunger returns to normal position. The cycling, or back and forth movement of the plunger, vibrates one of a set of two pins (see lower right pic near solenoid coil) causing a "stepper" wheel to advance which is fastened to one of two worm gears. The second of the two pins hold the stepper in place until it is ready to advance again. The worm gear in turn is fastened to a brass pinion which advances the second worm gear the other end of which has a similar pinion fastened to it. This pinion engages with the center shaft pinion which advances the minute hand.
Balance wheel motion is a co-op method between the magnetic field created by the energizing of the solenoid coil and the balance spring. When the magnetic field of the coil is energized, as discussed above, it "assists" the recoil motion of the balance wheel oscillating the balance clockwise. When recoiling counter-clockwise, the strength of the balance spring in the opposite direction is more than sufficient to defeat the "pull" created by the magnetic field created by the energized coil, and the balance wheel swings in the opposite direction. Thus the balance spring and the magnetic field created by the solenoid coil, keep the movement running. The other purpose of the balance wheel is to keep time. Regulation is accomplished by lessening or strengthing tension on the balance spring. A small (customer adjustable) device with a closed loop surrounds the outside coil of the spring....more tension, clock runs faster and vice a versa.