Old Is New A large number i of the calls we r . receive are for help | | F » with hook-up, testing ae ry and troubleshooting | “== y of older alternators, particularly those with mechanical voltage regulators. On the surface this seems ridiculous since everybody has been rebuilding these units for a lifetime… ight? Well, think about that 20-year-old kid you just hired. He was born after the major automotive manufacturers stopped using mechanical regulators. He sees anything that was manufactured before 1973 as an antique! Consequently he needs a little help with an alternator that uses a mechanical regulator. Be sure to start the “new kid” off on the right foot. “Bypassing” the regulator is a much used technique to determine if an alternator is operational. However, explain that he must first disconnect the field wire from a mechanical regulator before bypassing it. Let’s make sense of this by describing the operation of a mechanical regulator. Refer to the diagrams: 1. When the movable contact is in the upper position, maximum field current flows, producing maximum alternator output for the given speed of the alternator. 2. As battery voltage increases, the coil magnetism increases. This pulls the movable contact to the middle position. Field current flows through the resistor, reduc- ing alternator output. 3. At high alternator speeds, or low current demand, alternator volt- age continues to increase. This produces an even stronger coil magnetic pull. The movable con- tact is pulled down all the way to the bottom point. As Diagram 3 shows, the opposite polarity is supplied to the field. This com- pletely shuts down alternator out- put. The resistor limits the amount of current flowing through the lower points to a safe amount. When the alternator is shut down, voltage drops, the movable con- tact returns to the upper position, and the process starts all over. The above is a “slow motion” descrip- tion. In actual operation the movable contact vibrates rapidly between the three positions to maintain voltage at the set point. What does this have to do with bypassing the regulator? When we bypass the regulator with the field wire connected, voltage increases (if the alternator is good). As described above in step three the movable contact is pulled down to the lower point. This supplies the opposite polarity to the field. Since the jumper wire used to bypass the regulator doesn’t limit cur- rent flow, pow—the movable contact welds to the lower contact. Another possibly good mechanical regulator just bit the dust. Juan Grube is president of JIMCO, Inc., Raymore, MO.