Know Your Solenoids By Dan Smith Editor’s Note: For those who weren’t able to join us at Niagara Falls, we’ve sneaked a copy of Dan Smith’s notes from his train- ing station on solenoids. So pull out some parts and set up your own sta- tion while you learn everything there is to know about solenoids. You might also want to check out Dan’s “‘Sole- noid Tech Tips” on page 7 for help with specific applications. Webster’s Collegiate Dictionary defines a solenoid as “a coil of wire usually in cylindrical form, that when carrying a current, acts like a magnet so that a movable core is drawn into the coil when a current flows, and that is used especially as a switch or a con- trol for a mechanical device.” The most common solenoids used in automotive starters are made by winding two coils of wire on a com- mon bobbin. When current is passed through the wire an electromagnet is created. This in turn causes a steel core to move. These two coils, a pull-in and a hold-in coil, each have a specific makeup and function. They share a common feed point but separate ground termination points. The pull-in coil is made of a heavy- gauge wire and terminates at the field connection “motor stud.” This coil has a high amperage draw and creates a strong magnetic field. Its primary function is to induce enough force on the movable core “plunger” to shift the starter drive into the ring gear. When the switch contacts close they carry current to the motor stud. At this point, the motor stud becomes a live termi- nal and cancels out the ground for the pull-in coil. This eliminates its draw and makes the amperage it was using avail- able for cranking. The hold-in coil is made of a lighter gauge wire and terminates to a case ground, or in the case of an insulated solenoid such as a 40 or 42MT, toa ground stud. Its primary function is to hold the movable core in place while the starter is working. The coils are wound parallel, and when current is applied to the com- mon feed terminal, both exert a same direction magnetic field on the plunger. When power is removed from the feed terminal, a reverse flow of cur- rent begins at the motor stud, and trav- els through the pull-in coil to the feed terminal, then on through the hold-in coil to ground. This creates two op- posing magnetic fields that cancel each other out. This allows the drive to dis- engage from the ring gear. The coils must be balanced so that one does not overpower the other and cause the plunger to hang up and keep the drive engaged. Switches Webster defines a switch as “a device for making, breaking, or chang- ing the connections in an electrical cir- cuit.” The switch portion of our sole- noids consists of the moving contacts and the stationary studs. The moving contact is mounted on a spring-loaded stem and when shifted by the plunger, is used to bridge the stationary studs. The moving contact has to carry full battery current to the starter motor. The initial contact pro- duces a heavy arc and heat caused by resistance. Additional circuits, such as a relay terminal, are used to switch battery voltage to secondary loads MAY, 2000 during cranking only. These “R” ter- minals can be used to feed ignition coils, cold start injectors, and fuel pumps. Common Problems Solenoid chatter or rapid clicking can have several causes. Look for the following when chatter is evident. Faulty hold-in coil. The hold-in coil may not have a solid ground. It may have a break in the coil or a poor con- nection to the feed terminal. This can be checked by testing from the feed terminal to the case of the solenoid or to the ground stud of insulated circuit units. An amperage load test will give best results. Poor connections. An insufficient system ground, loose, dirty or cor- roded feed wires, may provide enough of acircuit to energize the solenoid. But when the heavy draw of the starter kicks in, the ground or feed can be lost and the solenoid drops out. As soon as the contacts break, the circuit re- connects and the solenoid pulls back in and the process repeats. Voltage drop testing will locate this type of problem. Low battery. Batteries with bad internal connectors, dirty jumpers or that are undercharged, will not be able to supply sufficient voltage to keep the solenoid engaged once the starter load is placed on the system. Battery volt- age should be checked at the batteries while the starter is activated. Six-volt batteries that are hooked in series should be tested individually and as a bank. This will qualify the jumper con- nection. Faulty starter. Ashorted or me- chanically bound starter may cause enough of a voltage drop to cause the solenoid to kick out. Be sure that the starter will free spin independently of the solenoid. Page 1 of 2