Solenoid Contact Sticking: Whats to Blame? From the Ace Tech Corner An explanation of solenoids contacts and why they fail leads to a wide range of subjects in the starting system. The solenoid contacts are one in a system of interdependent components. A lot of variables must be considered. Solenoid contacts have to make and break anywhere from 100 amps to 2,000 amps reliably. If we assume that the average car is driven 10 miles each time it is started, in 100,000 miles the sole- noid will operate 10,000 times. Most cars have it much worse. Every time the contacts make, an arc causes a weld to occur, and some of the contact material is vaporized. Each time the contacts break, another arc occurs, and more material is vaporized. Additionally, some metal is trans- ferred from the positive contact to the negative contact. Usually the positive contact (commonly the battery contact) shows the most wear, followed by the contact disk. The negative contact may accumulate material. In some solenoids, the negative contacts may be smaller and often 24-volt solenoids, only the positive contact may be silver. Let’s use the terminology “spot weld” meaning every time the solenoid has current going through the contacts, technically a spot weld occurs. High current levels involved are the cause for a spot weld. The goal is for a spot weld to be large enough to carry the current without excess voltage drop and heat; but also small and weak enough to break. Contacts that are crooked and make on an edge will create a long dura- tion arc because of the reduced surface area, resulting in contact sticking. A simple no-load bench test will not detect this condition. As a result, many contacts are designed with domed or machined surfaces. In years past, many older starters used cadmium plating on the contact disk to reduce arc formation and make the spot weld weaker. However, due to environmental regula- tions, this has fallen out of favor. Higher melting point copper alloys are com- monly used because they get much hot- ter before they melt and form a strong spot weld. Silver is another example of a high melting point alloy. Some of the newer starters are using steel studs. Rough surfaces do not pro- duce a strong spot weld, so many times one or both contact surfaces are dimpled or serrated. After wearing through the serrations, the contacts are still rough, so sticking does not begin to occur until large non-uniform erosion occurs, which creates the same situation that occurs when you get an edge contact. Naturally, contact chattering leads to a huge amount of heat, and since starter current is higher under these conditions, sticking or melting is almost assured. If the contact area is contaminated with oil, grease, and road gunk then many undesirable events occur, such as carbon spots and extra contact erosion and arcing, which also decreases sole- noid life. It is common in this industry to allow cost reduction pressures to reduce the quality of or eliminate gas- kets, and often this “seems” to work. Remember, some increased failures will occur due to water, salt, oil and grease getting into the cap, so this can be a false economy. There are two common springs in the solenoid—the return spring and the cushion spring. The return spring is used to break the spot weld and force the con- tacts open. This spring should be strong, but there are limits. If the spring is too strong you will have solenoid chatter, or break the cap. If too weak, you can have sticking. For this reason, some PMGR starters have a non-removable plunger so the plunger spring force can add to the return spring to help break welds. The cushion spring provides the neces- sary pressure to make sure the contact disk can force the contact through various kinds of debris and form the right size of spot weld. If the contact spring is too weak you will have problems with contamination, which prevents starts, along with excess arcing and sticking. If the spring is too strong, you will have sticking from good spot welds, and/or chattering. Copper increases in resistance about 20 percent for every 100 degrees F. Since amps and turns determine mag- netism, if you have a 100-turn, 1-ohm coil drawing 10 amps at 10 volts at 77 degrees F the tums amps would be 1,000. This same coil at 170 degrees F would be 1.2 ohms, draw 8.3 amps and turn amps would be 833, so we have lost about a fifth of our pull force. Go to 377 degrees F and the situation is much worse. Now the coil is 1.6 ohm, draws 6.2 amps and turns amps is 625, or 62 percent as strong as when at room tem- perature. It is true that springs get weaker when hot, but this is only about two per- cent per 100 degrees F, so it is not real significant. Since solenoids can easily hit 500 degrees F and more, they have to be a lot stronger than needed for room temperature operation. Weak coils usu- ally only cause contact sticking at high temperatures, sticking at room tempera- tures is usually due to something else. The angle o