Fault Versus Responsibility: Starter Run On COLUMN BY NATHAN UNGER “But the key switch is fine.” I’m sure we’ve all heard some version of that after a starter has been returned, . completely destroyed (@ ) from running on with Ke the engine after it started. The matter is more complicated if the unit was sold through a distributor or through another mechanic’s shop. Whose fault is it? Who takes responsibility for it? The best example of no-fault respon- sibility I can think of was portrayed so vividly on the blockbuster film, “The Passion of the Christ’? There was no fault on Jesus’ part, yet he took respon- sibility for the faults of everyone. We see that fault and responsibility do not necessarily rest with the same party. Starter run on may be the mechan- ic’s fault for not knowing to replace the key switch, or maybe the customer’s fault for not telling the mechanic that it felt kind of ‘wiggly’ or had caused inter- mittent problems before. In spite of the unmistakable racket that a starter makes when it’s running on, the untrained ear may not become aware of it. We the rebuilders have to make sure there is no fault on our part, even though we may sometimes have to ‘warranty’ the claim just to keep our customers happy. The last time this happened to one of my rebuilds, I split the cost with the mechanic just to show some goodwill, even though I could have let the respon- sibility stay with him. It is important to understand how to tell if run on is caused by the starter/solenoid, or if something in the vehicle is causing it. That way, even if you end up with the responsibility of the comeback, at least you will not be at fault, and you can work toward educating your customers to prevent these occurrences. There are at least three main prob- lems inside the starter that cause run on. Solenoid coil problems; coils imbal- anced, or the hold coil not strong enough. If the hold and pull coils do not cancel each other’s magnetism in that instant when the key switch is released but before the plunger has moved, then the remaining magnetism might be enough to keep the plunger in. This will keep the starter engaged even with the key switch off. You can easily test if the coils cancel each other by connecting B+ to the motor stud, B- to the case or ground terminal, and manually inserting the plunger to see if it sticks at all. If it does, then your coils are not canceling each other out, and might cause run on. If the hold coil is not strong enough against the various springs involved, then the solenoid will chatter, leading to possible welding of the contacts. This could cause run on after the key switch is released, even if the drive itself disen- gages the ring gear—run on can occur whether the drive is engaged or not. Also, any type of chattering could, over time, burn out or even weld the key switch contacts because the pull coil is activated repeatedly during the chatter- ing action, and the pull coil draws considerably more current than the hold coil. Many key switches are not designed to handle this much current for too long. Linkage too tight. Parts interchanged or replaced during the rebuilding process could cause the shift lever link- age to be too tight, causing run on. When the starter is free run tested on the bench, this problem will not show up because no torque load is applied to the drive. In the vehicle, the spiral splines of the armature shaft cause the starter drive to stay engaged as long as the pinion is cranking the engine. Then even though the ‘S’ terminal voltage is removed, the solenoid contacts will be forced closed by the drive which pulls on the shift lever which in tum pushes the plunger onto the contacts (a form of positive mechanical feedback). This will result in the starter continuing cranking with- out the key switch being on. If the cus- tomer cannot get his battery disconnect- ed in time, the starter could overheat. Tapping the starter during this kind of malfunction will not release it, as it might in the case of the contacts being welded. If the engine starts up, the tight linkage fault will not be detected because the drive is then released. You can check this condition with a battery and a test light. Connect your ohmmeter or continuity checker between the B+ terminal and motor stud of the solenoid, with the battery disconnected. Then hold the drive fully extended with a pry bar or vise grip. If your meter shows continuity, then the linkage is too tight. In this type of a malfunction, the starter drive and solenoid coils may survive OK, but the solenoid contacts, field coils, armature, and brushes may show signs of heat. By the way, one thing I have seen done when starters were being tested is to connect the ‘S’ terminal to the B+, and then pulse them together to free run the starter. This is incorrect. The B+ should have continuous power during the test, and the “S’ terminal only should be pulsed. What happens when pulsing the B+ and ‘S’ together is that if there are unba