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A starter motor uses the basic principle of like magnetic poles repelling each other, which causes it to rotate and turn the engine over. The strength of the magnetic field, and thus the starter’s power to turn over the engine, is determined by the amount of current flow to the starter. Therefore, the battery is a good place to start your diagnosis. Before looking at the starter itself, make sure the batteries are good.
In the good old days when you pushed on a big button on the floor of a truck, that closed the big contacts sending current to the starter. These days, a solenoid does the same thing when the key is turned to the “start“ position. (See the diagram.)
The wire from the ignition usually goes through a neutral safety switch and/or clutch pedal switch. These safety switches ensure the transmission is in neutral or the clutch pedal is pressed before current is allowed to pass through to the starter, which activates the solenoid. To test this circuit, disable the fuel/ignition system on the vehicle, and then conduct voltage drop tests on both the start and main battery circuits.
For the start circuit, make sure the vehicle will not start and it’s in neutral. Then, have an assistant turn the key to the start position while you take a voltage drop reading from the starter (S) terminal to the positive terminal on the battery. If there is more than 0.2 of a volt, there is a poor connection somewhere between the S terminal on the starter and the battery. (See the starter photo to locate the S terminal.)
Next, do the same thing, but go from the motor terminal on the starter to the battery positive terminal. Finally, go from the battery connection terminal to the battery positive terminal. There should be less than 0.2 of a volt in all of these tests. This test could reveal high resistance in the solenoid, which is a fairly common problem. If so, the solenoid will need to be replaced.
Don’t forget to do the same thing on the ground side of the circuit. On the ground side, there should be no more then 0.1 of a volt drop between the starter and the battery. A starter motor is a fairly simple device. If you have good batteries and no voltage drops to the starter, that indicates the brushes are bad, there is a worn armature or worn bushings. Many times you simply need to polish the armature and replace the brushes to get a starter to perform properly.
If a starter motor turns but the engine does not, a new starter drive is probably required. The starter drive has a one-way clutch in it, which allows the starter to spin the engine flywheel (and, therefore, the engine’s crankshaft.) But it won’t allow the process to work in reverse; the flywheel cannot turn the starter. So if you forget to let go of the key after the engine starts, the engine will not damage the starter by forcing it to turn too fast. It is not uncommon for these clutches to stop working and the starter will no longer turn an engine over.
But there is one other final check to make. Ensure that the engine oil is not as thick as molasses. It is hard for the starter to turn the engine with thick oil.
Dietrich Schellenberg, Marty Zuzens, Peter Lung and Dennis White are instructors at Assiniboine Community College’s School of Trades and Technology in Brandon, Man. If you’re interested in more information about the training programs offered there, check out www.assiniboine.net.