Now learning and understanding is such a "sweet" job. But the main purpose of that is what can we really do with the component. Therefore a dedicated dismantling and analyzing are required to fully fortify understanding of the topic. On May 6th we had our starter motor bench testing and repair assessment. So i'm gonna go through what we did to test it and why we did that.
As I explained earlier, there are 2 main circuits in a pre-engaged starter motor, they are plunger circuit or the "control" circuit and the Armature circuit, which both consist of many components. But, there is only one input and output out of the whole thing, plus the two circuits are logically connected, therefore the first test we must do be4 dismantling is testing the winding using multimeter. On the body of the plunger circuit, we need to distinguish 3 terminals: one is for the Ignition Switch(S), one is Battery Input(B), and the other one is M-starter motor supply in.
Winding(Coil) test consists of 2 sub-tests: Ground test and internal circuit test. With the ground test, set the meter on 2k Ohms, connect one lead to any end of the winding and the other to the body. This test is to find out if the winding is shorted to ground or not. If it is shorted, the reading should be a number amount of resistance, hence the winding is faulty. Short-circuit is the most common, most deadly enemy of electrical circuit. It makes the conductor contain an overloaded amount of current through it, hence this leads to overheating which fatally damage the circuit and any component within. Therefore, the correct reading for this test should be infinity, indicating there is no circuit between the internal circuit and ground.
With the internal circuit test, the resistance reading should be low, just as the specification so that the resistance is controlling the amperage. A lower resistance reading results in higher amperage, hence the risk of overheating is higher. This could be caused by a short circuit inside the internal winding, causing the resistance to fall as electricity is taking a shorter path. And such a high resistance reading tells us that the circuit does not have enough current flow, hence the power output is insufficient for the whole operation. This caused by damaged or corroded conductors. Or significantly, an infinite reading tells us that there is a break in the circuit, which could be caused by those reason.
With great result from that "surface" test, now simply we need t know if this starter can work. So we put it on a bench tester to simulate its operation. The way we hook it up the tester is relevant to how we actually put it in our cars. So Battery to B terminal, Ground to Battery negative, the Ignition Supply to S terminal. It is actually easier to run the starter on a bench than trying to run it off-car, which is also COMMONLY POSSIBLE. Because with the bench when the switch is hooked up, we have our switch buttons for both battery to kick in and ignition to close the circuit. But with the off-car test, unless or even you have a switch simulator, battery is always "on" once you hooked it up, and without the switch simulator, you'll have to touch the switch cable to the S terminal, which is quite unpleasant and it is easy to short-circuit the whole thing.
OK the starter on the bench test is called a "no load" test. This is simply because it does not crank the flywheel, in which a higher power out put required is higher. When the switch is on, the voltage supply should drop, but not below 11V, hence the current provided must be also high enough between 30-50 Amps. When it is actually on a car, the voltage minimum required to crank is 9.5V, so 11V is a safety margin when testing with no load.
Now, disassembly! This is a very practical part of the whole assessment, also very relevant in some real life situation, when you have to pull it out to see whats inside. Instruction is ok but nowhere to be needed because as you take the screws out, the coils and springs start to disassemble themselves, pushing almost everything apart. So what i learned from doing this is: try to do this slowly, remember where to put the bits( O-rings n stuffs) back, alignment is also important as when you put it back. It is actually more organized(which is the whole point of doing this) if you try to take one out of a time and test it. The only pain i got was a dirty/old starter is hard to distinguish and a real pain to put back due to alignment problems.
THIS IS WHERE ALL THE DEDICATED INDIVIDUAL TESTS BEGIN!!!
Visual inspection is always the first thing we do. it helps us to quickly determine the problem if it is obvious.
I Armature test:
The commutator is an in-contact component, therefore we must check its circuitry. Commutator segments and armature shaft must be fully insulated, hence the reading is Infinity. There must be connections between commutator segments to ensure ducting, so a low resistance reading is expected between 0-1 Ohms, between every segments. Thats why this is called continuity test, by putting a lead to a segment, and the other lead moving around the commutator bar. Also, diameter and depth of the commutator bar and the mica undercut are important as they decide the correct contact condition with the brushes. Alternatively, a 48V test light can be used 2 check its continuity.
For checking internal short circuit, we use the "growler". This device is able to check for short circuit using the V seat and a metal strip holding above and along the shaft. When the short circuit segment is in place, it will ignite some sort of electromagnetic surge that is strong enough to move the metal strip and hence the short is detected.
II Field coils and Pole shoes
Field coils are like force-multiplier for producing magnetic torque. Therefore between each end of the field windings, conduction must be good, it means small resistance. Commonly, if field winding is insulated from the body, which in this case it is, the resistance test should be Infinity.
III Brush Holder Assembly:
Firstly, brushes should be long enough to ensure there are firm contacts with the commutators, as described above. Remember the key is the shape of the contact, and the minimum length. Measure the length of the brushes, if they are close to the minimum, should be replaced.
With the brush holder, the key thing is between the body and the brushes must be insulated, otherwise the power circuit for the armature is sabotaged. We can check this by using Ohms meter, n it should read "I".
IV Solenoid magnetic switch.
By using a 9V supply tester, we can check the operation of pull-in and Hold-in winding.
In Pull-in test, put the 9V supply between S and M terminal. This simulates when the switch is closes, the current flows through both pull-in and hold-in, but pull-in wins, therefore, the result of the plunger got pulled in fast is expected.
In hold-in test, 9V is connected between S terminal and the body. This simulates when the contacts are closed by the plunger, the pull-in winding is shorted, only the hold-in winding in operation, and 9V flows from the switch, through hold-in, to ground. Note that when carrying out this test, plunger must be pushed in, so when the lead is grounded, releasing the plunger won't coil it out.
Those are the key things that need to be remember during disassembling and testing. And the rest, testing pinion gear and Overrunning clutch, bushes...re-assembling, just follow the instruction, because they are mechanically easy to comprehend.
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