Thursday, 31 March 2011

Starter Motor bench repair and on car testing

The task to be done was to strip down a starter motor and test the individual components of the motor to make sure they still worked, then rebuild the motor to condition that it can put back on a car.

First we tested starter motors off car by doing a no load test to make sure the motor worked then stripped the motor down to test the internal components of the starter motor, first was the no load test to make sure the starter was working properly before it was stripped down and tested.

The starter motor I worked on passed the test as it was above the minimum voltage requirement and was within amp draw specification. After the starter had been stripped down visual checks were done to see whether any damage had been done to the motor that might require replacement, first to be checked was the armature the visual checks that were done were to look for physical damage things like chips and scratches on the armature the insulation and the coil windings around the armature. Overheating this would be seen as discolouration on the metal, burning would be seen as black soot marks around the armature, and poling this is done by  the armature scraping on the pole shoes, srape marks would be visable on both the armature and the pole shoes.

The next test was to test whether there is a short to ground from the commutator segments and the armature core, the reading should be open circuit if there is a reading this would indicate that there is short in the insulation and reduce the performance of the starter motor as the magnetic fields would be affected that are produced from the armature, and the armature core would need to be replaced.

 The next test is a continuity test in which you check for resistance of the commutator, as the commutator connects to the brushes this means resistance must be low or the peformance of the starter will be affected and the armature would have to be replaced. The continuity test was done to check whether the commutator segments had a circuit, and checking the resistance of the segments, if there was resistance this would affect the magnetic field that is produced and thus reduce the performance of the starter motor and the armature would have to be replaced.

Next the commutator diameter was measured using a micrometer if the commutator came up to be to small in diameter this would make a poor connection between the brushes and the commutator this would create resistance and cause sparking if this was the case then the armature would have to be replaced. Then the mica undercut was measured using the micrometer again this is done to make sure that there is enough space to stop any two segments from shorting together and if the mica is not deep enough it can wear out the brushes very quickly.

The next test was to check for armature shaft runout this is done by putting the armature on some "V" blocks and using a gauge on the side read how much deflection there is whilst spinning the shaft around. If there is to much runout this could lead to poling on the pole shoes and the armature core or it could do physical damge by chipping insulation or windings or the core itself if there is to much deflection then the armature shaft needs to be replaced.

Then an alternative test to checking continuity of the commutator segments was done, this test applied 48 volts to the segments giving a much greater load to the windings so that any faults would show under greater load than normal. This test is done by using a 48 volt test light if everything is in good condition the light should glow then using the same machine the armature ground test was done if the test light did not glow then the armature shaft is not grounding.

Then using the 48 Volt test light do the ground test by placing one probe on a commutator segment and the other on the armature core or shaft the light should not glow.

Then using a machine called the growler the the armature core was tested for internal shorting, (internal shorting is when the insulation is damaged on a winding of wire and the current flow misses some of the windings) this is done by turning on the growler machine spinning the armature and holding a hacksaw in over the top of the armature if the hacksaw vibrates during the test this will indicate an internal short this could affect the performance of the starter motor and the armature would need to be repalced.

The next test was to test continuity of field coils this test only counts if the field coils are not grounded, set the multilmeter to ohm's and place the probes on each end of the winding and test the resistance of the field coils if there is to much resistance then the magnetic field would not be as strong and the starter motors performance would be reduced and the field coils would need to be replaced. If the field windings are grounded then place the multimeter probe on the field wire and the black probe on the starter motor body. This is check the resistance of the field wires and for grounding if the field wires are grounded this would give a reading on the multimeter and the field wires would have to be replaced.

Next the length of the brushes was measured this is to check for wear and tear if the brushes are to short this will give bad contact onto the commutator and this would cause sparking and create resistance.Next the insulated brush holder assembly was tested for grounding the reading should come up open cicuit if it doesn't this means that the commutator and armature are not getting the full voltage from the brushes and the performance of the starter motor would be reduced this means that the brush holders would have to be replaced.

Next the starter motor solenoid was tested to check the condition of the pull in and windings, this test is done to check whether there could be resistance within the winding connections or lack of resistance caused by internal shorting this is tested by checking the amp draw  if the pull in winding was shorting the magnetic field could be to strong and the pull in winding could burnout if there is to much resistance then the winding wont produce a strong enough magnetic field to to pull in the plunger in either case the solenoid would have to be replaced. Next the hold in winding was tested to make sure that it could actually hold in the plunger and checked whether the windings could have had to much or to little resistance.causing undesired operation of the plunger if there was to little resistance this could produce a very strong magnetic field and cause the plunger to stayin when it should let go if there is to much resistance the winding might not be able to hold in the plunger and the starter wont go this would mean that the solenoid would have to be replaced.

The last test to be done was to a visual inspection on the pinion gear, bushes, and the clutch, checking the pinion gear for cracks or scratches that could indicate that the pinion gear is engaging the ring gear of the flywheel in a undesirable way causing damage to both the pinion gear and the ring gear, then the bushes are checked for wear and tear and whether the clearance between the housing and the shaft are appropiate then the one way or overrunnig clutch is checked for free movement and its ability lock in place correctly, after all this the starter motor has been thouroughly checked and it is now ready to be put back together.

Once the starter has been put back together it is ready for a no load test in which the readings should be the about the same as they were before the starter motor was stripped this would indicate that the motor has been put back together correctly and all the connections are good.


The task to be done was to test starter motors on the car, checking for voltage drops, amp draw and testing the overall efficiency of the motor.

The first test do be done when testing starter motors on car, was the check the OCV of the battery to make sure there is no surface charge and that the battery is charged enough to carry out the tests this is done so that readings are as accurate as possible.

 Once this has been established de-activate the ignition or fuel injection system so that the motor will not start then having the multimeter wired up the same test as the OCV start turning over the starter motor and this will give a cranking voltage this voltage must not fall below 9.5 volts or the test will not be able to go any further. As results would be invalid.

 The next test is to check voltage drops along the starter circuit first testing voltage drops from the battery positive to starter solenoid input then across the solenoid and then the earth side of the circuit from starter motor body to battery negative. These tests are performed whilst the starter motor is turning over, in a perfect circuit the voltage drop across these areas would be zero however dirty contacts and corrosion can cause voltage drops across the circuit as voltage must be used to allow current to continue flowing.

The last test to do is to test the current draw from the starter motor this is done using an ampmeter and connecting it to the positve side of the starter solenoid terminal then having someone turn over the starter motor to get a reading off the meter, if the reading is above specification this could indicate resistance along the circuit and the motor may have to be replaced, if the amp draw was below specification, in the real world the starter would not fail but in the case of testing it in the assesment it must be failed.


  1. Hi Richard, thanks for this informative article! I have a permanent-magnet starter motor from a motorbike, which is acting strangely. There are no visible signs of damage to the windings, but when I run a continuity test on the commutator segments I notice a short circuit between every segment! This is my first time doing this test, but I had expected to find the commutator segments in pairs, not all electrically joined! Does this suggest there is a massive burnout inside?

    Thanks for any idea's.

  2. Hi Chris, unfortunately it sounds like there is short circuit, there is not much you can do except replace the armature or get a second hand starter motor from a reckers. Hope this is of some help.


    The 'Show Me Tell Me' section of the practical driving test. However the questions are actually quite basic and you should know the answers with hesitating.