M201 starter motor cleaning
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When the drive gear is pulled forward as shown above then released it should snap back quickly and freely. If it is reluctant to return, moving only slowly, as shown in the video clip, then it will be equally reluctant to come forward and engage the ring gear on the flywheel.
A typically 'sticky' shaft is shown in the photo above. Ideally, the starter should be dismantled for cleaning but good results can be achieved by spraying the shaft with WD40 and allowing it a little time to penetrate its way up the shaft. Pulling the gear forward and releasing it a few times (as shown in the video clip) will bring muck out onto the shaft that can be cleaned away before repeating the process several times until no more muck comes out.
At this point it is important to carefully inspect the shaft for any corrosion and or pitting that developed under the layer of grease (shown in the photo above). This will also hinder the free movement and must be removed with fine wet and dry paper lubricated with WD40. It is also well worth cleaning the surfaces of the gear. When this stage is completed the gear should snap quickly back down the shaft after it has been pulled forward.
The final stage is to lightly grease the shaft and work the grease between the gear and shaft by pulling it forward and releasing it a few times. When all moves freely the starter can be refitted and tested. Very bad cases may need the starter to be dismantled for cleaning but I've had the dreaded failure to engage fault with both of my jeeps and cleaning them as I've explained above seems to have cured the problem.
Other faults may lie with the solenoid which performs two functions: firstly to assist the movement of the gear towards the gear ring on the flywheel and secondly, to power up the starter motor at the correct moment. I have a motor waiting to be fixed where the fault lies with the solenoid but that will have to wait for another day.