I’m now going to try to attempt to answer my original enquiry following some great help from a neighbour – we’ve fixed the problem.
After removing, disassembling, inspecting the starter motor and solenoid we put it all back on the car and it seemed to work fine – then the fault reappeared. This is the embarrassing bit – by then bypassing the vehicle wiring we found a poor connection on the main fuse in the battery box. I thought I’d cleaned all the connections - but not well enough
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So my first tip is to undo all electrical connections that serve the starter, clean with WD40 and retighten – then retest.
If you have to remove the starter motor, it was quite easy on the 1.2 polo – it’s a small engine so there’s room to work. You must disconnect and remove the battery and electrical connections to the starter, remove the plastic battery box and under shield and undo the lower fixing bolt. This was an 18mm bolt with a stud on the head to locate a bracket for some other kit – just remove it all. The top fixing was similar – the stud being an electrical ground - but we had to loosen the splined gear selector lever to allow the motor and bolt to pass. Remember to mark the position of the splines so it can be refitted exactly the same.
We disassembled the solenoid by undoing the fixing screws and cleaned up the piston (Sorry - I don’t know all the proper names for all the components) and then checked that it operated when powered – the piston pulled in to close electrical contacts at the back. We also checked that the contacts worked.
We then powered the motor by connecting between the external heavy conductor and ground using jump leads – this seemed to work ok but we were going to take it to bits anyway.
We took it apart by removing the small cover to the bearing on the end cover, removing a circlip and then undoing the long bolts through the body and removing the end cover – this exposed the bearing end of the armature and the plate holding the brushes.
Make a note of how it all looks and take a photo in case you forget.
The plate pulled out and the brushes and springs pinged all over – do not panic – keep a clean workspace. The brushes were in good condition. We could then pull out the armature and inspect and clean. The commutator was also cleaned and found to be fine in this case – it should have clear grooves between the copper contacts.
Putting it back together was straight forward, if tricky. The difficult part was reassembling the brushes – we found that each brush could be restrained against its spring by a thin wire that was removed when all were in position on the commutator. We had to pull the armature up against the magnet’s force to refit the circlip, then regreased the end bearing.
Finally test electrically – support the motor in a vice – use jump leads to connect to the main terminal and the motor body and then a smaller lead to power the solenoid – hopefully it will all work. Be careful with the connectors – starter motors use very heavy currents and you may get sparks.
Putting it back in the car is a reverse process. It probably took us about 4 hours altogether, but this was because we were inexperienced. I reckon it should normally take a couple of hours. Hope this helps and good luck.
