The reasons for this phenomenon can be contacts attaching the power wires on the back side of the pull-in relay, which is oxidized, or burnt inside the retractable. As well as worn down starter brushes.
To thoroughly understand and investigate the cause, first remove the starter. If you are uncomfortable to Unscrew the third position with open end wrench the bolt from the clamp of the starter, take the head on 13, extension cord, and key drive. Although it is possible you will have to remove the retractor relay without removing the starter from the car.
If you took it, then on the cover of the housing and the retractor make marks with a pencil so that when you fold you do not have any problems.
Then remove the retractor relay and disassemble the starter. So you can be sure that the brush is all right, to the critical 10 mm, they still fall short.
If you have removed the starter, disassemble it completely, do your due diligence and brush. You can use a lithium grease and graphite.
It is considered that the pull-in relay is a sealed. The cover of ebony you can Unscrew the two screws, but it doesn't come off as preventing two soldered wires. But you remove the two screws under the cross and respeite contacts soldering iron. Only not to damage the sealing gasket carefully with a thin screwdriver podavaite cover. Do this very carefully. And it easily comes off.
After parsing the lid you will find out the true cause of the defect. If burnt contacts retractors, they must be carefully cleaned. Repeat this procedure, even if they're right, but there are only slight traces of erosion. And even if there is no obvious traces of dirt on the contact group, possibly in poor condition is not power contact.
Carefully spend the build stage of the site. After all, in a strong tightening the nuts on brass bolts can damage the cover. If this trouble happens, seal the crack with epoxy, and fasten fiberglass. Although this is, of course, a temporary measure.
Spend the rest of the Assembly in the reverse order. And after starter checks for serviceability be sure to liberally lubricate all external contact with petroleum jelly so they don't oxidize.