If I understand you right, the problem is at certain times
the engine will not crank, that is turn over when you turn they
ignition key to START. And at times it will start and run normally.
Might it be that the issue is with the starter system. The
starter system is initiated at the ignition switch when the key is
turned to start. At this point, the ignition switch then provides
+12VDC to a starter relay. The relay is but a remote switch with
slightly higher amperage capacity. The starter relay then engages and
supplies +12VDC to the staring solenoid. The starting solenoid then
performs two (2) functions. It provides the switching action (hundreds
amps) from the battery heavy cable to the actual starter motor. The
solenoid likewise pushes a small pinion gear along the shaft of the
starter motor to engage the ring gear. The ring gear is of course on
the edge of the flywheel which in turn is attached to the crankshaft.
Starter system problems could be corrected by:
1. check battery, battery cables and connections;
2. charge the battery if required;
3. clean brushes and guides of brush holders of the starter motor;
4. clean the commutator and commutator segments;
5. check/replace the armature or field coils if determined to be defective;
6. in some extreme cases it could be the drive pinion or the flywheel gear ring is defective.
7. often it is the big switch that the solenoid activates that looses contacts causing no +12V internally to the motor.
Most likely would be a defective starter relay or the IGN switch
itself. Relative to this, you might try and check the
connectors/terminals and electrical joints.
In the picture below, the upper portion is the starter solenoid while the one below it is the actual starter.
Given the amount of work to overhaul a starter, you might consider just replacing it.
This of course assuming that all other vital components of the car are working.
Hope this be of initial help/idea. Pls post back how things turned up or should you need additional information.
Good luck and kind regards.
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