[Mick Bailey]

I've done a post-mortem on two starter solenoids (each 10 years old when they failed) and both had considerable internal corrosion. The zinc plating had gone white and furry on the actuator and spring. In both cases the solenoid stuck 'on' - a far worse condition than the bike not starting and can lead to a lot of damage. The way the contacts operate is that the solenoid coil pulls a spring-loaded actuator that closes the contacts. When released, the spring pushes the actuator which opens the contacts. The force applied by the coil is far greater than the spring pressure, so that its possible for the actuator to become forcibly stuck inside the solenoid coil with insufficient spring pressure to overcome this.

This corrosion is puzzling. The top is firmly crimped to the rather heavy-gauge body and internally there's a decent-looking gasket. The solenoid is not positioned on the bike where it could get wet. Anyhow, somewhere moisture must get inside the body of the unit and cause problems.

My Wife's bike is now showing problems with the solenoid and I suspect for the same reason. My plan is to carefully drill and tap a small hole in the centre of the casing (at the bottom, making sure no swarf enters the casing) so that I can get a straw in to spray in contact cleaner/lubricant, then seal the hole with a fibre washer and screw. This will also keep the contacts clean and hopefully prevent it from sticking.

[Adrian II]

This corrosion is puzzling

Assembled in a hot, humid climate, sealed then left to age in a colder climate? Just my guess.

A.

[Mick Bailey]

I've take a really good look at the dismantled parts and have come to the same conclusion.

EDIT:

Drilling the casing did not work - this one is constructed differently to the others. It has a metal disc at the bottom which spins and prevents drilling. The casing is also too thin to take a screw. Instead I used a desolder pump to remove the soldered leads and carefully prised the retaining crimps away so that it could be dismantled. Cleaned up, lubricated and re-soldered. It's now working fine.