I manufacture test computer controlled test rigs for a UK company who manufacture Starter solenoid DC switches.
My CB750K0 starter solenoid switch is stripped down, as we speak, for rebuilding. To dismantleand evaluate / repair
1) Release the cable wiring from the clip by carefully unfolding it.
2) Remove the 2 crosshead screws
3) The two wires are sometimes soldered into rivets within the thermoset plastic cover but on the Denso one on my '69 they are internaly soldered to the enamelled electromagnet wire and are fed into little rubber grommet sleeves.
4) For the rivet type you have to simultaneously heat up the two soldered joints to remove the plastic cover, desoldering the wires from the rivets. For the Denso type with no rivets you gradually pull off the cover so that the wires feed through the grommets. You can then reveal the joint of magnet wire/cable and either snip this or desolder it. There are little bits of glass reinforced sleeve over the enamelled wire.
5) The cover has two copper studs which have rectangular contact faces (on the types I test these have precious metal tips). Examine these for pitting/electrical erosion or corrosion and if necessary dress with a fine file.
6) The moving contacts are a metal bar, again copper (on the types I test these too have precious metal tips). Again examine this bar for pitting/electrical erosion or corrosion and if necessary dress with a fine file. It may even be possible to turn this upside down if the surface is too bad. To do this you gently bend out the four tabs on the top 4-winged washer and then push this down to reveal a circlip which can then be removed. One important thing to check is the state of the phenolic thermoset plastic cover, particularly where the studs fit. If this is overheated and blisterd and smells fishy then the contacts have overheated. It may clean up but if not then you will need to find a spare from a scrap one.
7) Examine the the movement of the contact bar. When you grasp it and pull on it you should feel the compression of the movement of the solenoid armature though a few millimetres of travel against it's spring. If you push on it you should feel the pressure of the compression of the contact spring. These spring operations are crucial to the correct operation and prevention of arcing and overheating.
Whilst the solenoid is stripped down connect 12v from a battery or 3A PSU to the two magnet wires and you should observe the armature flying out due to the electromagnetic force generated. You should check the coil resistance with an ohm-meter. It should typically be 3-4 OHMs but this changes a bit (increases) as the coil warms up. There should be infinite resistance between the metal can (mag frame) and any of the two coil wires. If there is anything less than 1 megohm then there is either moisture in the unit or the magnet wire insulation is breaking down
9) These type of switches are intermittently rated at typically less than 25% duty cycle so be careful not to operate the coil for longer than a few seconds.
10) To use the old chestnut ' Reassembly is the the reverse of this procedure'. When reassembled make sure you have a good seal against moisture ingress, particularly where the coloured wires feed through the top cover grommet sleeves. This can be aided by using a little silicone grease on the grommet on reassembly.
11) If you need to clean up the copper contacts you can use lemon juice (or citric acid) by the way. Wash it off afterwards though
12) When you test out after reassembly, before you fit to the bike , power up the coil and measure the resistance between the two copper studs after you hear the click. It should read zero ohms on an ohm-meter. If you have any contact resistance, as soon as you pass the inrush current of the starter motor through it there will be localised heating and burnout.
13) The oscillation you get when you operate the solenoid with a weak battery is known as 'doorbeling 'and can seriously damage contacts. I have a patent in my name for an electronic device which prevents this but unfortunately it was developed whilst I was working for a company and I make nothing from it!!
There are lots of variants out there but all the same basic operation. If you have any specifics PM me or reply to this post.
The only reason I went to all the effort of repairing mine is due to the old 'K0 obsessive syndrome' of keeping everything standard. I could easily fit one of the ones my equipment tests millions of each year!!!!!!!!!
