Disassemble and Clean the Main Key If it Works Fine But Not Functioning as Expected

Qzaa01.jpg How to

The main key is an essential part of riding a motorcycle. Without the key, it is impossible to turn on the ignition, and it is impossible to operate auxiliary equipment such as blinkers, headlights, horn, etc. There are a wide range of problems related to the main key, but here I disassembled OEM main key for a 1970s Super Cub that was about to malfunction, and tried to restore the electrical contacts.

Is it possible to disassemble the main key of an old motorcycle?


When the key is inserted, it turns on and off smoothly, but sometimes the electrical circuit does not work as expected. The most likely cause of such symptoms is a problem with the electrical contacts built into the back of the key. First of all, with the key alone, check for continuity between the terminals according to the "color" of the wiring diagram in the service manual or owner's manual. In this case, the ignition circuit, which loses continuity when the key is turned on, was in a constant state of continuity regardless of the key position. The first step was to remove the ground contact solder from the body fixing part and remove the end cover.

Plastic nail hooking structure


Since the cover on the back side of the main key often serves as the terminal block for the key contact, start by removing the back cover by pressing in the nail part that holds the back cover in place with a precision flathead screwdriver. The copper plate contacts are pressed into the contacts by a spring, so to avoid losing the contacts when removing the back cover, remove it slowly on a table with the back cover upright. If you are worried, you can stick your arm inside a clear plastic bag and work on it to prevent parts from flying off.

Inside the back cover contacts are full of small parts.


Once the back cover is removed, the moving contacts, steel ball, spring, and rotating base of the copper plate press parts can be removed, but it is best to take pictures with a cell phone or smartphone before disassembling to ensure that the installation direction is correct when restoring. Or, use a magic marker to mark the parts. If the copper plate contact is incorporated upside down, it will cause electrical malfunction. After removing the electrical components, turn on and off the motorcycle with the main key inserted to make sure it operates smoothly.

Thoroughly remove dirt from back cover contacts.


A small amount of grease containing copper powder is applied to the contact points on the back cover to prevent corrosion and improve power feed (the image shows an overdone application), and dirt is removed by rubbing the terminal surface with the tip of a flathead screwdriver. After wiping off the grease and dirt with a rag, clean it with parts cleaner, and after it dries, apply a very small amount of the same grease to the contacts again.

Silicon-based grease is the best choice for plastic parts.


A small amount of silicon-based grease (also known as rubber grease), which does not alter the plastic parts, is applied to the shaft of the rotating base for the electrical switch before assembly and restoration begins. If you apply a lot of product, it tends to attract dirt, so use only a small amount.

Grounding circuit at the tightening position of a metal part


After restoring the swivel position, spring, steel ball, and copper plate contacts to the main key, push the back cover, which is the switch contact for the wiring circuit, firmly into place and click it into place. If the back cover does not feel comfortable during this restoration, you can quickly fix the back cover and the cylindrical body in several places with a glue gun (hot glue) while pushing the back cover firmly into place. After fixing the back cover, fix the grounding hardware of the tightening position to the ground contact of the back cover with solder. This solder fixation is important, and if the solder comes off (due to cracks, etc.) and the grounding is interrupted, it may be possible to kick-start the engine regardless of the key position (Flywheel Magneto Ignition Model).

  • POINT 1: If disassembly and maintenance is difficult, don't hesitate to ask a "Key professional" to do the work.
  • POINT 2: When the cause is wear on the electrical contacts, you can lightly tap the contact protrusion from the back side with a punch to push the protrusion out.
  • POINT 3: There are many problems with the grounding circuit of the key being interrupted, so when disassembling and restoring, use new solder to restore it properly.

When the main key malfunctions, can't you do something about it? It might be the Sunday mechanics who want to try to repair it, but it's hard to find new compatible parts. If you are able to exchange one key for another, it will be a "different key" and you may end up with a 2-key model where the main key is different from the handlebar lock and helmet holder (I bought a used motorcycle before that had a 3-key model). To be honest, many of the parts for models born before the early 80's have simple structures that can be disassembled and maintained by a professional Sunday mechanic. However, many of the motorcycle keys that have appeared since the unprecedented motorcycle boom (since the early 80s) have various hurdles, including anti-theft measures, and cannot be disassembled and repaired as easily and as desired. You can ask a key specialist to repair the keys of such models, but you should know that they may not necessarily be able to repair all problems.

I myself have disassembled and maintained several main key of models born before the 1970s. As soon as I twist the key, the fuse blows or I twist the key and the neutral light doesn't come on, so I wiggle the base of the key with the key on and the neutral light comes on. When I see such symptoms, I try to take apart the back side of the main key to check the condition of the harness contacts.

Incidentally, the cause of the problem of the fuse blowing as soon as the key is turned on is often caused when the + wire of the power supply in the electrical circuit is directly grounded (dropped to the ground) in response to the key being turned on. Is the problem located in the main harness or switches on the motorcycle side? Or is it in the harness contacts in the main key? If the cause of the problem was the contacts in the key, it could be repaired by disassembling and maintaining the key contacts.

One of the actual cases I experienced in the past was a problem where the my motorcycle suddenly stalled while driving. Of course, the main key is on. I pulled over to the side of the road and put the motorcycle in neutral, but the green neutral light did not come on. So I shook the base of the key with the key in the ON position, and the neutral light came on. After that, I was able to start the engine without any difficulty. The cause of this trouble was poor contact with the main key contact. After somehow getting home, I disassembled the back of the main key. The protruding part of the sliding contact of the copper plate press part was worn out, so I used a punch to slightly tap the protruding part from the back side of the contact to push it out. The contact points were hit harder and a small amount of grease was applied for corrosion prevention and power supply before restoration. Then, after that, it regained into trouble-free status.

What I' ve disassembled and maintained here is OEM key for a Super Cub from the 70s. I cleaned the harness contact points on the back side of the body, applied chemicals, and soldered a new ground slope on the tightening position, and the motorcycle has been working well ever since.

A main key with such a simple structure can often be cleaned and maintained, so I would like to encourage old motorcycle enthusiasts to try to repair the main key before replacing parts.

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