SMSU query by electrical ignoramus (me)

[Mike_Berkshire]

I am trying to repair my CB550 '76 K2 loom to get the bike running now the engine is back together & carbs, air cleaner etc. Eventually I will probably splash out on a new main loom as the one I have is pretty maimed. I am treating this as a learning opportunity; hoping I can get to the point where I understand how the bike works electrically, but I have a lot to learn. One thing is puzzling me. I understand that the '76 K2 wiring is unique in that it does not have the starter motor safety unit (SMSU) that the K1 has and has different connectors/handlebar switches to the later models (happy to be corrected if this isn't right). I have read that the SMSU works in conjunction with the clutch sensor and neutral sensor to prevent starting when in gear; according to the US SOHC forum in the '76 K2 the SMSU is replaced 'by a diode'. I think I have found the diode but I would be grateful if somebody could explain to me how this diode works with the clutch sensor and neutral switch. Can anyone explain or point me in the direction of something that explains this in simple language. Thanks

[McCabe-Thiele (Ted)]

This might help.

.http://forums.sohc4.net/index.php?topic=76893.0https://support.common-motor.com/hc/en-us/articles/9008352426639-The-Late-Electric-Starter-Safety-Switch-Circuit-Post-1974-Electrical-Troubleshooting-and-Basics-on-a-Vintage-Honda-Motorcycle


and the second link  https://support.common-motor.com/hc/en-us/articles/9008352426639-The-Late-Electric-Starter-Safety-Switch-Circuit-Post-1974-Electrical-Troubleshooting-and-Basics-on-a-Vintage-Honda-Motorcycle

[SteveW]

You can ditch the ssm unit and short two wires out, the starter will only operate if either it’s in neutral or the clutch is pulled in. If you use a piece of wire instead of a diode the only difference is every time you pull the clutch lever in the neutral light lights up.
The diode stops this happening.

I replaced the ssm unit on my 74 K0 with a 1N4004 diode and it works perfectly.

Here’s a pic, not very good but you can see how it’s done.

[Bryanj]

The diode is on the earth side of the solenoid coil system which goes to ground either via the neutral light OR the clutch switch, the diode stops the clutch switch putting the neutral light on      i think

[Mike_Berkshire]

This has been really helpful, thanks. I now understand what the diode actually does. Thanks for the picture Steve. The ‘76 K2 does not have the red connector on the harness. It has 2 leads which go to the diode. I guess that mod you show emulates the ‘76 K2 setup. I’ll have a proper read through the links Ted.