Topic: Starting Motor Safety Unit(SMSU)

[amitr0]

I have been looking at wiring diagrams for my 1976 CB550K(I think it is called a K2) US model because I wanted to order a reproduction harness for it.

There seemed to be none available for this year, seriously, I looked everywhere. The reason I found in many forums and reddit subs is that the '76 is a unique year and hence the problem. I do not know why that is, or how true it is, but it is true that there are no harnesses available.

So, the other option was to check the wiring diagrams of similar bikes like the 500 from the same year or the 550 from a different year for differences, before ordering a harness available.

Looking at the addendum for this year : http://manuals.sohc4.net/cb500/service_manual/HSM500550_15.pdf  and differentiating it with wiring diagrams from the previous year K1, and after starting at it for an hour I think the only difference is that the K2 has a diode that replaced the SMSU for the K1. Al future models like the 77 (K3) also seems to have the diode and no SMSU, but their wiring is significantly more complex and I might have missed other components.

After reading this excellent thread : http://forums.sohc4.net/index.php/topic,186383.msg2159976.html#msg2159976 and Hondaman's awesome explanation : http://forums.sohc4.net/index.php/topic,122711.msg1398205.html#msg1398205

it does look like a switch can be replaced by a diode (uni directional). My bike has the diode, I have verified it.

But it leaves other things unanswered, and also begs other questions:

1. This was a safety device that Honda installed to get the DOT checklist, why was it removed later, or is there a substitute circuit that I have not seen in the 76 and beyond achieving the same with a diode?
2. Does this mean I can actually buy the reproduction harness of a 75 https://www.ebay.com/itm/152632732922?hash=item23899eacfa:g:d24AAOSwQAphAbYo and use it in the 76 by rewiring just that part?
3. What is a definitive way to knowing which harness will work, other than eye balling the circuits, I want to learn a smarter/better way of going about this.

[Alan F.]

All very good questions, I'll be going through this with my 750k8.

[Stev-o]

You are not the first to have this issue.  Seems the best bet is to repair the the wiring harness that you have.

Look here at Matt's thread for more info...

http://forums.sohc4.net/index.php?topic=171114.0



EDIT: And if you need parts for your harness, there are a couple used ones on eBay...

https://www.ebay.com/itm/265420430935

https://www.ebay.com/itm/154521127365?_trkparms=amclksrc%3DITM%26aid%3D1110006%26algo%3DHOMESPLICE.SIM%26ao%3D1%26asc%3D235874%26meid%3Df457725726434066af2084c75f9fe2cc%26pid%3D101195%26rk%3D1%26rkt%3D12%26sd%3D265420430935%26itm%3D154521127365%26pmt%3D1%26noa%3D0%26pg%3D2047675%26algv%3DSimplAMLv9PairwiseWebMskuAspectsV202110NoVariantSeed%26brand%3DHonda&_trksid=p2047675.c101195.m1851&amdata=cksum%3A154521127365f457725726434066af2084c75f9fe2cc%7Cenc%3AAQAGAAACEIWTZoWNke6jRsD3ZfJUQjrBIL20qdHwGph2HFfTgI6z9WGFvUwjbqqVgy8fqzXA3d9WM8p6BCxkak2%252BtFUXBoyR9q3c6kv1X%252FiwTpp27QpJ5SDQutD3cEO7OLWrDDCcNcpbhR74gUaXvij4ouZMgTKnyVgjC7ZlwfEzOAbOdrnvTUJQcIhUcHH2%252FOE8uGj3WiTY%252BpjDXkTrRDmkc0pARI5Twk%252B0X5KhqFPDThYIJCn5brriXedAlFvQ5%252F2%252F1tQ5uwGXWijJAhrPVlJJ0WK2csfYD9l15LUqUXvwF%252FGISgizjDPIoaJa6Xbv643UfLyxS6bztGingS4bvdEgINtzn69t3LKaHZVi2brihwd3saKYUHs3JWySOjNGWwQb13qz1y%252Bcl8cJARNPMPAlGI622wdHZTPkMD9Z2j9tUBY9f%252FqlPt8r0Q7%252B0%252FkZAjvxzKBIWeKXZ%252FrNShqVhmOJFGYY5NcLlHkN7yGBk8Tv7MfAw3%252FdBAMUSeOJ877sA8HKXFEtOOqo0%252FaiUsECSmNNUCtlF2tf%252BjtCxqExgELpU%252Fyis%252FXkb3wHyTVm985o75Cc%252By6AXAK0fBkyySO84exTIrftoxeFlShennpERgmGuN85eoBbZL9c%252FOCRYo0ma8hPbTivACEZSGYUe%252Fj%252BhK9ZufCc%252BZNQ9THw1KdMMPExdd5%252Fd5fMStaf3CP9LXFcA7wXVD0QAQ%253D%253D%7Campid%3APL_CLK%7Cclp%3A2047675

[Bodi]

Guessing, here...
I think some government regulation somewhere required the starter motor not run if it would turn the wheel - ie when in gear with clutch engaged.
So the SSU was designed and put into production. It's relatively simple and pretty cheap.
Then someone looked at the wiring diagram and realized a single simple diode could do its job, and would cost quite a bit less. So the SSU went into the obsolete bin.