Topic: "Starting Motor Saftey Unit" ???

[goaarongo]

Hi guys -
I'm looking around for a Starting Motor Saftey unit for a 72-74-ish CB750.  Its the only component on the wire harness that came with red terminal couplers...

So I have a couple questions:
What exactly does this thing do?  It's sealed (I think) so I'm really curious whats inside that little black box.

Is this unit interchangeable between a 750 and 550 from the same era?     

Does anyone have an extra laying around?  I'm struggling to find another one.

[Duke McDukiedook]

They are still for sale (SW, STARTER MAGNETIC 35850-371-670) but it will run you about $70, it is basically a solenoid.

[TwoTired]

The unit keeps the starter motor from engaging unless you either have the trans in neutral, or have the clutch pulled in.

[Fritz]

Does anyone have an extra laying around?  I'm struggling to find another one.

Do you want it to make your bike 100% original or do you need it just to prevent you from starting the motor in gear?
You can substitute the whole unit with a single diode just like Honda did in later models.

What exactly does this thing do?  It's sealed (I think) so I'm really curious whats inside that little black box.

Actually it switches the ground path of the solenoid, using the clutch switch and an electronic one triggered by the neutral switch. The aim is to route ground from the solenoid through these switches in parallel without providing a ground path for the neutral light whenever you pull the clutch.

The CB500/CB550 workshop manual features a diagram of this unit.

[CycleRanger]

They can be found on eBay from time to time.

I got one there about a year ago. I think I paid 12 or 15 bucks for it.

here's one starting at $10.

http://cgi.ebay.com/ebaymotors/1974-Honda-CB550-CB-550-Four-Starter-Motor-Safety-Unit-_W0QQcmdZViewItemQQhashZitem53e381f246QQitemZ360299229766QQptZMotorcyclesQ5fPartsQ5fAccessories

[swellguy]

You can substitute the whole unit with a single diode just like Honda did in later models.

And this allowed you to start the bike without holding the clutch lever in?

[Whaleman]

1972 K2's did not have the safety switch. Dan

[goaarongo]

Awesome info.  Thanks guys.   

Fritz - I appreciate the detail, and the diagram. 

I'm not doing a meticulous restoration on the bike, so a simpler version might be the way to go.  But I'm not the worlds greatest electrician, so if I can buy the 550 version for 10 bucks and just plug it in I may jump at that.  While most of my bike is a '72, its clear to me after studying the microfiche that the harness and battery box came off a later model that did have this unit.

Can anyone confirm for sure that the same unit was used on both the 550s and 750s?   


     

[Tintop]

If a 550 unit will work on your 750 you can have mine.  It's from a 77.  I'm not using it in my custom harness.

PM with your address if you want it.

tintop

[Toxic]

Can someone confirm that you can bypass the starter safety unit by creating a jumper wire between the green/red and the green wire.?
Basically connecting these two together.

I believe I read that on here from a reliable source but I would like to confirm it.

[Tintop]

Haven't read that post, but can explain how SSM is wired.  Mainly because I'm not using it.

Short answer to your question is yes, but you would be bypassing all the safety switches.  I think you would also have to connect the 2nd solenoid wire to ground to complete the new start circuit. 

Don't have my 550 diagram handy.  Hope this makes sense.  Basically both the starter switch, clutch switch, & neutral are wired into the SSM (directly or indirectly).  For the start button to operate the solenoid, the clutch has to be in (closed circuit), and in neutral (closed).  The closed neutral circuit is what completes the grounding of the solenoid and starts the bike when the start button is pressed (closed).

And I thought English sports car harnesses were weird.  The nanny state at its finest.

[rangelov]

The clutch lever switch is in series with the starter button wire going to ground.  You don't need the Starter Motor Safety Unit (SMSU) for the clutch safety switch to function properly.

Honda has the internal schematic for the SMSU in the service manual in the "Supplement to CB750K1-K4" section.  You can download the manual from the 'sticky' post at the top of this forum.

It looks like the bike will work normally if the unit is unplugged.  Would someone try it?  My bike isn't running, don't have a battery and trying to replace fuse block.  Best spot looks to be where the "SMSU" is located.  BTW, I haven't had a clutch switch forever.

[Tintop]

I was referring to my 77 550 not an earlier 750 harness.  What you describe is more like the 500 harness.  Sorry for any confusion.

Offer of my 550 SSM still stands if it will work in a 750.

[Bodi]

Some SOHC4 models use a simple diode that allows the starter to work in neutral, the clutch switch is always the "master" switch and the starter should always work with the clutch disengaged. I think the starter safety unit avoids overloading the neutral switch, the diode system puts the solenoid coil current through the neutral switch while the starter safety unit uses a transistor to handle that load. This is really the only difference between the diode and the SSU. Honda must have had a good reason to use the fairly complex SSU rather than a simple diode, so I would avoid substituting a diode to replace the SSU: you might burn out the neutral switch.
Just connecting the neutral switch to the clutch switch allows starting whenever you're in neutral or the clutch is disengaged and also puts the solenoid coil current through the neutral switch, but it will turn your neutral light on whenever the clutch is disengaged. That's what the diode really does, isolate the two circuits so the neutral light works properly (the SSU does this as well).

[goaarongo]

Tintop - thanks for the offer - I may take you up on that.  From what I can tell based on the pictures I have seen, the units look to be interchangeable.   

Now that I know how the SSU works, it occurred to me that since I'm not using the stock clutch control, I will have to jump the clutch switch (since there isn't one on the control I have).  All my "Starter Safety" will have to rely on the neutral switch. 

If I jump the clutch switch, is there additional rewiring I will need to do?  Will the SSU still work?     

Ps - I'm just getting started on wiring my bike and sorting out the customizations I have made (obviously).  Thanks again for everyone's input and patience...     

[j squared]

Unless I am misunderstanding your question, if you leave the clutch safety switch as it is (not jumpered), your starter button will only work with the bike in neutral.  If you jumper the clutch wires the starter button will work all of the time, whether the bike is in gear or not.

[bollingball]

Don't forget you may want to kill the headlight with this circuit if you do not have a headlight  on/off sw. on your bike.

[Tintop]

Tintop - thanks for the offer - I may take you up on that.  From what I can tell based on the pictures I have seen, the units look to be interchangeable.     

No panic, I will take it off the old harness and put it on a shelf.  If you want it just PM me.

[lucky]

They are still for sale (SW, STARTER MAGNETIC 35850-371-670) but it will run you about $70, it is basically a solenoid.

What good does it do to tell the forum members a part is available, and the price, but not tell us WHERE?

[lucky]

So many years of the CB750 used the starter safety unit.

Why is it not available?
If it is available , who has them?

[lucky]

They are still for sale (SW, STARTER MAGNETIC 35850-371-670) but it will run you about $70, it is basically a solenoid.

It is NOT a solenoid.

I took it all apart and chipped all of the potting compound off of the components.