Topic: the most frustrating electrical issue ever

[mystic_1]

I believe the diagram is wrong.  Specifically, the diagram shows a light bulb in the center of the SMSU.  I believe this is the neutral light at the handlebars.

I also note that the diagram shows the wrong number of wires leading into the SMSU (dashed box).  Diagram suggests 7 wires, actually four.

I noticed the "light bulb" thing too.  I got this image from the Honda CB750 manual scanned by Honda4fun.

mystic_1

[Grinnin]

Update: left the key on only no lights or anything and it seems the smsu is definatley getting warm its been about 10 15 minutes on now and the heat seems to be coming from one side of the unit the side closest to the black wire entering it (not sure if thats really relevant because idk where the wires lead to inside the actual box just trying to be thorough.

I still dont see how all this can add up to my tailight not coming on with the key like it should as thats my only complaint with her right now besides smsu heating up which i wouldnt think is normal. Now that i think about the high beam and neutral indicator bulbs are blown but there older than i am i bet so thats to almost be expected.

There are two transistors inside the SMSU and one has a large heat sink.  That means the transistor dissipates enough heat to require cooling.  But the heat is still trapped inside the box.

Your opening post said the wiring on your bike has been modified.  I suspect that there will be no diagram of stock wiring that will show what those modifications are or how to get it to act normally again.  I think you'll have to make your own measurements and connections.

[Grinnin]

I also note that the diagram shows the wrong number of wires leading into the SMSU (dashed box).  Diagram suggests 7 wires, actually four.

I noticed the "light bulb" thing too.  I got this image from the Honda CB750 manual scanned by Honda4fun.

mystic_1

Sorry, didn't mean to point a finger at you.  I've used the same diagram from the same scan.  I think I wrote that
"the diagram in Honda's 750 manual is incorrect".

I'll be putting the coils back on the bike today, but have selected a diode to replace my (still working) SMSU.  I thought I'd try to colorize the diagram so I could relate Honda's SMSU diagram to the full-bike wiring diagrams.  That caused me to test each wire to every other wire, both directions, for ohms and the diode test.  I'll start a new thread for my SMSU esoterica although a simple diode is easier.

I also agree with you that the SMSU is not causing trouble at the taillight. I suspect that the dreaded PO wanted to ride in full-dark mode.

[DH]

Current is fed to your tail light circuit THROUGH the ignition switch. The IGNITION switch RECEIVES current thru ckt TL1 on the diagram, (a brown and white wire) and passes that current along to ckt TL2 which leads (thru a fuse) to the tail light.

Ckt TL1  AT THE IGNITION SWITCH SHOULD become live when the headlight switch is turned on.

EDIT:  MYSTIC, ya beat me to it.....

[mystic_1]

Sorry, didn't mean to point a finger at you.  I've used the same diagram from the same scan.  I think I wrote that
"the diagram in Honda's 750 manual is incorrect".

I'll be putting the coils back on the bike today, but have selected a diode to replace my (still working) SMSU.  I thought I'd try to colorize the diagram so I could relate Honda's SMSU diagram to the full-bike wiring diagrams.  That caused me to test each wire to every other wire, both directions, for ohms and the diode test.  I'll start a new thread for my SMSU esoterica although a simple diode is easier.

No no, it's all good mate i think this has been a very useful conversation.

I, also, have been thinking we could easily come up with an improved diagram.  In fact, I would even go a step further and say we could probably suss out the SMSU well enough to come up with plans for building a plug&play replacement unit.

I'll keep an eye open for your thread!

mystic_1

[Grinnin]

No no, it's all good mate i think this has been a very useful conversation.

I, also, have been thinking we could easily come up with an improved diagram.  In fact, I would even go a step further and say we could probably suss out the SMSU well enough to come up with plans for building a plug&play replacement unit.

I'll keep an eye open for your thread!

The plug&play replacement is easier than the diagram.  These 2 diodes should do everything the SMSU does.

The two diodes here are both rated for 6 amps.  One of them connects Dark Green w/Red to Light Green w/Red.  Silver band at the Light Green side.  This is the diode that provides a circuit from the starter button to the neutral switch without turning on the neutral light every time you pull the clutch handle.

The other diode connects Yellow w/Red to Black.  Silver band at the Black.  This diode works when you release the starter button and prevents the solenoid's collapsing magnetic field from creating a high-voltage spike that can burn switches.





(And yes, I'll crimp on real spades although I kinda want to put it on a full 6-circuit plug.)

I should be able to test this sometime this weekend even with the engine out.  There ARE a couple of differences.  The SMSU reduces the amount of current through the neutral switch.  The diode will put the full 3.3 to 4.2 amps through the neutral switch.  The starter button already carries the full current.  Honda uses the same part number for the neutral switch in '78 when they used a diode instead of the SMSU, so the neutral switch should be fine.

EDIT:  Tested the circuit today and it acts like a functioning SMSU.

[mystic_1]

I was also thinking that coming up with plans to replicate the SMSU completely would:

1. be a fun intellectual exercise
2. appeal to some of the purists out there



mystic_1

[mystic_1]

Had a little fun...

[stockscreamer]

Oldhat- okay i hope were all on the same page here i have nothing but good conversations with everyone on here, i know their is no magic cure for my problems life aint easy.
Mystic- im at the shop again till morning but im almost certain that nothing works but the tailight with the key switched to park mode soooo im VERY happy about that news. I will be back with my results soon.
Thank you everyone in pointing me to the right direction of the correct 4th or 5th wiring diagram for 75 : )

[70CB750]

I replaced my diode with a piece of wire. Actually I ran one wire from start button to the relay and connected the other side of the relay to + on the battery.

The rest of the wiring including clutch switch I removed, I dont like safety features of this kind. 

All of this on F3 harness.