Author Topic: CB550 M-Unit Wiring Schematic  (Read 4580 times)

0 Members and 1 Guest are viewing this topic.

Offline shads

  • Enthusiast
  • **
  • Posts: 135
CB550 M-Unit Wiring Schematic
« on: October 06, 2018, 05:40:26 pm »
I was hoping you fine gentlemen could possibly give my wiring diagram a quick go over to check I haven't seriously messed anything up!  Pretty much everything is new components apart from the stock handlebar controls.  Ive read through a lot of posts on here and all the manuals and I think (hope) everything is correct, but would be great to have someone more experienced have a look.

A couple of points/questions:

 - My bar end indicators have a white and yellow circuit, so im using the white for running lights which are turned off when the indicator on that side is on, via the function in the original controls.
- The horn switch ground through handlebars doesn't work as ive got powdercoating and clips on, so rather than scratch through everything im running a separate new ground wire which will attach to one of the inside screws to provide ground.
- Im trying to find some striped wire that I can use to replace some of the doubled up colours.
- Anyone got any ideas for what I can use for the ground block in the headlight?  I seem to have a lot of ground connections.
- What thickness wire should I use for the ground connection between the ground block and the frame connection at the coils?
- Do the three yellow connections from the stator to RR have any particular order?  Im assuming they don't.
- Am I right in thinking that the yellow and blue wires to the points are connected to the ground terminals of the coils?

Thanks very much in advance!  :D

The PDF is full resolution by the way.

« Last Edit: October 06, 2018, 06:01:24 pm by shads »

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 20,784
  • I refuse...
Re: CB550 M-Unit Wiring Schematic
« Reply #1 on: October 06, 2018, 06:17:20 pm »
A couple of points/questions:
- The horn switch ground through handlebars doesn't work as ive got powdercoating and clips on, so rather than scratch through everything im running a separate new ground wire which will attach to one of the inside screws to provide ground. Skip the ground block, and move all your grounds to the main frame behind the steering neck. This eliminates any current running through the neck bearings.

Every switch on your stock handlebar controls with the exception of KILL/RUN, needs to have its wiring routed to ground. Every input on the left side of the M-unit (except LOCK) is a ground trigger. Do not route power to the left side.

- Anyone got any ideas for what I can use for the ground block in the headlight?  I seem to have a lot of ground connections. See above
- What thickness wire should I use for the ground connection between the ground block and the frame connection at the coils? Since all the stock controls only need to signal a ground to the M-Unit, you can actually use as small as 24AWG from frame to control to M-Unit. I used 22AWG specifically.
- Do the three yellow connections from the stator to RR have any particular order?  Im assuming they don't. Correct, they do not.
- Am I right in thinking that the yellow and blue wires to the points are connected to the ground terminals of the coils? Yes. BLK/WHT from KILL/RUN provides 12v (or IGN from M-Unit) depending upon how you wire it.
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline 540nova

  • Hot Shot
  • ***
  • Posts: 593
Re: CB550 M-Unit Wiring Schematic
« Reply #2 on: October 06, 2018, 06:49:51 pm »
This is how I brought all my grounds from the handle bar switches to a central location.  There are actually more grounds than this. I collected many grounds into molex connectors, in the headlight bucket,  such as 4 in, ganged together, on a 4 wire plug,  with one output wire to this ground bus.

Sent from my SM-G955U using Tapatalk
« Last Edit: October 06, 2018, 07:00:42 pm by 540nova »

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #3 on: October 06, 2018, 07:03:28 pm »
A couple of points/questions:
- The horn switch ground through handlebars doesn't work as ive got powdercoating and clips on, so rather than scratch through everything im running a separate new ground wire which will attach to one of the inside screws to provide ground. Skip the ground block, and move all your grounds to the main frame behind the steering neck. This eliminates any current running through the neck bearings.

Ok, ill change it to that, thanks.  However there is no ground going through the neck bearings as the current wiring collects all the grounds in the headlight bucket and then connects them to the frame ground at the coils with one wire.

Every switch on your stock handlebar controls with the exception of KILL/RUN, needs to have its wiring routed to ground. Every input on the left side of the M-unit (except LOCK) is a ground trigger. Do not route power to the left side.


Yep, I currently do have every left side input connected to ground, including KILL.  Im using KILL/RUN to switch ground to the KILL input.  I dont have any power to the left side except LOCK, have I missed something?

- Anyone got any ideas for what I can use for the ground block in the headlight?  I seem to have a lot of ground connections. See above
- What thickness wire should I use for the ground connection between the ground block and the frame connection at the coils? Since all the stock controls only need to signal a ground to the M-Unit, you can actually use as small as 24AWG from frame to control to M-Unit. I used 22AWG specifically.
- Do the three yellow connections from the stator to RR have any particular order?  Im assuming they don't. Correct, they do not.
- Am I right in thinking that the yellow and blue wires to the points are connected to the ground terminals of the coils? Yes. BLK/WHT from KILL/RUN provides 12v (or IGN from M-Unit) depending upon how you wire it.  Yeah my +12v for the coils is coming directly from the M-Unit IGNITION out (brown wire)

« Last Edit: October 06, 2018, 07:11:17 pm by shads »

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #4 on: October 06, 2018, 07:04:49 pm »
This is how I brought all my grounds from the handle bar switches to a central location.  There are actually more grounds than this. I collected many grounds into molex connectors, in the headlight bucket,  such as 4 in, ganged together, on a 4 wire plug,  with one output wire to this ground bus.

Sent from my SM-G955U using Tapatalk

Ah, using a molex plug!  Thats a great idea, thanks!  I like the earth bus bar too :)
« Last Edit: October 06, 2018, 07:11:59 pm by shads »

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 20,784
  • I refuse...
Re: CB550 M-Unit Wiring Schematic
« Reply #5 on: October 07, 2018, 04:48:55 am »
Looking at your gauge, the RPM input needs to come from either BLUE or YELLOW from the coils, not a 12v signal from IGN.

Are you disabling the KILL/RUN switch function by tying it to GROUND? It should be IGN->KILL->RUN->12v@Coils. This enables the handlebar to interrupt the power to the coils in an emergency. No ground termination needed.
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #6 on: October 07, 2018, 02:33:01 pm »
Looking at your gauge, the RPM input needs to come from either BLUE or YELLOW from the coils, not a 12v signal from IGN.

I did wonder about this as it seemed a bit odd to take it from the +12V side.  But the instructions do show the RPM input connecting to +12V...

Are you disabling the KILL/RUN switch function by tying it to GROUND? It should be IGN->KILL->RUN->12v@Coils. This enables the handlebar to interrupt the power to the coils in an emergency. No ground termination needed.
Yeah I was wiring it up as per the manual.  But I prefer your method so ill change it to be an interrupt switch



url picture


Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 20,784
  • I refuse...
Re: CB550 M-Unit Wiring Schematic
« Reply #7 on: October 07, 2018, 02:48:12 pm »
You’re misreading the tach signal. It’s not coming from the M-unit IGN, it’s coming off the coil Yellow wire. This is the signal from the points plate, not providing “power” as the IGN port does.

What that signal is looking for is the firing speed of the coil, and the M-unit IGN port only shows 12v constantly when the key is activated. The Yellow wire from the coil/points cycles with the motor speed, firing the plugs as the motor turns. That’s the info the gauge wants.

Make sense?
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #8 on: October 07, 2018, 02:51:58 pm »
You’re misreading the tach signal. It’s not coming from the M-unit IGN, it’s coming off the coil Yellow wire. This is the signal from the points plate, not providing “power” as the IGN port does.

What that signal is looking for is the firing speed of the coil, and the M-unit IGN port only shows 12v constantly when the key is activated. The Yellow wire from the coil/points cycles with the motor speed, firing the plugs as the motor turns. That’s the info the gauge wants.

Make sense?

Gotcha.  Makes perfect sense, thanks again :)

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #9 on: October 16, 2018, 02:02:54 pm »
Updated diagram.  Thanks for the help guys  :D


Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 20,784
  • I refuse...
Re: CB550 M-Unit Wiring Schematic
« Reply #10 on: October 16, 2018, 05:52:38 pm »
You've got all the wiring from your momentary switches wrong.

From the switch, you wire straight to the input side of the M-Unit. From the output side, you wire straight to the component. Your momentary switches have 2 terminals on them; one to ground and the other to the M-unit. Your diagram depicts running current through the switches ala stock.
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #11 on: October 16, 2018, 06:17:32 pm »
Hi Cal, my understanding of the OEM indicator switch is it has two function.  One function CLOSES one of the circuits when moved (ie the grey, blue/brown and orange wires) and the other one OPENS a separate circuit when moved (ie white/brown, white/orange and white/blue wires).  In my diagram (copied from the one from the manual), the left contacts are the CLOSE circuit and the right contacts are the OPEN circuit.  The two circuits are independent as far as I have tested and as per what the switch connections show on the diagram.

Im using the CLOSE circuit to provide ground to the indicator left and right inputs of the M-Unit (via the orange and light blue wires), and also using the OPEN circuit to provide power to the white lights on the front indicators (via the white/orange and white/blue wires).  This means that the white indicator lights are ON when the main headlight light is activated, but switched off when that indicator is activated by the control switch. 

No power hits the M-Unit as far as I can see.  Im very happy to be wrong though!! 
« Last Edit: October 16, 2018, 07:23:35 pm by shads »

Offline BomberMann650

  • Holy Cow! I'm a
  • Really Old Timer ...
  • *******
  • Posts: 6,319
  • Dr. Bovinestein iBa#80333
Re: CB550 M-Unit Wiring Schematic
« Reply #12 on: October 16, 2018, 08:26:44 pm »
Must have been working on the yamaha too much.  Seeing red go to turn sigs tripped me up. 
75 CB550F Bloo
03 DS650/720 Bombardier
76/78 XS750 Oscar the Grouch
07 FJR1300; Lydia
05 YFZ450/470 Stroker (sold)
73 Norton Commando 850 Cafe
99 Roadstar Silverado (rebuild)
Certified Harley Davidson Tech

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 20,784
  • I refuse...
Re: CB550 M-Unit Wiring Schematic
« Reply #13 on: October 17, 2018, 03:04:44 am »
Hi Cal, my understanding of the OEM indicator switch is it has two function.  One function CLOSES one of the circuits when moved (ie the grey, blue/brown and orange wires) and the other one OPENS a separate circuit when moved (ie white/brown, white/orange and white/blue wires).  In my diagram (copied from the one from the manual), the left contacts are the CLOSE circuit and the right contacts are the OPEN circuit.  The two circuits are independent as far as I have tested and as per what the switch connections show on the diagram.

Im using the CLOSE circuit to provide ground to the indicator left and right inputs of the M-Unit (via the orange and light blue wires), and also using the OPEN circuit to provide power to the white lights on the front indicators (via the white/orange and white/blue wires).  This means that the white indicator lights are ON when the main headlight light is activated, but switched off when that indicator is activated by the control switch. 

No power hits the M-Unit as far as I can see.  Im very happy to be wrong though!!
You miss the point. With an M-Unit, no power needs to hit a switch. The sole purpose of the switch is to trigger the M-Unit to transfer power from L side to R side. Once the power is available, send it directly to the component . This eliminates the need to run power thru switches at all.

The manner in which you have yours wired, you might as well ditch the M-Unit and replace it with a fuse block. A major benefit of the M-Unit is to completely separate the harness into a “trigger” loom and “power” loom. Less wires running to and fro. Completely dead nuts simple to maintain and organize.

You can do it the way you are diagraming, but you’ve made a sh!tload of more work for yourself. Also, be certain the momentary switches you buy are rated for the amperage you plan to send through them. If you’re sticking with stock controls, fine. Most push buttons won’t enjoy the headlight current and the terminals on those are awfully small for the gauge wire you’d need to run.

As an example of a straightforward M-Unit install, here’s what I’m describing.
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #14 on: October 17, 2018, 02:23:19 pm »
Hi Cal, thanks again for the input.  Let me just say that I really value your opinion and you are one of the legends of this forum.  Maybe im being dumb here but im struggling to follow the issues you raise.

- I actually used that Gordo schematic to make mine.  Mine is functionally almost identical to that one as far as I can see, however I wanted to show every wire and connection (including ground wires and where they terminate) which is why mine looks a lot more busy.  When I compare both, the Gordo one has 6 'power' wires outputting from the M-Unit to the components at the front of the bike whereas I have one more wire due to the M-Unit Blue also having an IGN output.  Both diagrams have 6 'trigger' control wires sending ground from the front of the bike to the M-Unit inputs.  There is very little difference between the two, other than I have some different components such as a tacho/speedo with more features, M-lock, and im using the white lights on the indicators as running lights.

- The additional work to make my indicator white lights work with the headlight is simply connecting three wires that will already be there, no additional wiring or work is needed.  As im using the OEM switches and wires from the switches to the headlight bucket, all I need to do is connect the white/brown from the indicator switch to the white LIGHT wire that in the headlight bucket (powering the headlight itself) and then connect the white/orange and white/blue from the OEM switch to the indicators.  All these wires will be in the headlight bucket whether I used them for this feature or not.  How would I make this indicator feature work without putting current through the indicator control switch?  Looking at the OEM diagram it looks like this is what the secondary circuit of the indicator switch was designed to do (ie interrupt a white running light on the indicators when that indicator is on), so im struggling to see the issue here.

- The only power going through the indicator control switch is for the two white LEDs on the indicators, which draw very little current.  The wires on the LED are minuscule.  There is no headlight current going through any switch, the headlight is directly connect to the M-Unit LIGHT output!

- Also, power does hit a switch in the Gordo one - the Kill Switch.  Why is putting very low power through the second circuit of the indicator switch any different?

Not trying to be confrontational here, just trying to understand where ive gone wrong!  :)

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 20,784
  • I refuse...
Re: CB550 M-Unit Wiring Schematic
« Reply #15 on: October 17, 2018, 03:04:32 pm »
So my view of your diagram shows a lot of wires connected where I don’t understand why you’re doing it, but as long as you’re happy with how you have it wired, so be it.

As for having your indicators powered, you can set that feature within the M-Unit without a separate switch to toggle them. You determine if you want them to burn always, and at what percentage of total power. They switch OFF with TURN activation, and only the side you activate flashes. Else, they burn.

(To address the KILL switch question, this is because Gordo wanted that FEATURE on a bar mounted switch, like stock. So routing IGN thru that switch controls power to the coils. With a momentary, a double tap kills the bike. Whichever the rider prefers.)
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis

Offline shads

  • Enthusiast
  • **
  • Posts: 135
Re: CB550 M-Unit Wiring Schematic
« Reply #16 on: October 17, 2018, 03:23:57 pm »
Sorry, im not referring to the setup of the orange indicator flash rate, I mean the secondary white lights that my indicators (and the stock setup) have.  These indicators have TWO sets of lights each, one orange LED for the turn signal and one white LED which is used as running lights that are ON when the headlight is ON, but momentarily switch OFF when the indicator is functional...  That's what im doing with the additional wiring, in using the stock controls to switch the indicator WHITE running lights off when the flashing indicator ORANGE light is on.

I think this is where the confusion is coming in!!

Offline calj737

  • Really Old Timer ...
  • *******
  • Posts: 20,784
  • I refuse...
Re: CB550 M-Unit Wiring Schematic
« Reply #17 on: October 17, 2018, 05:12:00 pm »
Yup, sure is.  ;)
'74 550 Build http://forums.sohc4.net/index.php?topic=126401.0
'73 500 Build http://forums.sohc4.net/index.php?topic=132935.0

"Of all tyrannies, a tyranny sincerely exercised for the good of it's victim may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated, but those who torment us for our own good will torment us without end for they do so with the approval of their own conscience." - C.S. Lewis