CB750 DOHC '81 Build - Wiring Help!

BrandonP

New Member
Hey guys,

This is my first post on the forum, so sorry if I'm being a fool...

I'm currently working on a CB750 DOHC '81 - my first ever build - and I guess the strange part is before this; I had a 125cc, bought it off the shelf and wouldn't even have known how to change the lights.... Oh, how times have changed!

I bought the bike a few months ago and have made slow and steady progress since.
It was stripped down to the frame and rebuilding from base up (minus the engine), with a few chops and welds to nail the Brat look I'm after.

I'm really keen to post a build series at some point to see what others think and to try help any folks going through a similar process to help them avoid a few mistakes I've made along the way.

For now though - I need some wiring help.
@Calj737 has been a huge help in fielding some of my stupid questions recently, but he's a busy man and opening this up to others makes sense too.

I'm using the following:
Switches - Motone 3 Button (x2)
Gauge - Acewell ACE-2853
(Including the appropriate Speedo cable)
M Unit - M Unit Basic V2
Bike - CB750 DOHC 1981 (USA)
Wiring - M Unit wiring kit (no M Button)
Start Button - Single push start
Ignition - universal unbranded (Acc/OFF/ON)
Battery - Antigravity 8 Cell
Starter Solenoid - Original
Headlight - eBay (White, Blue, Green in connector & Brown with bullet connector)
Rear Light - LED hoop strip light
CDIs / Coils - Originals
Front Indicators - From CafeR webshop
Horn - basic unbranded, eBay

I've created a diagram online (sorry about the quality, I can't make it any bigger!), and looking for some guidance on whether this is correct....

Please let me know your thoughts!!

Thanks,
Brandon
 

Attachments

Sonreir

Oregon
Looks pretty good to me. Don't forget to get power to your gauge for the oil pressure and neutral lights. I would normally use one of the AUX outputs.

Also, I can't remember if the setup menu supports using the brake output for the brake light as well as the tail light, so you might need your tail light running from an AUX output as well.
 

Sonreir

Oregon
I usually skip the diode. It's only there to allow the bike to be started in neutral or with the clutch in, without having the neutral light turn on when you pull the clutch lever. If you wanted to remove the clutch lever switch and safety cutout for the starter motor, that's definitely an option. It'll clean up the wiring a bit and it's what I usually do.
 

BrandonP

New Member
Perfect - I’m good with cutting it all out.
I just want the neutral light to be function for general use, but not be relevant when starting, so taking it all out doesn’t affect that?

Sorry if that seems like a stupid question you’ve already answered!

Thanks,
Brandon
 

Sonreir

Oregon
No worries, man. I live for this stuff. ;D

So the neutral circuit is now greatly simplified as well as the starting circuit. You can remove the clutch switch and the diode by running the light/green neutral wire from the neutral light directly to the neutral switch from the engine (it usually shares a connector with the ignition pickup and oil pressure wires). The other change is to ground the green/red wire coming from the solenoid.

Then you're done.
 

BrandonP

New Member
Ok, that seems straightforward enough.

For Reg/Rec, the connector has 3 wires -
Red/White - Starter Solenoid
Green - Earth
Black - ?

I’m aware you mentioned connecting it up to Ignition (Out) - is this via that black wire in to the connector?

Also, for the Starter/Solenoid - when I took a look at the old harness the other day I think my Red/White wire for example was on a different side. Can you shed any guidance on the correct layout for the 4 ‘pins’ the connector block connects to on that?

Really appreciate the help man, as a rookie I really overshot how complicated this can work out!!

Thanks,
Brandon
 

Sonreir

Oregon
For the R/R, the black needs power whenever the bike is turned on. AUX or IGNITION will work.

The solenoid's four pins are two "always-on" to feed the main ignition switch and for the regulator/rectifier. The remaining two pins (yellow/red and green/red) are for the actuation of the solenoid.
 

BrandonP

New Member
Hey guys (Sonrier)...

I'm still not 100% on this Starter Solenoid situation, hope you can help!!

For diagram / explanation help, this is the original Starter/Solenoid layout I'm working with:
MOTOR | BATTERY
PIN 1 - | PIN 2 -
PIN 3 - | PIN 4 -

The existing loom lay-out has the S/S set up as the below:
MOTOR | BATTERY
PIN 1 - CLUTCH SWITCH | PIN 2 - START SWITCH
PIN 3 - KEY IGNITION | PIN 4 - REG/REC

However, as per my diagram I created and approved by forum members - this is:
MOTOR | BATTERY + KEY IGN 'BATT'
PIN 1 - START (OUT M-UNUT) | PIN 2 - EARTH
PIN 3 - M-UNIT POS+ CONNECTION | PIN 4 - REG/REC

My issue is that START (OUT - M Unit) is a thick blue wire and this is signalled to connect to PIN 1 - however, the existing PIN 1 wire is Green/Red thin wire.

So my query is really - is my wiring diagram correct and should I swap out this current thin wire in PIN 1 for the thicker START (OUT) Blue wire?
Also, are the other connections wired correctly?

Any help is really appreciated!!!

Thanks,
BP
 

Sonreir

Oregon
One quick way to settle this is to hook the solenoid up to the battery and use your multimeter to find which two of the four pins have power. These two pins are for the R/R and the ignition switch (doesn't really matter which goes to which). The remaining two pins are for the activation of the solenoid (and it doesn't matter which two you use for this, either). For the solenoid activation pins, one of them goes to the output START of the M-Unit and the other is ground.
 

BrandonP

New Member
So - provided testing the S/S works out with the same 4 pin layout as the original loom, the set up would change from:

MOTOR | BATTERY + KEY IGN 'BATT'
PIN 1 - START (OUT M-UNUT) | PIN 2 - EARTH
PIN 3 - M-UNIT POS+ CONNECTION | PIN 4 - REG/REC


TO:

MOTOR | BATTERY
PIN 1 - START (OUT M-UNUT) | PIN 2 - EARTH
PIN 3 - KEY IGNITION 'BATT' | PIN 4 - REG/REC


And I assume the M-Unit Positive connection goes to the S/S Battery STUD?
Rather than PIN 3 - and swaps with the KEY IGN 'BATT' connection?

Sorry again for all the Qs - finally got round to putting my diagram into action yesterday and was going well until this - hoping to try get her going this weekend all going to plan!!!
 

Sonreir

Oregon
No worries.

To be honest, the order doesn't really matter too much on the red and red/white wires. It's all the same circuit. You could run the Reg/reg, m-unit positive, and the ignition switch all to the "hot" stud on the solenoid and have pretty much the same effect.

That said, the two pins on the solenoid are fused, so my preference would be the following:
Pin 3 -> Ignition switch
Pin 4 -> M-Unit positive
Hot Solenoid Stud -> Regulator/Rectifier
 
Great help, this topic... THANKS!
One more Q: what wire Gauge/diameter do you use from the starter relay to the M unit positive (as Motogadget guide recommended at least 6 mm2, and I wondered whether this would be compatible with connector at relay)??
Did anyone take a pic of their final situation pertaining to starter relay connections?


Thanks in advance!!!
 

Sonreir

Oregon
My preference would be to use 8 gauge wire, but 6 gauge is also a good choice. That said, for the starter circuit, I would hook it directly to the battery and not to the M-Unit.
 
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