1981 CB750C “M-Unit Blue” Wiring

Stootz

New Member
I am completely lost.. (First time wiring..) I bought an M-Unit Blue, starter solenoid, and a wiring kit. I was hoping to use the stock breaker points, coils, R/R, and stator/alternator. I also have two “shock units”?? I need some serious help and a shove in the right direction would be so amazing!

<a href="https://ibb.co/ipkdgH"><img src="https://preview.ibb.co/hvjtFc/00_BC89_A2_1052_424_D_BB19_08_C13_E04756_B.jpg" alt="00_BC89_A2_1052_424_D_BB19_08_C13_E04756_B" border="0"></a>
<a href="https://ibb.co/fvDaTx"><img src="https://preview.ibb.co/emmvTx/D231240_E_F1_E3_407_F_95_E2_EEFB2_F68_AE75.jpg" alt="D231240_E_F1_E3_407_F_95_E2_EEFB2_F68_AE75" border="0"></a>
<a href="https://ibb.co/eyLYFc"><img src="https://preview.ibb.co/jjqB1H/AD7_EEF96_9325_46_A1_A675_6_A6_D77_D00_F2_B.jpg" alt="AD7_EEF96_9325_46_A1_A675_6_A6_D77_D00_F2_B" border="0"></a>
<a target='_blank' href='https://imgbb.com/'>multiple image hosting</a>
<a href="https://ibb.co/cOE4MH"><img src="https://preview.ibb.co/krmvTx/CD14471_F_12_FF_4_EB5_A453_78_A7_E9_C69526.jpg" alt="CD14471_F_12_FF_4_EB5_A453_78_A7_E9_C69526" border="0"></a>
<a href="https://ibb.co/dmEh8x"><img src="https://preview.ibb.co/kEkYFc/987_CC0_A2_0_E2_A_45_FD_8_D2_C_63682862_AFC5.jpg" alt="987_CC0_A2_0_E2_A_45_FD_8_D2_C_63682862_AFC5" border="0"></a>
 
OK... we'll need to take this one step at a time to make sure nothing gets lost.

First up, the two spark units are necessary. Hang onto those. Your bike doesn't have points, it uses a magnetic pickup, sends that signal to those spark units, then the spark units control the spark on your ignition coils.

M-units use low side signalling, meaning each circuit is activated by completing a connection to ground. For instance, if you ground the START connection on the input side, the m-unit will send out 12V on the START port on the output side.

If you don't have one already, I highly recommend grabbing the m-button. Without using the m-button you're going to be making two harnesses (nearly). The m-button will reduce this by running a single wire from the handlebars to the m-unit and most of the input side on the m-unit can be skipped.

We can/should tackle this one circuit at a time. Start with something simple like the lights or horn.

First things first, hook up the m-unit to the battery and to ground. Make sure you keep your battery on a trickle charger while testing so you don't run it down (don't run the bike or the electric starter while the battery is connected to your charger). The next step is to turn the m-unit on. This is done by supplying power to the LOCK input. Running power from the positive cable of your m-unit, to your ignition switch, then back into the LOCK input will achieve this task. You should see the m-unit's lights come on when you're successful.

Let's tackle the horn first. If you look at your wiring diagram, you can see that the horn has two wires. A black wire on a Honda means power, so you know that the horn has power whenever the ignition switch is turned on. The light green wire leads to the handlebar switches and the diagram indicates that pressing the horn button will ground the horn, completing the circuit. This is already setup for low side signalling, so this make this circuit a bit easier than the others. Simply run a wire from the HORN input (or the HORN wire on the m-button if you're using one) to the light green wire on your left controls. Pressing the horn button will now activate the HORN output on the m-unit. So now lets run a wire from the HORN output of the m-unit to the horn. The horn won't work unless you complete the circuit, so one additional wire from the horn to ground and we should be good to go. Pressing the horn button on the bars should now activate the horn.

Repeat these steps for your headlight, turn signals, and brake light and you're halfway done. The only difference is that the controls for these lights are not usually setup for low side signalling. The additional wires for all of these switches will need to to be grounded rather than hooked up to power like in the original diagram.
 
Thank you so much for the reply, very grateful! ;D
Hope I can figure all this out without adding too many dumb questions to the World Wide Web.

I forgot to mention I am replacing the handlebar controls with some cheap buttons.. they are normally open and when pressed, closed. 2 on the left, 2 on the right. Start button possibly under the seat. Do the buttons need to be able to handle a certain amount of power?(amps?) The button posts aren’t labeled, are they both negative wire, one grounded and one to the m-unit (doesn’t matter which?) If I should get better quality buttons I can.

I’ve seen something about the ignition possibly needing a fuse, probably going to purchase an m-lock. Will the m-button work with the m-unit “blue”?

What gauge wire where? The m-unit came with a medium sized cable (8-10 gauge?) and I ordered a 6 gauge red and black cable from revival cycles (arriving tomorrow). Does the medium sized cable go from the battery positive to the m-unit? And I’ll have the 6 gauge red cable also going from the battery positive but to the solenoid? (Or we don’t worry about that part yet?)

I bought the deluxe wiring kit also from revival cycles that came with black and red 12 gauge wire, tons of 18 gauge wire (inputs?), and tons of 22 gauge wire (outputs?). Would I splice in a certain gauge wire to the ignition from the medium sized cable that came with the m-unit?

Bolting the m-unit down to the metal tray through the grounding holes will ground it? And using the cable that is stock bolted onto the frame to ground the battery?

Again thank you so much!

<a href="https://ibb.co/ixUjBH"><img src="https://preview.ibb.co/fcshyx/955_D2203_6410_498_F_8198_E34_C1109_EED2.jpg" alt="955_D2203_6410_498_F_8198_E34_C1109_EED2" border="0"></a>
<a href="https://ibb.co/itT0kc"><img src="https://preview.ibb.co/dkpUdx/AA397_E89_DF37_441_E_84_CA_A1_CDBF6_F3300.jpg" alt="AA397_E89_DF37_441_E_84_CA_A1_CDBF6_F3300" border="0"></a>
<a href="https://ibb.co/hOKW8x"><img src="https://preview.ibb.co/ndxJox/96681_AB5_95_FA_47_EC_8_F54_11_AD2_BA56_CDF.jpg" alt="96681_AB5_95_FA_47_EC_8_F54_11_AD2_BA56_CDF" border="0"></a>
 
Stootz said:
Thank you so much for the reply, very grateful! ;D
Hope I can figure all this out without adding too many dumb questions to the World Wide Web.

I forgot to mention I am replacing the handlebar controls with some cheap buttons.. they are normally open and when pressed, closed. 2 on the left, 2 on the right. Start button possibly under the seat. Do the buttons need to be able to handle a certain amount of power?(amps?) The button posts aren’t labeled, are they both negative wire, one grounded and one to the m-unit (doesn’t matter which?) If I should get better quality buttons I can.

I’ve seen something about the ignition possibly needing a fuse, probably going to purchase an m-lock. Will the m-button work with the m-unit “blue”?

What gauge wire where? The m-unit came with a medium sized cable (8-10 gauge?) and I ordered a 6 gauge red and black cable from revival cycles (arriving tomorrow). Does the medium sized cable go from the battery positive to the m-unit? And I’ll have the 6 gauge red cable also going from the battery positive but to the solenoid? (Or we don’t worry about that part yet?)

I bought the deluxe wiring kit also from revival cycles that came with black and red 12 gauge wire, tons of 18 gauge wire (inputs?), and tons of 22 gauge wire (outputs?). Would I splice in a certain gauge wire to the ignition from the medium sized cable that came with the m-unit?

Bolting the m-unit down to the metal tray through the grounding holes will ground it? And using the cable that is stock bolted onto the frame to ground the battery?

Again thank you so much!

In order:

1.) No minimum requirement for current rating on any of the wires or switches that go to the input side. They are all low current, so anything that can handle even a single amp will be just fine. For wire gauges I prefer 18 because it's a bit chunkier, but you can go as low as 22 if you really wanted. That small of wire is fairly fragile for a motorsport application, though. 20 might be a good compromise if you're wanting to go as small as is feasible. For almost all buttons and switches, their polarity doesn't matter. There isn't a positive or negative unless the button/switch is designed to ground itself during use.

2.) A fuse is recommended between the positive terminal on the m-unit and your ignition switch. M-button will work with all of the m-units, include the blue.

3.) On the input side, 18 or 20 gauge wire is good. On output, I use 18 for almost everything, but 14 for high, low, and ignition coils. Don't forget to use 14 on the grounds returning from those components as well.

4.) I don't think you'll need the 12 gauge wire for anything. 22 gauge can be used for input (you've seen my thoughts on that already) and the 18 would be good for most of the outputs. You can probably use the 12 gauge for the headlight and coils if you didn't want to have to buy more 14.

5.) Bolting the m-unit will ground it, yes. Retain your existing ground strap and ground the battery to the frame as well.
 
Sonreir said:
In order:

1.) No minimum requirement for current rating on any of the wires or switches that go to the input side. They are all low current, so anything that can handle even a single amp will be just fine. For wire gauges I prefer 18 because it's a bit chunkier, but you can go as low as 22 if you really wanted. That small of wire is fairly fragile for a motorsport application, though. 20 might be a good compromise if you're wanting to go as small as is feasible. For almost all buttons and switches, their polarity doesn't matter. There isn't a positive or negative unless the button/switch is designed to ground itself during use.

2.) A fuse is recommended between the positive terminal on the m-unit and your ignition switch. M-button will work with all of the m-units, include the blue.

3.) On the input side, 18 or 20 gauge wire is good. On output, I use 18 for almost everything, but 14 for high, low, and ignition coils. Don't forget to use 14 on the grounds returning from those components as well.

4.) I don't think you'll need the 12 gauge wire for anything. 22 gauge can be used for input (you've seen my thoughts on that already) and the 18 would be good for most of the outputs. You can probably use the 12 gauge for the headlight and coils if you didn't want to have to buy more 14.

5.) Bolting the m-unit will ground it, yes. Retain your existing ground strap and ground the battery to the frame as well.

So if I wanted to use the solenoid similar to the diagram below, would all of the cables coming from the solenoid spots 3, 4, 5, and 6 be the same gauge? So the wire powering the m-unit would be smaller than the one it came with but handled by the fuse? Then the same gauge wire running from the m-unit battery spot to the ignition switch and back to the LOCK input?

<a href="https://ibb.co/jXg88x"><img src="https://image.ibb.co/c49H1H/312501_F0_EFC0_48_D1_9399_2_BD16_B062716.jpg" alt="312501_F0_EFC0_48_D1_9399_2_BD16_B062716" border="0"></a>
 
3 and 4 can be 18 gauge, but 5 and 6 should be 14 gauge because they're handling the entire load from the charging system. Power is generated by the alternator, rectified and regulated by the R/R, then passed through pin 6 to pin 5 and to the battery. All of the power your bike will use (aside from the starter motor) will be passing though the wire from pin 5 to the m-unit, so go big.
 
I have more done than in the photo but the starter kicks on and I want to start figuring out the stock spark units, coils, stator, and reg/rec. I have no idea where to start!

<a href="https://ibb.co/gSeqS7"><img src="https://preview.ibb.co/fecMZn/DA852_DCD_6317_4037_9939_969433_D0_E15_F.jpg" alt="DA852_DCD_6317_4037_9939_969433_D0_E15_F" border="0"></a>
 
Back
Top Bottom