Topic: Power Killed When I Pressed Start

[Ellz10]

1977 Honda CB750K

m.Unit, lithium 8 cell AG battery, ricks reg/rec, ricks starter solenoid, m.lock, momentary switches.

Today I pulled the spark plug wires off, turned the gas off, made sure my battery was fully charged (I just got back from having it tested and it's cranking at 240a), turned the ignition on and then pressed the start switch and all I heard was a loud clack or click and then everything powered off. The m.Unit wont turn back on at all. I disconnected the battery and tried reconnecting and still nothing.

The m.Unit functioned properly and everything worked before I hit the start button.

I thought perhaps the battery was toast, but it tests out perfectly.

If my m.Unit isn't powering up, does that mean it's fried? I put the volt meter on it and it has power at POS so I'm just very confused.

I have a video of the whole encounter showing the sound and everything when I pressed Start if that would help.

[Ellz10]

So I just got home and had the opportunity to do some troubleshooting. I removed the m.Unit and tested it independently and it is not fried or malfunctioning THANK GOD. I also tested the battery for voltage and its sitting at mid 13.

So I went ahead and plugged everything back in and connected the battery.

No power to the m.Unit, just as before. Hm. Then it hit me and I thought to check the 40a inline fuse that runs from Battery POS to the Starter Solenoid. Fuse was blown. So I checked the 40a inline fuse that runs from the Starter Solenoid to the m.Unit and that one was still intact.

Placed an order a little bit ago for a few of the 40a inline fuses that go in the Motogadget cable that blew. Once I get them I'm going to install a new one then take a circuit tester to see what happened, I'm guessing I have a bad ground somewhere.

Could a bad ground have caused the fuse to blow and cause all this headache?

[HondaMan]

Hmm...has the starter been somehow connected thru the 40A fuse? They can draw about 70 amps, worst-case scenario.
If not, it sounds like there's a cross-connection to ground somewhere, but I don't know enough about the M-unit to talk about it.

[Ellz10]

Hmm...has the starter been somehow connected thru the 40A fuse? They can draw about 70 amps, worst-case scenario.
If not, it sounds like there's a cross-connection to ground somewhere, but I don't know enough about the M-unit to talk about it.

A cable goes from starter motor to starter solenoid. Then from the starter solenoid I have a 40a inline fuse that connects to the m.Unit and from that same side of the starter solenoid I have the cable that goes to battery positive thru 40a inline fuse motogadget battery cable.

[calj737]

Why do you have. Fuse inline between POS and the solenoid? 40 amps wouldn’t cover that anyway… abdandon that fuse altogether and use the POS to mUnit fuse for safety.

[HondaMan]

Hmm...has the starter been somehow connected thru the 40A fuse? They can draw about 70 amps, worst-case scenario.
If not, it sounds like there's a cross-connection to ground somewhere, but I don't know enough about the M-unit to talk about it.

A cable goes from starter motor to starter solenoid. Then from the starter solenoid I have a 40a inline fuse that connects to the m.Unit and from that same side of the starter solenoid I have the cable that goes to battery positive thru 40a inline fuse motogadget battery cable.

There should not be a fuse in the line between Battery (+) and the Starter Solenoid.

[Ellz10]

Why do you have. Fuse inline between POS and the solenoid? 40 amps wouldn’t cover that anyway… abdandon that fuse altogether and use the POS to mUnit fuse for safety.

Hmm...has the starter been somehow connected thru the 40A fuse? They can draw about 70 amps, worst-case scenario.
If not, it sounds like there's a cross-connection to ground somewhere, but I don't know enough about the M-unit to talk about it.

A cable goes from starter motor to starter solenoid. Then from the starter solenoid I have a 40a inline fuse that connects to the m.Unit and from that same side of the starter solenoid I have the cable that goes to battery positive thru 40a inline fuse motogadget battery cable.

There should not be a fuse in the line between Battery (+) and the Starter Solenoid.

Well I believe it's safe to say I found the problem! I'll make it all correct tonight.

Thank you both so much!!

[Ellz10]

On to the next issue now that I've fixed the fuse cable problem.

When I turn the ignition on and press the start button, the headlight comes on and I hear this loud click or clack coming from the m.Unit that lasts for a split second then stops. Each time I press start it does it. I hooked the volt meter up to the battery and it drained slightly each time I pressed start.

I've used my circuit tester and tested all my grounds and everything is fine so that's not the issue like I thought it could be.

When I press start the click/clack sound comes from the m.Unit like I said, but I don't hear any other sound coming from the starter motor or anything else.

Not sure if it's anything, but I noticed that of the 2 ignition coils, only the coil for 2 & 3 are warm to the touch, the one for 1 & 4 are cold.

What am I missing?

[HondaMan]

On to the next issue now that I've fixed the fuse cable problem.

When I turn the ignition on and press the start button, the headlight comes on and I hear this loud click or clack coming from the m.Unit that lasts for a split second then stops. Each time I press start it does it. I hooked the volt meter up to the battery and it drained slightly each time I pressed start.

When I press start the click/clack sound comes from the m.Unit like I said, but I don't hear any other sound coming from the starter motor or anything else.

Not sure if it's anything, but I noticed that of the 2 ignition coils, only the coil for 2 & 3 are warm to the touch, the one for 1 & 4 are cold.

What am I missing?

The last item first: the engine may just be sitting in the position where only one coil is currently active. That's the one that will get warm to the touch. Normally, just one coil at once is ON in these bikes, unless the ignition triggers are like the Dyna - which are always ON except at the moment when the magnetic trigger piece swings past the sensor to briefly turn it OFF and make the spark happen.

If the M.Unit has a relay inside it (again, I don't know those gizmos) for the headlight power, there will likely be both an NC (Normally Closed) and an NO (normally Open) contact from that relay for the headlight. If that's the case, the headlight is currently on the wrong one: it should be on an NC contact if it is like my Switch Saver Relay kits.

[Ellz10]

On to the next issue now that I've fixed the fuse cable problem.

When I turn the ignition on and press the start button, the headlight comes on and I hear this loud click or clack coming from the m.Unit that lasts for a split second then stops. Each time I press start it does it. I hooked the volt meter up to the battery and it drained slightly each time I pressed start.

When I press start the click/clack sound comes from the m.Unit like I said, but I don't hear any other sound coming from the starter motor or anything else.

Not sure if it's anything, but I noticed that of the 2 ignition coils, only the coil for 2 & 3 are warm to the touch, the one for 1 & 4 are cold.

What am I missing?

The last item first: the engine may just be sitting in the position where only one coil is currently active. That's the one that will get warm to the touch. Normally, just one coil at once is ON in these bikes, unless the ignition triggers are like the Dyna - which are always ON except at the moment when the magnetic trigger piece swings past the sensor to briefly turn it OFF and make the spark happen.

If the M.Unit has a relay inside it (again, I don't know those gizmos) for the headlight power, there will likely be both an NC (Normally Closed) and an NO (normally Open) contact from that relay for the headlight. If that's the case, the headlight is currently on the wrong one: it should be on an NC contact if it is like my Switch Saver Relay kits.

That's good to know, thanks Mark!

However the main problem here is the fact that the starter isn't operating when I press the start button. It should be functioning and showing oil pressure on my gauge, but its not. All I get is a loud clack then nothing, every time I press it.

[calj737]

Take a picture of the mUnit and post it to show you have it wired.

[Ellz10]

Take a picture of the mUnit and post it to show you have it wired.

First pic is a current overview (taken this morning) and it shows that I fixed the cable issue - moved battery POS from starter solenoid to m.unit

Second picture is what the m.unit shows as soon as I turn the ignition on.

Third pic is what the m.unit shows when I press start.


I would also like to mention that now the 40a inline fuse I have going from the solenoid to the m.unit blew when I pressed start. Replaced it with a new one and same thing when I pressed start again.

I also went ahead and hooked my volt meter up by placing one clamp on the end of the solenoid where the starter motor attaches and then grounded the other and its reading zero volts with the ignition on as well as zero volts when I press start.

[calj737]

Remove the fuse between the mUnit and the solenoid. This is what we posted yesterday. You only need a fuse between POS and mUnit.

By the way (once again) a 40amp fuse would not handle the load of the starter (closer to 60+ amps). Hence, no fuse.

Fix that, and try again. The loud “click” you hear is the fuse blowing. Or, it’s the internal fuse on the START circuit resetting because of incorrect wiring.

[calj737]

To be CRYSTAL CLEAR:
You need a single fuse between mUnit + and battery POS. That’s it. No other fuses ANYWHERE. REMOVE THEM, DELETE THEM, IGNORE ANYONE WHO TELLS YOU OTHERWISE.

The mUnit is individually fused by circuit internally. Only the output side sees voltage and only 12v. Every circuit provides less than 20 amps total because the mUnit is designed to provide managed power to all circuits beneath the inline fuse from battery POS and mUnit +. You could never support added fuses totaling more than that single fuse and expect to operate the device.

[Ellz10]

Remove the fuse between the mUnit and the solenoid. This is what we posted yesterday. You only need a fuse between POS and mUnit.

By the way (once again) a 40amp fuse would not handle the load of the starter (closer to 60+ amps). Hence, no fuse.

Fix that, and try again. The loud “click” you hear is the fuse blowing. Or, it’s the internal fuse on the START circuit resetting because of incorrect wiring.

So I just got finished removing the fuse between the solenoid and m.unit and replaced it with just a normal 8awg cable and then went to start it and again all I heard was a loud snap and all the power went out just like in the beginning when I did it for the first time. Checked the 40a fuse that's inline from battery POS to the m.unit + and it blew.

So I'm not sure what to do now. It doesn't make any sense.

Attached is a pic I just took showing I removed the inline fuse and replaced with 8awg regular cable (no inline fuse).

[Tracksnblades1]

Ellz 10,
 Nice professional looking wire coverings.

Looking at your last picture. The ground wire from m unit to frame. Where does the auxiliary smaller ground sharing the frame ground bolt go. It looks like it’s going to the solenoid starter cable but I can’t tell for sure.

[Scootch]

It seems like you have a short that happens when the starter solenoid or a relay in this m unit is activated.

[Ellz10]

Ellz 10,
 Nice professional looking wire coverings.

Looking at your last picture. The ground wire from m unit to frame. Where does the auxiliary smaller ground sharing the frame ground bolt go. It looks like it’s going to the solenoid starter cable but I can’t tell for sure.

It's not very good, but in my defense, this was my first time ever working with electrical!

Are you referring to the ground wire that's under the ground wire from m.unit to the frame? If so, then that ground wire is coming from the starter solenoid (green/red).

[Scootch]

Looking at the diagram the 40 amp fuse is going to blow every time you try to start bcz the starter is going to draw big current. I would supply a heavy gauge cable direct from battery + to starter solenoid.

[Tracksnblades1]

No the other end at the frame.
Where does the small ground that is terminated at the bolt with the m units ground wire.  Where does the smaller wire come from.

[Ellz10]

No the other end at the frame.
Where does the small ground that is terminated at the bolt with the m units ground wire.  Where does the smaller wire come from.

It comes from the starter solenoid. This Rick's starter solenoid has 2 wires that comes from it - yellow/red & green/red. The yellow/red went to START output on the m.unit and the green/red went to ground (attached to the same bolt that the m.unit ground uses).

[Tracksnblades1]

Is your bike still wired as the diagram shows.
The positive battery cable needs to go to the solenoid (starter side terminals) first as 737 writes with no fuse. Then your m unit power supply + cable 40 amp fuse could share that terminal only as a battery junction (tie) point. Or you could go back to the battery’s pod I’ve post for the same potential.

Does the last picture represent your schematic and your latest updated mods recommended  by 737 and Dave ?

[Tracksnblades1]

No the other end at the frame.
Where does the small ground that is terminated at the bolt with the m units ground wire.  Where does the smaller wire come from.

It comes from the starter solenoid. This Rick's starter solenoid has 2 wires that comes from it - yellow/red & green/red. The yellow/red went to START output on the m.unit and the green/red went to ground (attached to the same bolt that the m.unit ground uses).

I agree that’s the control side of the starter solenoid.

[Ellz10]

Is your bike still wired as the diagram shows.
The positive battery cable needs to go to the solenoid (starter side terminals) first as 737 writes with no fuse. Then your m unit power supply + cable 40 amp fuse could share that terminal only as a battery junction (tie) point. Or you could go back to the battery’s pod I’ve post for the same potential.

Does the last picture represent your schematic and your latest updated mods recommended  by 737 and Dave ?

Bike is currently wired exactly as the diagram shows.

So I need to run a separate cable from battery POS up to the starter solenoid as well? Dang. Alright well I'll go ahead and try my best to run that cable.

[Tracksnblades1]

Should have read I agree the control side of your starter solenoid is correct.
Still need to know post #21. Because I don’t see the battery cable feeding one side of the starter solenoid on the schematic. Like Dave and 737 indicated..