Topic: Fuse issue?

[mrector]

I had to replace the battery on my 78 cb750k and accidentally blew a fuse hooking It back up. Bike wouldn’t start at all. Running lithium battery

Got a new 30a fuse and installed It and
immideatley the bike went into this weird trying to start mode. It sounded like It was trying to start up but wouldn’t. Light was on. It has a wireless key and m unit and neither appeared to respond (checked with Bluetooth feature as well)
No button functionality worked (horn, blinkers etc)
I disconnected the fuse box from the m unit and It stopped. Then reconnected and the exact same thing happened. Fuse box was getting pretty hot, I disconnected and walked away. 

Any idea how to solve this?


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[calj737]

What did you do that caused the fuse to blow when hooking up the new battery?

If you’re running an mUnit, why are you also running a fuse box? The mUnit eliminates the fuse box and should only be protected by an inline 30a between it and battery POS.

Perhaps you can provide better details on how you’re wired up?

[mrector]

To be honest I wasn’t even aware of that. I purchased the bike with the m unit in It (and I hate It so far). I have no idea of the wiring schematic but I put in a picture that might help

Additionally I just wasn’t paying attention and hooked up my negative terminal then my positive terminal and created a surge.
I replaced fused and It appears to want to start but It just seems stuck


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[Scootch]

Did you reverse the power (+) and ground (-) from the battery? Normally devices like this have a diode on the input to protect from reverse polarity.

[mrector]

I hooked up the negative terminal and then went to hook up the positive terminal and It sparked and created a surge, blowing the fuse.
I unhooked the battery and connected in the correct order (positive first then negative) then replaced fuse and got the weird response I described above


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[calj737]

To be honest I wasn’t even aware of that. I purchased the bike with the m unit in It (and I hate It so far). I have no idea of the wiring schematic but I put in a picture that might help

Additionally I just wasn’t paying attention and hooked up my negative terminal then my positive terminal and created a surge.
I replaced fused and It appears to want to start but It just seems stuck


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Can you take and post a picture of the “fuse box” also please. And see if you can tell if any wires from it go somewhere.

[mrector]

Once I connect the munit to the thing with the fuse in It. The light turns on, speedometer stays off, I don’t see m unit click on, and bike starts to try to start on its own. Wireless ignition does not function either


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[calj737]

It sounds like the starter solenoid is activating. Perhaps it was damaged with the battery fiasco. Do you know where it is and what it looks like?

[mrector]

No clue but if I got a picture I could maybe track It down in this mess! Do you think It needs to be replaced?


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[calj737]

I think you have some diagnostics to perform first.

Look at the right side (top as oriented) of the mUnit. Find the START port. Trace that wire to the solenoid. Locate it and post clearer pictures of what else you have?

[newday777]

To be honest I wasn’t even aware of that. I purchased the bike with the m unit in It (and I hate It so far). I have no idea of the wiring schematic but I put in a picture that might help

Additionally I just wasn’t paying attention and hooked up my negative terminal then my positive terminal and created a surge.
I replaced fuse and It appears to want to start but It just seems stuck


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Mark
Are you near the person who wired the m-unit/built your bike to get them to supply the wiring diagram they used or have them help you with it? If not, you will need to trace backwards the wiring and make your own diagram or find someone to do it that knows how to diagnose the problem in the m-unit? Hopefully you have some electrical knowledge of tracing and diagnostic testing??


To me it sounds like you may have fried something in the m-unit(electronic control) when you zapped it with the reversed polarity hookup.

Cal
Should this be moved to the m-unit section rather than here in the sohc/4 bike section? Can you do the moving to there?

[bryanj]

Sounds to me like something electronic is terminaly dmaged, it should matter in what order you connect battery as long as pos is to pos and neg to neg.
If you revesed the polarity your m unit , full of electronics ,will be toast

[calj737]

If you revesed the polarity your m unit , full of electronics ,will be toast

Nah. Happens all the time and they survive due to their design.

[mrector]

First off,
Thanks to everyone for trying to help me figure this off. I’m clueless electronically and help goes a long way for fixing and learning about my bike (which I’m trying to take up as a hobby)

Secondly, I reached out to the previous owner for a schematic. So hopefully he has one that could help us

Third. I traced the starter wire to a covered area in the bike. I’ll have to remove It when I have some time today. So I’ll get a picture today

I don’t believe the entire system is fried, I didn’t reverse the polarity, It just looks like I shocked the system somehow.


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[calj737]

Not the wire for the starter, for the STARTER SOLENOID. Trace the output wire from the mUnit that is labeled START to a round thing with 3 wires on it.

You said you have a fuse box and an mUnit. Can you take a picture of the fuse box and post it?

[mrector]

My lack of knowledge is showing,
When I said starter I meant the starter solenoid

I don’t have a fuse box, I misspoke earlier. I posted a picture of the m unit and hookups with the single 30a fuse that I replaced already, did It not come through?


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[calj737]

It has posted, twice. Can you take a picture from a bit closer to it with better light? It’s a bit murky to distinguish what all is going on.

[mrector]

Hopefully this helps


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[mrector]

Update: I pulled the spark plug at the advice of a friend and am not seeing a spark after plugging the relay back in. Additionally when I did this and, the battery dropped from 11.8 to 8.6. At this point the bike stopped trying to start up and the noise It was making (like it’s trying to fire up) ceased. The battery is now back to normal . I have a video I could send someone if you could PM me your phone number? Any help is appreciated!


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[calj737]

You won't see spark unless the mUnit is energized (which you stated it is not as no lights were coming on). If your battery is 11.8, it is HEAVILY discharged. At 8.6 (if it's a lithium type) its done, garbage, kaput.

So, charge your battery slowly. Get it to 13.4v. Let it rest off the charger for 2-4 hours. Then take a meter and grab a reading of the battery directly on the +/- terminals. Post that please.

It appears the solenoid for your bike is on the left side of the seat pan. Perhaps you could snap a picture while standing on the right, at a flatter angle towards the left so I can see a bit more of the wiring (not what's connected to the mUnit)?

[Scootch]

I might be misunderstanding... Are you saying that plugging stuff in causes the battery voltage to drop to 8.5 volts? If that is the case you have a very heavy current draw caused by something. If that is the case you will want to selectively disconnect stuff to find the culprit. If I misunderstood back to your regularly scheduled programming.