Cb750c electrical headache. Main fuse blowing

[saben]

Having a very frustrating issue with my 81' cb750c. Every time I hook up the battery, I blow the main 30a fuse on the starter relay. I thought it was the relay so I bought a new one, same thing. I thought it was a bad ground so I reran all the ground cables I could find to a central ground and all tested clean with my multimeter. Disconnected my ignition and stator, still blows the moment the cable touches the positive. Lastly I checked the cable to the starter motor and it is in good shape. I'm not sure where this grounds but I assume it's straight to the engine by the mounting bolts. My eyes are melting from looking at wiring diagrams, any help would be great.

- saben

 

[mindactivated]

In no particular order...

1. re-test all grounds for continuity

2. test the battery itself

3. double check the actual fuse & what comes stock on the bike (I guess 30amp in this case?)

4. Here's one you won't think of... how's your horn switch? I had a bad horn switch cause me blown fuse nightmares for a better part of a year. I just didn't know what the hell was wrong and then I realized it... disconnected that wire and bam - no more blown fuses! Check your wiring diagram for other items that are grounded but have constant power to them.

5. Put a little light in lieu of the fuse and then you'll know when you've fixed the problem (can someone chime in on doing this correctly?)

6. Do you have any mods or upgrades, like LED lights, different after market items on the bike? Disconnect them and double check their wiring while you're diagnosing this issue.

7. Make sure your multi meter works properly.

8. Look at your wiring diagram... check continuity of each wire. Print out your diagram and colour in wires that you tested with a highlighter or something. Keep going until you're done.

I found a colour wiring diagram for my motorcycle and printed it on legal paper, then took it to a office store and had it laminated for $2.25. So useful when things are getting greased up in the garage.

9. Go have a beer and have a buddy take a look at it.

 

[Swapmeet Louie]

One thing you didnt say you checked or test is the starter

 

[saben]

Awesome! A ton of useful stuff, I made some notes below. Thank you so much for your detailed response. There were several things that I needed to double check that this pointed out.

1. Retested, everything tests clean I'm thinking about replacing the battery ground cable just in case the frame ground is testing clean but not clean enough for the amperage for the starter

2. Brand new battery, plus the old one was turning it last week. Tried both, same result

3. 30a per the manual. I stuck in a 16g wire to test and it caught fire

4. Disconnected horn, brake light switch, throttle switch, speedo, tach, idiot lights and the ignition. Still blowing

5. This sounds very interesting, and cheaper than blowing a pile of fuses.

6. Someone did a pretty serious hack job on the wiring. There are no after market parts but she has had quite a few unskilled hands on her (myself included)

7. Tested with my second meter. Same result. (good call though, this has burned me before)

8. This is my biggest fear. All this work is being done in the ally way behind my apartment as I live in the city and don't have a garage. Completely rewiring the bike is not going to be fun at all.

9. I hate to let something like this beat me but I'm a few troubleshooting steps from hauling it to a pro.

Starter ground is my number one suspect right now. I'll do some research on how to test this and see what I can find.

Thank again for your help, any more ideas come to mind please shout them out.