Topic: Melted wiring - solenoid terminal to frame ground? CB750K7

[flybackwards]

in the process of my restoration I came to examine the wiring harness on the bench - while everything was working prior to disassembly I notice that the red/white originating at the starter solenoid and routing to the rectifier  is fried and then the green from the rectifier  to the frame ground by the coils is also toasted. The red/white is unfused battery current so the wire became in effect the fuse - looks like current went from solenoid terminal to frame ground via the r/w and the green.  The rectifier appears original and its internal red and green jumpers look like they were overloaded as well. Anyone else seen this and have an explanation? My only theory at this point is that something metallic got dropped into the rectifier assembly and bridged the r/w and the green otherwise the diode assembly would have had to fail and been replaced...

[flybackwards]

That is a possibility!

[69cb750]

My guess is reverse battery.

[bryanj]

Only reason i know of for that, may have fried rectifier

[flybackwards]

Thanks guys - looking at the diode directions - I'm going with reverse battery at some point in history. Rick is sending me one of their combined regulator / rectifier units as part of the upgrade and I'll be replacing the wiring that needs it and fusing that feed.

[Bodi]

The wire goes to the main fuse (to power the bike) and to the rectifier. That fuse would blow way below the current needed to cook the wires, so the prime suspect is high current through the rectifier to ground.
The rectifiers rarely fail in service. They fail pretty much instantly if the battery power is reversed... either from connecting the battery backwards (ie installing a seemingly identical to stock battery which has the terminals reversed to suit a different bike) or connecting a boost battery reversed. Nothing else on the bike should be affected unless you have electronic ignition and the key was turned on (poof!).
Anyway the rectifier is pretty easy to test with a multimeter.

[maxheadflow]

The wire goes to the main fuse (to power the bike) and to the rectifier. That fuse would blow way below the current needed to cook the wires, so the prime suspect is high current through the rectifier to ground.
The rectifiers rarely fail in service. They fail pretty much instantly if the battery power is reversed... either from connecting the battery backwards (ie installing a seemingly identical to stock battery which has the terminals reversed to suit a different bike) or connecting a boost battery reversed. Nothing else on the bike should be affected unless you have electronic ignition and the key was turned on (poof!).
Anyway the rectifier is pretty easy to test with a multimeter.

While you guys are right on about the melted wires, it worth noting that rectifiers do commonly fail.  When these bike were relatively new, the rectifier would fail when one of the connections from the alternator to the rectifier became flaky or loose. The peak inverse voltage would be exceeded on one of the diodes and it would short out..  I suppose that the same could happen if power or ground were loose but the main one I saw was to the alternator.  I probably saw 4-5 of them fail that way including my own CB550F. 

[flybackwards]

Thanks to the reverse battery knowledge - I can see what happened here.

There are two wires on the connector at the solenoid inside a cable sheath and a two pin plug on the harness. The solid red is attached to supply the main fuse at the fuse block - the red/white stripe goes unfused to the rectifier and is I think the main charge lead to the battery using the solenoid post as a connection point - it has no overcurrent protection.

Checking the wiring diagram you can see that if the battery is hooked up with reversed polarity, current will see three sets of two forward biased diodes in parallel from frame ground to complete the circuit back to the battery - and no overcurrent protection. Current will be split three ways across the diodes which are a good size on a fair sized heat sink - they will probably last at least long enough for a doofus to see smoking wires and unhook it which is what I think happened here now its pointed out to me.

I am going to replace the pieces with a Rick's combined reg/rect and convert the fuses to ato style box with an extra fuse position for the charge lead. I'll replace the cooked wires with next gauge up - the frame ground looks a bit light for my liking. I think max alternator charging current plus 50% should be a good fuse rating? I'm a marine electrical technician and used to that kind of thing - first time into a vintage CB electrical though.Thanks all..

[bryanj]

There was never a fuse there becauseHonda did not exect us numbnuts to reverse battery polarity, also should that fuse mechanicaly fail whilst engine is running, or rather alternator charging, the rectfier will be destroyed as the current can go nowhere

[Don R]

A couple years ago Advance auto was selling batteries for our bikes with the poles reversed. As I was leaving after noticing it, a guy was returning one he had bought for a sportster. Same battery and problem.

[flybackwards]

and the last word goes to Honda..

Service Manual page 102 charging system "Battery polarity should be strictly observed. Do not connect the battery in reverse. Reversing the battery connection will cause the battery to become shorted, resulting in a large current to flow through the electrical system and damaging the silicon rectifier as well as burning up the wiring harness."

Thus endeth the lesson..