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Author Topic: '80 CB750F Electrical Issues  (Read 915 times)

'80 CB750F Electrical Issues
« on: Aug 11, 2017, 10:48:58 »
Greetings all,

First of all, this my first post, so let me know if I start stepping on any toes. Last week, I picked up a 1980 CB750F Super Sport with only 4,300 miles on the clock (picture attached). As this is my first street bike, and I was rather impatient to get out and ride it, so I managed to put another ~450 miles on it before it broke down on me. Unfortunately, on my way back home a few days ago, I came to a stop sign, stopped, and when it was my turn to go, the bike shut off, and the gauge cluster went dark. No power.

I rolled it off to the side of the road, turned the key again, and I could see that the oil light and headlight were both coming on, but were extremely dim. Though, luckily, a friend was able to bring his truck out and carry it home for me. Later that night, I started poking around and got my jumper cables out. I connected the battery to my car (engine off), and everything came right back to life. Full electrical function. I left the battery connected for a few minutes, then turned the bike's ignition on, still working. Pushed the start button, and all I heard was a *click* and everything went dead again. This time, the main fuse was blown.

I just finished replacing all of the fuses, and put in a brand new battery. After doing that, I turned the key and still had no power. However, the new main fuse was, again, blown. I have new left and right handlebar switch assemblies arriving today (along with 'superbike' bars, grips, and euro mirrors), as it sounds like the starter button on these bikes can tend to be problematic. Beyond that, based on the other electrical threads I've read here, all I can think of is a shorted regulator/rectifier unit, starter solenoid, or possibly a bad ignition switch.

I do have the service manual that was helpfully provided by Hoosier Daddy, but I feel like I'm only guessing at this point. I'll be the first to admit I'm a novice with electrical systems, so any ideas/advice would be greatly appreciated!  :-\

Offline Sonreir

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Re: '80 CB750F Electrical Issues
« Reply #1 on: Aug 11, 2017, 12:55:21 »
Sounds like you maybe have more than one issue.

Dead battery (especially one that can be charged back up again) is almost always a result of a faulty charging system. Let's shelve that for now, though. Until you can get the bike running, troubleshooting the charging system can be difficult.

The first issue we need to tackle is the short circuit. We need to know exactly when the fuse is blowing and which fuse is the issue. Does the fuse blow only when you're pressing the starter button or will it blow before then if you just leave the bike on?
Sparck Moto - http://www.sparckmoto.com

Audaces fortuna iuvat.

1977 Honda CJ360 - Café SOS - Stage One™, Café SOS - Stage Two™
1976 Puch Maxi - APuchalypse Now
Suzi T500 Cobra Resto

Custom Gauge Graphics
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Re: '80 CB750F Electrical Issues
« Reply #2 on: Aug 11, 2017, 13:12:11 »
Hi Sonrier,

The first time the main fuse blew was when I hit the start button with the jumper cables connected. When it blew the second time, all I did was connect the battery and turn the key to the 'on' position, and back to the 'off' position. Though, I'm not sure exactly when it blew, as I only checked it after playing with the key. I have brand new OE Honda fuses coming today, so I am planning on redoing all of that step-by-step to see exactly when it blows.

However, the second fuse was the original back-up fuse that was already waiting in the fuse box, so it may have been a bad/old fuse to begin with. That said, none of the glass fuses up front were blown, but I went ahead and replaced them all anyway.

Hope that helps!

Thanks!

Offline julian.allard66

  • Posts: 278
Re: '80 CB750F Electrical Issues
« Reply #3 on: Aug 11, 2017, 14:55:03 »
The fuse blowing is caused by a live connection after the ignition switch going straight to earth, it will be hard to find.
Charging problem is most likely to be regulator rectifier.

Nice bike though, if it were mine I'd be trying to find a new wiring loom for it or rewiring it myself, a 37 year old wiring loom is going to be full of faults.

Sent from my SM-T560 using Tapatalk


Re: '80 CB750F Electrical Issues
« Reply #4 on: Aug 11, 2017, 15:52:56 »
Thanks Julian,

I had considered a new wiring harness. Given the limited number of components (compared to today's bikes), it shouldn't take too much time to install. Likely, less time than it would take to check each connection individually.

Offline julian.allard66

  • Posts: 278
Re: '80 CB750F Electrical Issues
« Reply #5 on: Aug 12, 2017, 05:26:56 »
Rewiring my 82 KZ1000 took about 1 day at a fairly leisurely pace, I really enjoyed doing it and I now know my wiring well and that everything is good quality, I also simplified the wiring a lot and got rid of things I didn't want, like the side stand cut out switch.

Sent from my SM-T560 using Tapatalk


Re: '80 CB750F Electrical Issues
« Reply #6 on: Aug 12, 2017, 23:33:54 »
Just wanted to give a quick update on the situation...

I managed to get the bike running with a new positive battery cable and fresh OE main fuse. I'm seeing 12.34 volts on the battery with no load, and 11.6 volts with the bike at idle. I couldn't get a stable resistance reading across any of the three tabs on the stator connector, though it was definitely over 10 Ohm. Voltage output was 0.2 at idle, and 0.3 at 2000 RPM. I think it's safe to assume I have a faulty stator.

I didn't get a chance to test the regulator/rectifier unit, as I ran out of daylight, but I'll try again tomorrow.

Offline Sonreir

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Re: '80 CB750F Electrical Issues
« Reply #7 on: Aug 14, 2017, 12:24:09 »
I concur. Stator sounds bad. I'd double-check the connectors, though. Make sure they're clean and clear of dirt and corrosion. They're cheap and easy to replace, if needed. Since you're possibly ditching the stator anyway, it might be worth cutting the connectors off and measuring directly on the bare wire.
Sparck Moto - http://www.sparckmoto.com

Audaces fortuna iuvat.

1977 Honda CJ360 - Café SOS - Stage One™, Café SOS - Stage Two™
1976 Puch Maxi - APuchalypse Now
Suzi T500 Cobra Resto

Custom Gauge Graphics
Custom Wiring Harnesses

DTT Red, White, and/or Black 360 Club - Better than those Blue guys

Re: '80 CB750F Electrical Issues
« Reply #8 on: Aug 29, 2017, 11:14:07 »
Update #2...

I've installed both the new regulator/rectifier and a new stator, but I'm still only seeing 11.68 volts at idle, and ~11.85 volts from 3,000-5,000rpm (when tested at the battery terminals). The only pieces left that I can think to replace are the charging rotor and brushes, neither of which are difficult or terribly expensive to fix. Anyone have any tips/tricks for replacing the brushes? Obviously, the rotor is plug-and-play...

Thanks for all the help, gents!  ;D

Offline cxman

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Re: '80 CB750F Electrical Issues
« Reply #9 on: Aug 29, 2017, 17:53:52 »
stator ?? regulator??

its a dohc its the damn rotor

its always the rotor

the first time

then the regulator goes from the new rotor lol
1978 CX650 Super Deluxe
1979 XS1100 Special
1974 xl350
1983 cx650 Custom
1973 cb750
1980 cb750
1981 cb650
1982 cb900 c
1974 kawasaki 350 bighorn
1983 GL1100 aspy full dress
1983 GL1100 Nekid
and a bunch of others