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Blood Sweat Tears and Grease => Electrical => Topic started by: GroupBEnthusiast on Aug 11, 2017, 10:48:58

Title: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast 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!  :-\
Title: Re: '80 CB750F Electrical Issues
Post by: Sonreir 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?
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast 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!
Title: Re: '80 CB750F Electrical Issues
Post by: julian.allard66 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

Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast 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.
Title: Re: '80 CB750F Electrical Issues
Post by: julian.allard66 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

Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast 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.
Title: Re: '80 CB750F Electrical Issues
Post by: Sonreir 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.
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast 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
Title: Re: '80 CB750F Electrical Issues
Post by: cxman 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
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Aug 30, 2017, 10:00:06
cxman,

Yes, I'm now realizing I started at the wrong end of the system. lol This is my first CB, so I'm sort of learning as I go. Guess I won't have to worry about charging issues ever again!
Title: Re: '80 CB750F Electrical Issues
Post by: cxman on Aug 31, 2017, 20:16:13
the rotors cook from heat and no ventilation

 
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Sep 11, 2017, 12:47:55
Okay, so I installed the rotor and new brushes. The bike seemed to be holding a charge, so I rode it 100 miles yesterday, checking the battery every 20 or so. I had 12.5 volts, consistently, at each stop (ignition off), but when I got it home, it was reading 10.5 volts and couldn't even turn the starter.... I'm quite stumped at this point. The entire charging system is brand new, and it rode perfectly for 80 miles before the battery started draining.

I did notice that there is a small rectifier on the wiring diagram that connects the clutch switch, neutral switch, and starter solenoid(?), but that's the only thing left that I can possibly think to replace at this point...   >:(

Any ideas, gentlemen? I'm starting to think I need to just take it over to the stealership.
Title: Re: '80 CB750F Electrical Issues
Post by: Sonreir on Sep 11, 2017, 12:54:54
The charging system shouldn't be too tricky to test. You have your multimeter handy?

Start at the battery and work our way back.

1.) With the bike idling, measure voltage across the battery terminals. It should be in the low 13s, but anything over 12.5 will suffice. Give it some revs and you should see the voltage increase up to 14.5 max.

No change when revving? On to the next test.

2.) Switch the multimeter to read AC instead of DC. Check between each of the yellow wires going from the alternator to the regulator/rectifier. You should see somewhere between 20V and 40V.

No voltage? Time for the next test.

3a.) Switch back to DC voltage and check the black wire running from the regulator/rectifier over to the alternator. You should get 12V on this one.

No voltage? Check the black wire going into the regulator/rectifier from the harness. You should have 12V on this one, too.

3b.) Next, check the white wire coming back from the alternator to the regulator/rectifier. This wire should also have 12V.

If no voltage on this one, you have an issue with the rotor and/or brushes.
Title: Re: '80 CB750F Electrical Issues
Post by: cxman on Sep 11, 2017, 21:24:52
did you do a resistance test on any of this ?

rotor ohm test stator test

wiring continuity test

battery test
Title: Re: '80 CB750F Electrical Issues
Post by: esmoojee on Sep 12, 2017, 01:45:17
Get a cycle x charging system and you won’t have any of these problems.  Mine provides a consistent charge while riding and idle. I get easily 12.5 or higher at idle The charging systems on the dohc were not great as already stated. You could have a bad battery as well. I'd take it and have a load test done at the auto parts store. 
Title: Re: '80 CB750F Electrical Issues
Post by: ChopperCharles on Sep 12, 2017, 11:56:35
Even with a new rotor, the charging system in the DOHC is abysmal. It will. not. charge. at. idle. Period. If you're in stop-and-go traffic, you will eventually run out of battery. I replaced the rotor and regulator in my 1982 CB750C, then I re-wired the charging system with 12ga wire, just to be overkill. Still didn't do it. Changed all my turn signal and tail light bulbs to LED -- even the gauge cluster bulbs. Still just barely above 12v at idle. At 3000rpm it was charging fine. Eventually bumped my idle up to  ~1600rpm and that was just barely acceptable. If I had kept the bike, my next step was to find a Euro style left handlebar control, which allows the headlight to be turned off.

I loved my CB750C, but damn that charging system. Damn it to hell!

Charles.
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Sep 12, 2017, 12:40:40
ChopperCharles,

The more I read, the more it sounds like I'm always going to have this issue, even if I do get the system running "properly". I've been trying to keep the revs at 3500 and above when riding it, but that's only feasible for so long... Now, if I were to go out and trade up to a CB900F, or (dare I say it?) a CB1100F, would I still be encountering these same charging issues, or do those bikes have a better system?
Title: Re: '80 CB750F Electrical Issues
Post by: Sonreir on Sep 12, 2017, 12:41:58
The charging system is the same for all the 80s DOHC Hondas.

That said, how are you not keeping the revs above 3500 when you're riding? Unless you're idling for really long periods of time, all riding is usually done at like 5K or more.
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Sep 12, 2017, 12:45:00
Sonrier,

Let me clarify... What I mean is that I keep the tach over 3500 even at low speed (i.e. 15-30mph). Anything above that, I'm definitely cruising in the 5-6K range.
Title: Re: '80 CB750F Electrical Issues
Post by: Sonreir on Sep 12, 2017, 12:46:39
You shouldn't have to keep the tach above 3500 at all times, no. You can run off the battery for a long time and it will charge back up when you're cruising.
Title: Re: '80 CB750F Electrical Issues
Post by: ChopperCharles on Sep 12, 2017, 13:23:42
One thing that does help is to ditch the cheapie batteries you have to fill with acid yourself, and get a sealed battery. It helps, but the charging system is still marginal. Honestly, if I had just bought that control pod that lets me turn off the headlight, I don't think the re-wiring or going with would have been necessary.  Just that and the LED bulbs I think will be enough. I mean, you have 1157 bulbs in the turn signals for front running lights, and 1156 bulbs in the rear signals. Plus two 1157 bulbs in the taillight housing. That's 8.3W for each front running light and taillight, plus 23W for each brake light and turn signal. With two taillight bulbs, that's 33 watts of power with the brake lights off. 62.6 watts with the brake lights on. Plus an intermittent extra 46 watts when the turn signals are on. And then 55 watts for the low beam.  Going to LED bulbs does really help, and turning off your headlight in stop and go traffic does too. There's also now the option of LED headlight bulbs, which consume a lot less wattage and are still hella bright. They're pricey though.

Ohh, plus each bulb in your speedometer and tach housing is 3.4 watts. Two in each bucket, plus one for each idiot light.  So going all LED will definitely help you a lot. It did help me back in the day.... back then though LED bulbs were a lot dimmer than they are now. You can really get some great brightness for not a lot of money these days.

I mean, I had mine working well. If I let it idle too long voltage would still drop, but it was honestly SO much better than stock. SO. MUCH. BETTER. The only times I really had trouble was when I was in stop-and-go traffic for 30-45 minutes.

Charles.
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Sep 12, 2017, 22:19:04
Quick update... I just charged the battery and ran through the voltage checks again, as you suggested, Sonreir. Everything checked out except for the voltage at the yellow stator wires. I was reading 10.2-11.8 V (AC) across the all three. (I did get 12V at the black and white wires)
Title: Re: '80 CB750F Electrical Issues
Post by: Sonreir on Sep 13, 2017, 00:33:29
How about when you rev it? Does the AC voltage go up?
Title: Re: '80 CB750F Electrical Issues
Post by: cxman on Sep 13, 2017, 08:05:12
and was the regulator hooked up while you were checking

Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Sep 13, 2017, 12:34:23
Yes, the regulator was connected when I checked. I believe I did try rev'ing it, but I don't remember it ever getting near 20 V. Maybe 16, tops. I'll double check on that when I get home tonight.
Title: Re: '80 CB750F Electrical Issues
Post by: cxman on Sep 13, 2017, 15:27:40
and check rotor ohms thru the brush wires at the regulator to make sure the brushes and teh wiring are working
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Sep 13, 2017, 19:37:36
So, at idle I'm getting ~10 V from the stator, and 12 V at 4500 RPM. I did read 9.9 ohm at the black and white wires, though.
Title: Re: '80 CB750F Electrical Issues
Post by: cxman on Sep 13, 2017, 20:26:44
ok that aint right

you wont  be exciting the magnet properly

better pull the cover and recheck the rotor

if the rotor is still good the check it at the wire connector for the brushes

then thru the brushes and figure out where the drop is

dont forget to test for continuity to ground there should not be any

charge the battery and have it load tested
Title: Re: '80 CB750F Electrical Issues
Post by: GroupBEnthusiast on Oct 02, 2017, 14:51:09
Alright gents, the verdict is... still questionable. I ended up just throwing in the towel and taking the bike to a local shop that specializes in vintage motorcycles. They went through all the new parts, and each connection, only to find that everything was working as it should... When I went to pick it up, I spoke with the mechanic, and I explained that there was a loose pin in the rectifier connector that I had fixed before I brought it in. He seemed confident that that was what must've fixed the issue.

Now, I didn't bother to re-check the system after fixing the pin, as it seemed to have come loose after my constant fiddling with the connector. That was an oversight on my part, but it is what it is.
The bike is now working perfectly. I've ridden it another 500 miles since getting back last week, and everything seems to be good to go.

Thanks for all the help and advice, everyone! I'd also like to give a hat tip to Cafe Racer in Ypsilanti. If you're in the area, I can't recommend them enough.  ;D