1980 Honda CB750 DOHC Electrical System Explained
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- Опубліковано 20 лип 2024
- In this video I explain the oft-feared electrical system. A lot of the information in this video applies to general electrical troubleshooting on motorcycles and cars, and all of it will apply to all DOHC CB750s produced between 1979-1983.
To skip ahead:
Intro 0:00
DC Voltage + Grounding 1:14
Charging System + Batt chat 6:45
Starting System 16:36
Ignition System 29:24
Turn Signals + Outro 32:38
There is much that I'm sure I left out, and different DIYers will litigate the methodology differently; what I know for certain is that this methodology has worked for me up until this point. For example, some of the components, like turn signals, don't have a ground wire; they ground through the actual bolt that holds them to the frame. I also left a little easter egg in the starting circuit in the form of the neutral switch. Check your wiring diagram to see if you can figure it out and post it in the comments below. Remember, the proof of your work is in the ride! Always gather information from multiple sources before attempting your own maintenance.
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This is a good video on general electrical troubleshooting of this model. The white plastic block with the alternator wires is referred to as a coupler. It is much better to probe these couplers from the back as it prevents damaging the connecting part of the wire terminal. The starter circuit also includes the neutral light switch so that the engine can be started in neutral without pulling in the clutch lever. I think it used the neutral switch as a ground circuit as well as the clutch lever switch.
This model's alternator can generate more then adequate power to keep the battery charged, but with time and use they can get weaker. As well as testing the stator coils, check the rotor for insulation from ground, resistance of the winding and good continuity through the brushes. Since the rotor is always spinning, it can have shorts in the winding that reduce the output. Rotors and stators can be rewound. If the bike is dropped on the left side and the side cover is moved, the stator can rub against the rotor and damage both parts.
I don't think testing the open circuit AC voltage from the stator is a valid test. I would rather test the AC voltage between the yellow wires with the engine running and the coupler connected to see if they are similar. You could also test the battery voltage with the engine running, it should be above 12 V and not go over 14.6 V. A long time ago we use to test battery charging amperage, but this test is no longer used much.
The ignition on these models is a transistorized system not CDI. The spark units above the battery were notorious for failing. The symptom is that the engine will not rev to red line and the bike acts like it is governed. The coils are probably good, but you may need to cut some of the spark wire off at the plug cap end as the plug caps use a screw that threads into the wire and over time the connections get bad. Peak Voltage testing is the best way to test ignition systems. TecMate has a good tester for this.
That was a wonderful excellent video! I am pretty intimidated when it comes to a bikes electrical system and that was the clearest easiest to understand explanation I have yet to see. Thank you very much!! Btw, love the shirt. Namaste.
That is one hell of a good video. Lots of details no BS just amazing. Thanks for putting in the hardwork!
This IS one of the best videos of how to work on ANY motorcycle electrical system. Especially custom bikes that need custom wire harnesses. I am an electrician - and I DO suggest that you get a better digital multi meter, and/or NEW tester leads. A 1 ohm reading with just the leads is VERY bad. That will cause volt tests to be wrong - because you are adding resistance into the test. E=IxR (or volts=current x resistance) is Ohm's law. Your leads (or internal meter resistance) is causing bad readings.... Other than that - this is a really great video. I am also glad and impressed that you mentioned in the beginning that electrons flow from negative to positive - just like lightening actually goes from the ground up - lol.. Thanks..
I know right?!
I saw the one dude a few days ago and I thought he was great and in no way am I taking credit back, But I actually think I'm going to take care of the clicking.This is my 2nd street bike. My last one was a 1980 Honda CB750 F. Had a little accident and that hurt got rid of it now I figure try it again.i have a 1981 now . Both very fun
Oh and I'm not a mechanic at all But I'm learning from these videos.
Some stuff
Just wanted to thank you for the great work - it is definitely difficult to find good videos on motorcycle maintenance, and if you have a classic bike, much more so. I was gifted a 80s DOHC Honda bike, and you've definitely improved the chances that I will be able to keep and enjoy it, which is how I feel is the best way to thank the person that gave it to me.
Awesome, clear, concise explanation!!! Great video!
The signal to noise ratio of this video is extremely high. Everything about this format, delivery, and content is exceptionally thoughtful of the viewer. Much obliged.
I'm an audio engineer by trade, hah!
@@Gumaamug Well that is extremely obvious from the video. My first impression was enjoying the absence of background noise and the otherwise dead silence behind the lesson. It translates very well
I bought a 1980 by the way. It was an excellent video I have a 1980 CB750K when I bought it. The battery was bad, and then the guy hooked up in a car battery to it, but he hooked it up backwards. And before that, all the lights were working on, then after I noticed that he had it on backwards none of the lights would work and then I replaced the 30th fuse. I was born if you knew something else that could have went bad or what could have happened.
You replaced the 30A fuse in the starter solenoid? Did you also check the fuses in the handlebar fusebox?
Your video saved my life. I work on aircraft avionics and couldn't figure out why the harness in my buddies 80 CB750 wasn't giving any power anywhere. Was especially concerned that it wouldn't activate the starter solenoid. After beating my eyes on the same wiring diagram, and knowing I was right, your video made me 100% sure. Went right back to the battery with a test light, worked my way around, found that I had a loose connection to my ignition and the ground wasn't good for the neutral safety switch. Cranking good, coils got power, and lights work!
Ride on!
You are the first to explain that the whole frame is the earth..back to the battery.
For those challenged earths you can use a jumper wire from the negative- side.
Although my battery was charged the low V at start up was a key record that the battery was old...2011 it was changed I later found out so 12 years old.
I checked my starter to 12v IV it worked, the stop/start switch had continuity so worked.
The starter relay was not working correctly.
Thx for your help that was excellent.
Fantastic video! Really really like your calm and thoughtful approach of describing everything.
I mean this in the most positive and complimentary way when I say you’re like the Mr. Rogers of bike mechanics. And the bonus is the comment section of folks adding to the post. I just replaced the reg/rec in my 1981 CB750 Custom with the help of this video. I’ve never worked on bikes before. This is so helpful. Your video gave me the confidence, and it worked!
Cheers to that! Just doing my part to try and keep knowledge accessible and friendly
For the first time I understood the purpose of each component of the ES on this bike. Thank you very much!
I'm glad this was helpful
I learned allot from this video. Thanks for the content!
Fantastic video. Thank you so much for presenting in a clear and thoughtful way. I have 1980 CB750 Custom. A while ago the charging system failed and in troubleshooting I discovered the router was bad. I’m not a “handy” person but enjoy learning and I’m committed to doing all of the maintenance and repair work on my bike. I wish I had seen your video when I was troubleshooting the charging system - would have saved a whole of time! (And to your point on lithium ion batteries - I upgraded when I replaced the router but didn’t realize that I needed a regulator rectifier that worked specifically with lithium ion. That mistake was lesson learned #2!) Thanks again and hope you continue to make more videos.
Thanks J! I'm glad I could illuminate some concepts for you. I'm not an expert myself but also enjoy learning and being self-sufficient. Good luck with your bike and remember to enjoy the ride!
Another GREAT video! Thanks!
If you do a complete rebuild then a Motogadgett control box is the way to go. It simplifies wiring hugely.
Mr. Masters - I appreciate your review of the CB750 Honda Electrical System. Thanks - I have two 750s. My 79 is fine. The 81 is a Custom for my wife that we just got. Although the turn signals work correctly, The Right Turn Signal ( Blue wire side ) [ will lite up and stay lit if the Running Light ( tail light ) Fuze is okay ] unless used normally as a turn signal. In other words, the Turn Signals work regardless.
After about a minute or so, the running light fuse blows, and the right signal side is no longer lit up but is off unless the turn signal is selected.
In reviewing the schematic I concluded that the turn signal switch is leaking a positive from the run light brown wire to the blue wire side of the switch.
Observations: When the run light fuse blows, the run light does not receive power otherwise. The Turn signal fuse which is adjacent to the Run Light Fuse has not blown - turn signals work normally -
Your opinion is most welcomed -
thanks !
Hi, nice to hear from you. My gut says there's a pinched wire (somewhere in the headlight bucket perhaps? Or maybe under the seat where the harness for the tail light and running lights clip to the fender).
@@Gumaamug Thanks Ill check them out -
@@Gumaamug Hi I checked under the seat and the connectors looked undisturbed since 1981. Ill have to check the headlight bucket, as having ordered a replacement switch. The blue line is the only hot as expected. Will see what happens -
You video's are excellent - I just cant wait to tune two sets of 750 valves some day. This weekend Im going to use a new timing light -
btw: in reviewing the schematic , the blown fuse in question also is for the instrument lights and front run lights which don't work. The front run lights share the same switch vicinity as the turn signal switch. When the fuse is not blown, the right turn signal lights stay on unless the turn signal is selected - this I figure shorts the run light fuse path to ground thereby blowing the fuse. Hence, the turn signals can now operate without the intrusion of the run light power source shorting over to the blue wire. If in the headlamp bucket, will review first, but I got a switch on order -
Oh- something else -no where is there documentation of the vacuum fuel regulator that sits on top of carb 4 - please explain its operation in a separate video - thanks !
Good info! Thanks
Well done, thanks!
What a Stud! Thank you!
Thanks 🙏 that’s a great video
excellent ,thank you
Thank you good class.
A question about your ac stator test...If the regulator applies voltage to the rotor via the black and white wires in order to energize the stator, how do I measure AC stator output without disconnecting the Regulator from the field coil? (they share a connector) Thanks.
Matt great question-- I should have mentioned this in the vid. You'll need to use your probes on the back side of the connector and push them in where the wires go in. You *should* be able to make contact that way. Emphasis on "should" :)
Watching your video leads me to believe I have a bad battery. I have a new solenoid and when I try to start the bike my voltage drops to 10 and doesn't come back up. Starter doesn't spin but solenoid clicks
I have an 82 CB750C- bought with a charging system issue (battery wasn't recharging itself after running for about half an hour). I replaced the battery, rectifier, stator and starter solenoid. Now the bike has no power whatsoever, but I did discover that the 30amp main fuse continues to blow. Any suggestions of where else to look?
My cb750 DOHC was having some issues at idle with uneven temperatures on cylinders 1&4 (running hot) and 2&3 (almost 100*F colder). I have a dyna 2000 but what looks like cheap ignition coils. I tested the resistance on the coils and plug caps and they were all wacky. A plug cap on cylinder 4 had a resistance of about 200k ohms. The secondary resistance on both coils was roughly 150k ohms. I’m thinking I need to order some better coils but I also noticed my ignition spark unit melted all the potting out. Could this be caused by the coils being screwed up? Or is there an electrical issue somewhere else that’s causing excess current to the ignition spark unit?
Great video, learned a lot!!
I have a 1980 Honda 750f for 43 years now...got it on my 18th birthday..., great bike, but now I am about to give it up..
When I turned the key ALL lit up as usual, and as I tried to start the bike ALL went dark (NO power). I checked the fuses and they are all good, so I tried to test the solenoid and did not spark so I check the battery and it was bad. I replaced with a new solenoid and a new battery and still NO power at all. I decided to get a new ignition and still NO power. The engine does turn when I put the screw driver in the solenoid. I checked the ground connection and I cannot see anything wrong with it. At this point I have no idea what else could be wrong. I really need help
Hi - engine turning over when you bridge the solenoid is a good sign! Have you checked all the fuses with the ring/continuity mode on a multimeter or just inspected them by eye? It's possible that one or more is blown but doesn't look that way. They should ring out when you touch the probes to either end.
So you've replaced battery, solenoid, and ignition switch. Next I would look at the right side handlebar switch where the killswitch and starter button are. Take it apart and make sure it's not full of spiderwebs or corrosion or anything like that. Clean the contacts inside with spray contact cleaner. Without looking at the bike, I suspect this switch as you mentioned that everything was fine when you turned the key and then went bad when you tried the button.
I have a 1979 cb750k it would not charge when I got it I replaced battery stator and the rotor now it will run and charge for about 30 seconds it pops at the rotor and melts one wire off of the rotor and quits charging any ideas anyone
I am having trouble wiring my CDI to a m.unit. I thought I had everything right but my green wire is reading 12v. Do you have any diagrams specifically for the CDI?
Hi Ben, no diagram exists specifically for the CDI, but you shouldn't need one as all wires are clearly labeled in/out on the main wiring diagram. Here it is: www.dropbox.com/s/uo3y2ok342xhhvz/Wiring_Diagram_CB750F_79_80.pdf?dl=0
This bike does not have CDI but MDI ignition system which is completely different
Question, what size relay would I get to get to wire for my led turn signals? And would I just replace whole relay? Thanks in advance!
Hey Derek, I'm not an expert in LEDs on bikes. I know that in addition to replacing the mechanical relay with a electronic one (yes you do need to replace the whole relay), a diode needs to be added in the circuit somewhere. But this is where I'm unclear because I think some manufacturers sell lights with diodes built in... I'd Google it, and obviously since this is not a model-specific upgrade there should be plenty of info online to guide you. Good luck!
@@Gumaamug thanks for help much appreciated!
This is a fantastic video, very helpful. I have the exact bike and I have a problem. The previous owner cut the connector off of the ignition switch and I’m trying to rewire a new plug. I have no idea where what wire goes into what position on the plug. Also it seems the switch has five connections and the plug has six positions, but it seems I have an extra brown/white wire. Can you explain what I should do for this? Thanks!
My ignition connector is a square connector that appears to have, from left to right on the *back* of the connector, from top to bottom: 2x brown, empty, 2x brown/white; black, red, 2x brown
So you are saying the brown wires both terminate to the same position and the brown/white both terminate to the same position? 4 connections into the switch?
@@tblackwell1 Yes, but five positions total. I have two brown wires in the top left port and two more brown wires in the bottom right
Ah, thanks for the clarification. Mine is a tad different for some reason. There are five connections on the switch. I paired the brown/white wires and put them in the bottom opposite corner to the black and one brown next to the power and the remaining brown in the corner behind the black.
It kinda works but…
The headlight comes on, the tail light comes on and when you switch the turn signal to the right the front right comes on constant and the indicator on the dash is dark and the rear turn signal is blank. The left side is totally dead all three lights. The horn works, and the ignition with energize the starter solenoid. So we are close. But I have some chasing to do.
I had to refurbish the turn signals so I need to check their connections. And I need to check the brakes connections as I need to rebuild them so I’m not sure they are right. Any advice on the position of the brown or brown/white locations?
And by the way, I really appreciate your conversation on this. What a huge help! Thanks
Hey, thank you for the video it’s a big help. So my 1982 Honda CB650 nighthawk has a charging problem. Before watching your video I replaced the stator and magneto hoping it would fix the issue but it didn’t. After watching your video, when doing the test at time mark 13:24, I found continuity on the black wire. What exactly does this mean and what should I do from here?? I would greatly appreciate the help, thank you.
Also am I supposed to test the connector that is coming from the stator windings
Hi, it's hard to give a definitive answer without being there with you and seeing exactly what you mean, but if you have continuity between the one of the lines in that connector and your battery ground, it could mean that the cable coming from your regulator/rectifier is pinched or damaged and is somehow making contact with the frame. Could also mean that something inside the reg/rec is damaged and is making contact with the body of the reg/rec, which is mounted the frame, i.e. the ground. In any case, I would look at replacing the reg/rec. Charging issues are more likely to come from that component than the stator + rotor, but it certainly doesn't hurt that you've replaced those things.
I’m getting continuity from the copper windings when I put one lead on the negative terminal and the other on the connector terminal I’m testing. Could that be that the stator is making contact with the frame somehow??
@@_fredoo.mShould not be getting continuity between stator and frame. Don't know exactly what the mount looks like on the CB650 but make sure none of the windings are exposed (usually they're coated in clear epoxy) and especially look at where the bolts mount it to the engine as I guess something could be pinched or making contact there. You said you replaced it - did you buy a new one or a used one?
Any idea what wire in my head light would cause bike to die when put in gear. Runs fine in neutral. Only thing i did was remove some wiring in head light. I eliminated the left switch and wires low beam direct white and blue wire. Either something there almost like a kick stand safety but doesnt have one 81' or clutch just went..... no clue.
There is a dead man's switch in the clutch handle that will allow the bike to start in neutral with the clutch out, or if the bike is in gear, only with the clutch pulled in. If you took that out, this explains why shifting into gear causes the bike to die.
I looked at the wiring diagram and the clutch switch is just a part of the starter solenoid circuit, meaning it prevents the starter from turning at all if the bike is in gear without the clutch pulled in. It doesn't look like it would cause the symptom you're describing.
Just thinking out loud at this point... the only thing that changes electrically when you shift into gear is the contact with the neutral switch. The symptoms you're describing sound like the ignition circuit is somehow grounding out. The neutral switch cabling runs up to the headlight to feed the neutral light in the dash. I would check all wires in the headlight bucket and compare them with the diagram. Make sure the neutral switch wires (Light Green/Red) are correct and not going anywhere they shouldn't be.
@@Gumaamug think i found it ... the orange wires were grounding to the inside head light frame sound right?
Hi will this cover the cb750fn 96 reg .
Donyou happen to have a link to the pdf for the wiring diagram in this video its exact diagram i need for my bike.
Hey Kyle, it's not easy to share files on UA-cam but if you google "1980 CB750 wiring diagram" this diagram is very easy to locate online.
I found it thank you so much I was able to get it in color as well. So I'm rewatching this again and something you had mentioned was getting the upgraded regulator/rectifier do you have a recommended brand for that. As I am going to be getting a newer battery for the bike
I'm not going to skip ahead. So far so good.
UPDATE AT THE END OF THIS VIDEO: HOLY crud I'm glad I did watch the entire video. If he didn't break it down i would be lost.
Actually i would be shit out of luck if it was an engineer explaining all this crap. Thank you for explaining it the way you did.
I recently got a 1984 CB700sc nighthawk. When I was riding the bike suddenly shut off and wouldn't start, the only thing that happens is the click from the solenoid. I already checked the new battery and it was good. Do you have any idea as to what the problem is? All the lights come on, and when I bridge the solenoid terminals it doesn't crank as well. Thanks!
I haven't checked the starter but in the case it's bad, I'm not sure if it'll cause the running bike to shut off.
Sudden shut offs are usually related to a ground issue. Remember that if the DC circuit is broken, even briefly, no power will flow. Are you blowing any fuses? Can you measure both sides of the solenoid and confirm 12v passes through, the way I do in the video? Can you measure the voltage from the (-) side of the battery to the end of the cable that connects to the starter motor? When you press the starter button you should see ~12v there. If all this is fine and still no crank, start checking ground connections.
@@Gumaamug Ok I just checked the solenoid and there is continuity. I also checked the voltage to the end of the starter cable, showing 12.3v. I'm not sure how many ground points are on the bike but I confirmed that the negative battery cable is grounded tight to two spots(engine and frame it looks like).
I guess my question is: will all the lights turn on if there is a broken DC circuit(It turns on)? What ground connection enables both the starter and the ignition system?Thanks!
@@YourBestFriend822 It's possible for some parts of the bike to work and not others since different circuits are somewhat isolated and usually have a fuse for each one. The two grounding points you mentioned are the main ones for the bike, so you're probably fine there. Have you checked all your fuses? If any are blown, start by troubleshooting only that circuit, looking for frayed wires, a screw gone through the harness, etc.. I don't have a wiring diagram for that bike and though it is a CB750, the Nighthawk series of 750s are different from the 79-83 series so I don't really know the location of everything. But- if you have continuity through the solenoid and you see 12.3v on the end of the wire that attaches to the starter motor, then you're almost all the way there... make sure the motor is firmly bolted to the engine, which is its grounding point. This still does not account for why the bike died on you suddenly, and so I would also go sniffing around the killswitch to make sure that is wired correctly and a previous owner didn't put some kind of hidden killswitch on the bike.
@@Gumaamug Ohh good call! Yes, I also believe it's not the starter, although it should be able to turn when I bridge the solenoid regardless of other circuits right? I just checked the fuse diagram and found that one of them is ignition start, and I'll be checking that for sure! Thank you so much for your help
My rear amber lights are only going on when the turn signal is activated. Is this normal for my 1972 CB750 K. The front amber lights are lit when the headlight goes on but not the rear. All the bulbs are good. What could be causing this?
Hi Mark, this video is specifically about the DOHC 750s, not the early SOHC models. But I know that on my bike, the rear lights are not dual filament; they do not act as running lights the way the front ones do, only as blinkers. Hope this helps.
@@Gumaamug Thank you, very much! I was thinking that I might have an electrical issue, but this brings me relief! Thanks again.
Hi.. Great presentation. What would cause both CDI boxes to cook ..so much so that the epoxy is drained away? Horrendous! The bike is a 79 CB750F. Thanks so much!
When they go, they go. That's the only explanation I can offer you in this case
do you know what i should look for if my bike keeps blowing fuses when the stater cover is tightened? it will only blow a fuse when it's tight.
Sounds like the stator is grounding out against the cover. Check the stator and make sure none of the windings are frayed or pierced by anything, especially near the bolt points.
@@Gumaamug I've got it sitting in front of me. i don't see any bad spots. it's also a new part.
@@zacharybartels3680 I can't tell you much if it's an aftermarket part as I don't have it in front of me. Does the company that sold it to you have a support number or email you can contact? Make sure the AC wires that power the stator (black and white on the connector) are going to the appropriate wires on the aftermarket part. Sometimes with aftermarket parts you have to modify the connectors.
@@Gumaamug it's a Honda part i got from a dealer. it's wired correctly. would stacking two gaskets work or do you think that would cause more problems.
@@zacharybartels3680 Are we talking about the same bike? I don't know anywhere you can get a new OEM stator for this bike- they're forty years old. I'm sorry, I don't think I can help you.
Just checked everything . All ok, then noticed outside rotor brush worn-out . This brush wears out first because of the difference in circumference and distance it travels over the rotor . Check this first . 5 dollars fo both brushes. Fixed .
Hi just found you channel,,,,can you please send a quick update on why the cdi,s in my bike have melted ,,I have just started to restore the bike and today come across them ,just to say the bike runs ok ,,,thanks Michael from the UK 🇬🇧
Hi Michael; are you sure they are melted? The CDIs have epoxy resin on the back to keep them sealed up and this could degrade over time. If the bike runs OK, it probably means the igniters are fine
Thanks so much for your reply,,the cdi,s have definitely melted ,,so today I noticed the bike has got a what seems to be one new coil so I wonder if one failed so that could have caused the melt down
I didn't see how to check the diodeps in the rectifier / regulator. Isn't this a common fail issue.
Hi Jack thanks for watching-- as I pointed out early in the video a reg/rec upgrade is only about $100 and so I did not go into testing it. If you suspect yours is failing, I would just replace the part. Otherwise, I'm sure there are many sources online that can tell you where to stick your meters to check current through a diode.
Does this video pertain to a 1980 cb900C ?
The concepts apply and many components will be the same. Search for a wiring diagram for your bike to address any specific questions.
What's the function of that generator with the stator
Do you mean the rotor that is attached to the crankshaft? The rotor and the stator work together to generate the electricity that charges the battery.
Can I have ur contacts soo that u may help me repair my electrical issues please
I own a handa cb 750cc 2007 limited
Where is the fusebox located
On the handlebars under the plastic plate that says "Honda" on it
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