1979 CB 750k Electrical issues
- Thread starter elbes14
- Start date
lurkmaster2000
Active Member
charging system is a common problem on that bike
lurkmaster2000
Active Member
Its probably the Rotor, thats the big culprit, either get a new OEM style one that works, or there is a really nice replacement for the entire charging system but it costs lots of money. Cant remember the company that sells it right now, look on the internet.
Also a cheap fix may be, clean all of the electric connections with the spray electrical cleaner, then use dielectric grease in each fitting. But its prolly the rotor
Also a cheap fix may be, clean all of the electric connections with the spray electrical cleaner, then use dielectric grease in each fitting. But its prolly the rotor
elbes14
New Member
Did an ohm test on the rotor, about 4 ohms resistance between the 2 sides... did a you tube search and seems like that is the norm number that should be coming out of it. Stator has continuity between all the yellow wires so there is no break in the winding's of the stator. Did black and white to other yellows no continuity. Need to get a battery on it to check what the stator puts out without being connected to the rectifier/regulator. Can i clean/test the brushes? or is it a direct replacement part?
Unless your brushes are worn past the end of life marks, it's unlikely that they're the problem. Also check the springs and wire connections on them. On your stator, You should see about half an ohm between any pair of yellow wires, and NO continuity between ground and a yellow wire.
And definitely get a known good battery in there before you continue to test the system. I own two DOHC bikes. The first one had a bad rotor and reg/rectifier. It ran like crap. The second one had the same symptoms as the first, but in the end it was only a battery issue.
Also, I'm active on three other honda forums, and the most common charging system failure on these bikes is either the rotor or the regulator/rectifier. Your rotor tested at 4 ohms, which usually indicates that it's okay. However it may still be shorting out when it's spinning. The only way to check for that is with the bike running. Refer back to the link I posted above.
And definitely get a known good battery in there before you continue to test the system. I own two DOHC bikes. The first one had a bad rotor and reg/rectifier. It ran like crap. The second one had the same symptoms as the first, but in the end it was only a battery issue.
Also, I'm active on three other honda forums, and the most common charging system failure on these bikes is either the rotor or the regulator/rectifier. Your rotor tested at 4 ohms, which usually indicates that it's okay. However it may still be shorting out when it's spinning. The only way to check for that is with the bike running. Refer back to the link I posted above.
elbes14
New Member
Got another Multimeter with a 200 ohm setting... got a reading on the rotor of 1.6 ohms, on the you tube thread the reading has to be between 4.0-5.0 ohms to get a proper charge. The other meter I had was measuring in 300 ohms so I guess I'm looking at replacing the rotor. Thanks for all your help much appreciated!!!
elbes14
New Member
also I forgot to mention that the reg/rectifier was replaced prior to me buying the bike, also ran all the checks on the stator and it checks out ok. Had a look at the brushes as well and I see the wear marks and they have continuity from the contact points to the wire connection so I guess they are fine.
Yep, 1.6 is indicative of a short. The epoxy they used on those wires did not stand the test of time.
Here's the best replacement available, at the moment:
http://racetechelectric.com/Honda_Alternator_Rotor_F901
You can get a 15% discount if you tell them you're a member of the cb750c.com forum:
And I highly recommend joining cb750c.com if you haven't already. They're the DOHC gurus. Just don't tell them that you're cutting up your bike. They like to keep things stock over there.
Here's the best replacement available, at the moment:
http://racetechelectric.com/Honda_Alternator_Rotor_F901
You can get a 15% discount if you tell them you're a member of the cb750c.com forum:
And I highly recommend joining cb750c.com if you haven't already. They're the DOHC gurus. Just don't tell them that you're cutting up your bike. They like to keep things stock over there.
It is possible, although if it's a newer reg/rec, then it is probably fine. I wouldn't go ordering anything until replacing the rotor first.
Along with the link I posted earlier in the thread, here is another great diagnostic chart:
http://www.electrosport.com/media/pdf/fault-finding-diagram.pdf
Along with the link I posted earlier in the thread, here is another great diagnostic chart:
http://www.electrosport.com/media/pdf/fault-finding-diagram.pdf
elbes14
New Member
So Rotor is installed started up the bike, getting about 11.7volts at idle 11.9 at 4000 rpm. checked the rotor before i put it on and it read 4.6 ohms. Ran the bike for about 20 mins and shut it down started right back up for once. Would love to take it for a ride but there's snow everywhere. Whats your take on those numbers?
Do you know your battery is good and fully charged? If it's a dead battery, that would explain the low voltage. Don't just assume that it's a good battery because you can charge it with a tender and it reads 12V. The plates can hold a surface charge that will make it appear to be good with a simple voltage tester. You'll want to get the battery load tested if possible. You can pick up a load tester at an auto parts store, or you can take it to an auto-zone and they'll do it for free I think.
Also, follow the diagnostic flow chart from the electrosport link that I posted above. Basically, if it's not your battery, it's either your stator or your reg/rec. It's probably not your stator, but you should still check it out. However my money is on reg/rec or battery. A new battery is cheaper than a reg/rec, so load test that first.
Also, follow the diagnostic flow chart from the electrosport link that I posted above. Basically, if it's not your battery, it's either your stator or your reg/rec. It's probably not your stator, but you should still check it out. However my money is on reg/rec or battery. A new battery is cheaper than a reg/rec, so load test that first.
