Topic: 1977 CB750A electrical problem

[JimS63]

Hi Everyone,

I just recently purchased a pretty clean looking and nice running 1977 CB750A with about 18,250 miles on it. When I bought it the blinkers would come on and stay solid sometimes and blick other times. The horn and highbean didn't work. (all I assume have to do with the left hand controls). I've put about 300 miles on it without any other problems, then just the other day as I was riding I noticed that the front light was out (or not working). I continued on my way home and as let off the throttle to come to a stop the bike stalled out and the oil light was off (no electrical?). I pulled over, turned the ignition off and on and no oil light/electrical. I checked the fuses and they looked good. I hauled it home and just checked the battery and it has about 14v. I rechecked the fuses and the middle (7a) fuse was blown (I don't know which fuse protects what part of the bike?). I replaced it and turned the ignition back on and still no power at all anywhere (no oil light, no headlight, blinkers, won't even try and start). What would be the next step? Pull out and check the ignition? Any suggestions (with pics if possible) would be greatly appreciated!

Jim

[raymond10078]

The middle 7 Amp fuse is identified on the schematic as the headlight fuse.

Check or replace the 15 Amp main fuse.  Fuses that "look good" can be bad.

Fuse holder clips are known to get dirty/oxidize and overheat.  Make sure that the clips are good and clean, and grip the fuses well.

Lastly - check your bikes ground wire - make sure both the battery negative wire and the loom ground wire are both connected to the frame well.  The battery negative - just follow it to the frame.  I recall that the loom ground wire is underneath the tank, bonded either at the ignition coils, or close to them (someone else will have a better memory, or maybe pictures to share).

[JimS63]

Thanks kandrtech,

The fuse holder looks good/clean/tight. I will recheck the 15a fuse. The main ground wire is good to the battery and engine/frame...I checked that last night. I'm not sure what the loom ground is, but I'll look back at the wiring diagram and then see if I can find it and check that out too. The bike is completely dead and from looking at the wiring diagram, after the fuse (and with a good ground) the igintion switch looked like it was next in the series....so I suppose that will be my next project...to take it out and test it and the wiring going to it.

Any other ideas/help anyone else has would be appreciated. I don't know much about electrical stuff.

Jim

[raymond10078]

Do you have a meter (that can measure resistance)?

If so, you could determine if you loom ground is bonded well by performing a test from any green wire to the battery negative.  A convenient place is just under the seat - thre is a green wire from the loom going back to the tail light and rear turn signals.  Disconnect a green (from the loom), and then measure from the green wire to the battery negative.  Should be a very low resistance ground - pretty much zero.

[JimS63]

Yes I do have a Volt/Ohm meter. I will perform the test you suggested. Test from a green rear turn wire to the battery negative terminal. If not, then on to the ignition testing I suppose. Thanks for the help/info kandrtech!

[tutnai]

May not be related, but, had very similar issue with dead regulator. All the grounds etc, looked fine. You can test the regulator or just swap one out.

[dave the welder]

you are correct the loom ground is under gas tank left side   very appairent when you remove tank

[JimS63]

I know very little about eletrical wiring and electronics.....

I did the ground loom test you recomended and it was good. I also looked at the shop manual and tested the regulator. It is good also.

I put my volt meter on the 15a fuse and it was bouncing all over the place. I turned the ignition on and it went to 12.9v. I did the same with the two 7a fuses below it and they showed no current either with the ignition off or when I turned the ignition on. I don't know if this means anything? Does the engine have to be running to show a current in those circuits?

Any ideas where to go from here?

[raymond10078]

• The 15A fuse test - jumping all over the place?  Doesn't make sense.  But - to test a fuse, you could either: remove it, and measure resistance through it; or make sure you have voltage on boths sides of the fuse.  Voltage should be rock solid stable.  If not stable, either your meter connections are not good, or you have a bad/intermittent connection somewhere else.
• I recall that the 7A fuses should be "dead" with the ignition switch OFF, but should be "hot" with the ignition switch ON.  So, you either have a bad ignition switch, or a bad connection somewhere.  Since ignition switches are easy to pull and even disassemble, I'd start there.

[RickP1]

Check the ignition switch, these can be a weak link on these bikes.  They are two piece construction and the plastic bottom part comes away from the top and the bike will die instantly

[JimS63]

Thanks guys. The fuse and fuse block both are ok. I started to remove the ignition, but I can't get the wires unplugged from it. I guess I'm going to have to loosen/remove the guages so I can get my fat hands in there to remove the plug of wires. I will then take it apart and ohm/volt test the switch and wires.

[JimS63]

Well, I took the ingition out, cleaned up the contacts, put it back in....changed the fuses just in case, and it works! There are still a few bugs in the electrical system, but at least it's running!

THANKS everyone for your help. I greatly appreciate it!!! 

[JimS63]

My bike seems to have a reoccuring, very intermittent electrical problem (probably the reason the previous owner sold it). It will blow the 15a fuse and die at random times (scary when on the road). I've removed/cleaned/tested/checked the grounds, ingintion switch, regulator, silicon rectifier, fuse block and looked at the wiring as best as I can for shorts. This last time I noticed that the plastic under the 15a fuse is melted on the left side so I found/bought one on eBay. I also pulled the points cover and found that the bike has a dyna s electronic ignition in place of the stock points/condensor set-up. Does anyone have any knowledge about those systems? Do they have an effect on the electrical system? Could this be part of the problem? I looked online at the dyna S website and there is no info there.

If it doesn't have to do with the electronic ignition, does this mean that the regulator is malfuntioning sproadically and sending too much juice to the main fuse, overheating it and blowing it out? Any other ideas on what to do/check? Any help would be great. It's not fun having to coast to a stop on the side of the road to replace the fuse....and not knowing when the next time will be....in 20 miles or two. Any help is appreciated!

[jukku]

My Matic blowed the 15A fuse every now and then. There was a shortcut every now and then in the right handle killswitch!
I opened the switch cleaned it and put it together again.
Now I also clean the fuseholder and renew the fuses every spring.
No blown fuse any more!
The fuseholder melts because the holdersurface against the fuse is corroded! It works like a heater element!

[raymond10078]

A Dyna S itself shouldn't cause a problem - as the electronics don't consume much more power.  However, if the coils were replaced with 3 ohm units, which draw more power, as compared to stock, or 5 ohm units, then that would put more load on the 15 ampere fuse.

Looking at the schematic, the following electrical loads are fed from the 15 Amp fuse:

- Coils;
- Parking indicator light;
- Oil pressure indicator light;
- "D" indicator light;
- "L" indicator light;
- Turn signal buzzer;
- Tail light (via master cylinder switch);
- Tail light (via rear brake switch);
- Turn signal "blinker" relay;
- Turn signals (via turn signal switch);
- 7VDC regulator (for fuel gage);
- 12VDC regulator (for bike charger system);
- Stator coils (via regulator);
- Speed Warning Unit; and
- Changing Relay.

Lots of stuff in that circuit that could intermittently fail.  Hopefully, this list will give you more to look at, and consider.

I haven't heard about intermittent regulator over-voltages, but maybe someone else will chime in.  Have you checked the output of the charging system?

Also - if the fuse runs hot to the touch, you could measure the resistance through the fuse holder connections, and if found high, try re-heating/re-melting the solder joints at the fuse holder to get a low resistance connection.

[Fran Fitzpatrick]

I was having similar issues with my CB750A.  Sometimes the bike would just die for no reason.  Sometime it was a blown fuse sometimes not.  While troubleshooting I removed my fuse box to check and clean the connections and found that the back of the 15 mp fuse slot was melting.  I think what was happening was the plastic  fuse box would heat up / melt allowing the 15 amp fuse clamps to loosen leading to a poor connection. 

I Purchased a new (home built) fuse box from a vender referred to me by one of the members of this site.  The new box works fine, but I have not put enough miles on the bike yet to give it a recommendation.   I also found that I had a poor connection with my ignition switch white plugs.  The plugs are old and no longer held a strong bond with one another.  I safety wired the plugs together which created a better connection and have not had the bike die since.   

[JimS63]

Thanks for all of your help you guys! Looks like I have more investigative work to do...I'm not all that great with electrical stuff....ugh! I love this bike and can't wait to get her running reliably!

[zboy]

had same problem with my '77. replaced fuse block with new style automotive 4 position fuse holder from napa. the forth position makes a good spare holder. clipped wires one at a time and spliced so as to not get mixed up. went ahead and disconected every wire in headlight,one at a time, and cleaned male end and coated with die electric grease from automotive store. did this to all the wires from front to back just so I would know what shape they were in. had just tuned this bike up when original main fuse blew. after putting new fuse holder in tried to start bike and found  not getting fire to #2,3 cylinders
 thought it was a bad coil, bought another off ebay, still no fire! finally found the problem was the "new" honda brand points I bought were just barely sticking open on #2-3 side. took it off and found it was binding on the pivot. took it apart and reamed it out with a 5/32 (4mm) drill, put small amount of die electric grease on it and reset gap and timing and bike fired up and runs great! hope this helps some one out there, kept me up late at night thinking about it!

[JimS63]

I pulled out the fuse block and it is melted on the left side of the 15a fuse. I ordered a "new" fuse block off on eBay, but I haven't had time to work on the bike lately. I also noticed that someone had added/resoldered each contact on the back of the fuse block since the "new" one I just got has a crimped style connection on the back side. I think I'm going to do what you suggested Zboy and pull the headlight out and check/clean every connection I can find and apply some dielectric grease on all the connections. Hopefully that will resolve the issue....

[JimS63]

Well, I took off the headlight and pulled apart and cleaned alot of the connections in there, as well as under the seat. The plug to the fuse block had a lot of corrosion on it so I cleaned that up. I installed the "new" fuse block that I got off eBay (which I noticed also has a small spot of melting under the 15a fuse) and put new fuses in. I turned on the ignition and the front light seems brighter. I fired right up as usual. Since I didn't find any obvious problems, I suppose I just need to get out and test ride it for a long while and see what happens. I'm crossing my fingers that it doesn't continue to blow fuses since there is no obvious problem!