1980 cb750 electrical mystery. Starter, solenoid and diode.

[epfromnyc]

Fellow cb750-ears. I have been trying to reincarnate a dead 1980 cb 750 supersport. When I first found the bike. I cleaned it up a bit and then slapped a new battery on it to see what would work. Signals worked, headlamp came on. Neutral light and oil light glowed. Checked the solenoid switch and the thirty amp fuse was popped. So I decided to slap an inline fuse on the solenoid switch... Easier to replace a more modern fuse. Right? Well then, at the next start attempt the wires to the diode cooked and smoked! After that I have nothing but nothing good. I pulled the starter off the bike to test it. It spins great when connected to a car battery. But when I do a direct connect to a motorcycle battery. The same starter motor does nothing.  A friend advised that the brushes on the starter may be going and that a car battery delivering more amps is the reason it has life when connected to a car battery. So I have a newly rebuilt starter arrive and i did a direct connect to the charged motorcycle battery and it did not spin or show sign of life. Okay so what I want to know is how much amps is generated when the starter is in the bike in order for it to engage. Considering that the spark units, regulator and diode is involved?  Definitely need to de-mystify this process. I only get a neutral light now. No signals, no headlamp.. No horn.  I see ground to the frame indicated on wiring diagram that just does seem exist on this bike.  Thank for any insights.

[shinyribs]

How are you connecting your starter motor to this motorcycle battery?

It's hard to diagnose something without putting your hands on it,but it seems like the common denominator here is the motorcycle battery and what ever you are using to connect that battery to your starter.

If you are positive that the battery is good I'd investigates the cables. Do they have continuity? I know that sounds like a long shot,but it's all I can think of for now. Good luck and keep us posted.


*edit* I reread your post looking for something I might have missed.Well,I totally missed the part about your ground. Make sure that your negative post of your battery is making a direct,solid connection to your engine case. The starter need this (it grounds through it's casing/attachment points) and so do your spark plugs to operate correctly. A poor chassis/frame ground will affect lights and such.

[motogrady]

Thats a dead short somewhere.
A component is fried, or wires are touching together somewhere.

I'd look reall good at that cycle battery first.
If a 12 v car battery is doing it,
the bike battery should.
Charge that thing real good,
get a cheap meter and see if it's holding 12 volts.
If not, bad battery.  Return for new.

Maybe try this, unplug all the electric connectors
under the side plates.  
New fuses everywhere.
Key on, plug in connectors one at a time,
checking what they supply or work.
If you pop a fuse when a connector is plugged in, look at every wire in that part of the harness,
or test any componet in that section with a meter.
Don't mess around with the key on, you don't want to give anything time to really fry.

Good luck dude.

[motogrady]

Turn it Up.......

[Lucky 1]

To diagnose any electrical problems you must have a solid source of 12 volts.

Every motorcycle needs a ground to the frame from the battery,
 and a ground from the engine to the frame.


Check all fuses.

Perform a good visual inspection of all connectors.
Un plug them, and look closely at them.

[epfromnyc]

Hey Shinyribs and all. Yep, rule one is make sure battery is a solid source. So yes thats the first thing. New Battery and volt meter confirms as good source. Battery trickle charger unit is a smart one too. It can sense if battery is bad and will display an indicator light to replace the battery if bad. So far its all good.  Still there is nothing like a volt meter test to confirm. Ground cable to battery has continuity... but will do more cleaning up around the washer end which connects to the casing and engine mounting bolt. Battery is YTX14AHL-BS. I will pull the battery off my 1983 kz440 ltd and try the starter motor direct voltage test again. Surprised me when both starters did nothing when connected to the YTX14AHL-BS.

I replaced the oem solenoid switch with an after market cheapy. Comes the standard type car fuse right in the top of the darn thing. But i notice that the four prong female reciever that leads to the rectifier did not like the male 4 prong tabs on the new solenoid switch. The plastic molding around the new solenoid switch did not receive it well. Looks like i have to pull the female wires out of the plastic casing and insert them one by one into the after market solenoid. But i figured that i would deal with that part later.

I redid the fuse box on this bike cause they can be alittle flaky. Regulator is new. But the wires leading to the diode got cooked, so i replaced the diode with a used one... but since the diode is not new, I will have to question it and test.

Spark units are not new. Coils are not new.

[Lincoln Highway Bullitt]

I have that solenoid. I fried my diode by hooking up the wires the same way I took them out of the plug to the new solenoid. I forget the pattern it was now that worked.

[epfromnyc]

Aha! Since you have the same solenoid switch could you take a pic of how you connected it and share so that I  Match your pattern? The cooked wires sure adds a layer of frustration.

[shinyribs]

Bingo! Sounds like you're on the right path now. Good catch Lincoln.

[epfromnyc]

motorgrady: Thanks for the Godzilla Vid! Enjoyed that one. Now if only there was a Gamera (giant turtle) one.

[Lincoln Highway Bullitt]

I am not going to be able to get a pic to you for awhile. Sorry. It wasn't that hard to figure out. I even reused the plug. You just have to trim of the little plastic nip that the clip attached to.

[epfromnyc]

Lincoln: No worries. I did that immediately after the first try. Had to shave the plastic lock latch down so that it would slide into the new solenoid switch. Also, reverse the plug direction. Anyway just wanted to make sure that i didnt miss anything. So I think its right. I have other electrical issues for this bike so the new after market solenoid switch added more questions to the mix. Thanks again.

[epfromnyc]

 

The beast lives! Okay thanks for all the advice. So first and foremost... a VOLT METER is a rider's best friend.  As it turns out, the new battery purchased about 3 months ago was one of the problems. The battery would charge up just enough to fool the SMART trickle charger. Funny how tools nowadays are suppose to be Smarter? So the battery charged up and would be enough to display a GREEN light on the trickle charger. Good battery right? WRONG.  The battery would discharge slowly.  Okay next problem... BRAND NEW SOLENOID Switch but it was a dud. How about that! New solenoid switch... and it was damaged. So you take for granted that the new parts you buy are good. Uh, so... Back to the basics. Use the dumber good ole volt meter!  Check Ground and Continuity. Oh yeah and a dirty run/stop switch. Slowly but surely... the parts I thought were new and good became the issue. Go figure. Replaced them and Bingo.... The Beast lives again. This bike had not been started in about 10 years. I didnt know if the issues were deeper... so I really had to go through it because of second guessing. Hope you like the pic. Didnt have time for spit n polish. I will make her shiny later. Just need to ride!

[motogrady]

man, that a nice looking bike.