Topic: I swear charging issues must be contagious (74 Cb750)

[ianlou]

My sohc died in traffic the other day from a dead battery and I am honestly at a loss. Heres where I'm at, its a 74 CB750 with mostly stock charging system, but reg/rec was upgraded to a solid state unit from 4into1 (this one: https://4into1.com/regulator-rectifier-honda-cb350f-cb400f-cb500-cb550-cb750/) and when I installed that, I also put in a new 8 pin connector because the old one was in less than great shape. Outside of that connector all other connections have been cleaned/checked and look good. So, on to some measurements.


All static tests on the stator were good, running test I got about 64v out of the stator.
As far as voltages:
(These were all taken at the battery)
I get 12.58v key off battery installed
~12.04v key on bike not running
13.5v at idle
Heres where it gets weird. ~14.5 when running at ~3000-4000 rpm, but not consistently. It'll read 14.XX volts for a second, then the reading drops to 2.XX then 4.XX then 10.XX, each of those only for a split second or so, then back up to 14.XX. I put the battery in an 86 Shadow I have, and it didn't do this, I just got a steady reading at just above 14 volts when revving on that bike. I know the Shadow and the CB have fundamentally different charging systems, but I still would say that this eliminates the battery as the problem.

Essentially, I've scoured the forum and I cant figure this out. Is this inconsistent reading just an artifact of the CB's electromagnetic charging system? If so, why did my battery die while I was out the other day. If not, what the heck is causing the issue??

*I should also mention that while I was in traffic when it died, I had just gotten off a highway and I didn't have the bike sitting at idle/low rpm for any extended period of time

[TwoTired]

First guess is wiring intermittents, ...maybe grounds.

However, you can check the voltage going to the Field coil to see if it is also oscillating voltage in unison to battery voltage changes.

Lead acid battery?

Cheers,

[ianlou]

It could be the ground, I'm getting continuity between the wire right where its coming out of the reg rec and the bolt for the chassis ground, but maybe continuity isnt inherently enough, so I'm going to clean some connections and check some wires to see if I can get better ground contact.

Is checking the voltage to the field coil as simple as positive test lead to the power input of the FC and negative test lead to any ground?

[rotortiller]

If the volt meter is accurate and connected correctly to the battery terminals I'd say you have an intermittent break inside the battery  or poor battery connections. There is almost no way a good battery will drop to 2 volts without smoking something. While I doubt it, maybe take the starter relay out of the circuit as it's about the only wire heavy enough to pull the battery down without melting.

[ianlou]

Rotortiller I know that it isn't the battery because the issue doesn't persist when switching the battery over to my other bike. My assumption is that the battery isn't actually dropping to ~2 volts, but rather that some kind of wonkeyness in the system is causing it to read that falsely. After all, I assume the kind of current required to drop my battery voltage from ~14 to ~2 in a matter of less that a second would basically blow every single fuse I could have on the bike.

[TwoTired]

A voltmeter displays the difference of potential between its probes.  As such it can measure voltage loss in a pathway that shouod lose none by placing the probes between source and destination of a power supply or return path.
Since you are concerned about battery voltage, the posts become one point of reference. The other probe gets placed at destination devices where it reads directly the voltage difference in that path way.
Example, between neg post to any grenn wire destination, or battery POS post to any black wire destination.
You wil note that with key switch off in the latter test, full battery voltage should displayed.  Switch on and pathway losses will be displayed.

Note that device to frame is only half the electrical pathway. Battery to frame is also an important path.

Cheers,