Topic: Cb750 charging problems

[Jonm99]

Hi guys,

My charging system all of sudden died on me. I've gone through the diagnostic steps on the forum, and here are my results so far:

0.5 oms (after accounting for probe resistance) on all combinations of the stator wires coming out of the stator.
No continuity BTW yellow wires and ground.
Same test with same results performed on the bullet connectors behind the shifter cover.
Field coil measured at 7 oms (7.2 before accounting for resistance on probes) BTW green and white wires coming out of the stator
Battery fully charged - when switch is off 12.8v (was 13+ before I started tinkering), switch on 12.35v (headlight on)
Engine at idle: 12.35v
2k,3k,5k rpm =12.35v (no signs of charging)
AC output test (headlight on) ~60v on all three yellow wires
Took a look at the stator and rotor, didn't see any noticeable damage, melting, broken magnets, etc...

So I'm thinking r/r or a wiring issue.

As for r/r, it's a ss Rick's r/r combo unit and it fails the reverse diode test (I'm getting a consistent reading on all three yellows, but no beep). This would seem to indicate a bad r/r but its got less than 30 miles on it, so even if it is a dud, I want to be sure I'm covering the issue and not just the symptom.

One last thing, alternator is magnetizing when switch is on...

So before ordering a new unit, is there anything else I can test to be sure about thr r/r (as the symptom or the source of my charging problem)?

Thanks!
J

[bryanj]

The only other test is disconnect just the yellow wires and test for AC voltage across each of the three pairs of yellow wires coming from stator with engine running. Should be 20 to 50 volt AC, if so rectifier suspect.
A bad ground or even a split second of reverse voltage will blow a silicone rectifier

[Bodi]

The other test is connecting the black and white wires at the regulator/rectifier together (disconnected from regulator/rectifier). This will fully power the field coil and the alternator will give 100% output. With engine running you should get charging - battery voltage rising (rev the motor to around 3K). This connection will severely overcharge the battery if you leave it there for riding but it's OK for testing.
Charging = rectifier good (at least partly), stator coil wiring or regulator bad.
No charging = wiring or rectifier bad, regulator may be good but that's not confirmed.

[Spanner 1]

On many of these old bikes a no charging issue has been caused by the 50+ yr. old 8-way alternator connector being burnt/corroded/ loose. I have seen this several times and all the charging components turned out to be fine. That's always my first go to, disconnect that plug and examine it closely ( yel-yel-yel-white-green-bl/r-lg/r ). Good luck !

[Jonm99]

Body - I'll give it a go tomorrow and report back.

Spanner - I know this reads like a silly question, but aside from a visual examination of the block connector, how do I test its integrity? Continuity with one probe on the reverse side of one block and another on the reverse side of the opposite block (when the blocks are connected)?

Thanks,
J

[Jonm99]

I may have found the culprit....

I first did Body's test, and as I still was not getting a charge, that raised my suspicion about Spanner's recommendation to go through the connectors. The red/white power line going into the 2 prong block in between the relay and alternator was somehow broken (i.e no connection). I'll fix or replace this and check charging again.

Thanks guys!
J

[bryanj]

You may find that with nowhere for the power to go the rectifier has failed

[Jonm99]

OK, so that connector was definitely the (a?) source of the problem as the system is now charging. However (there's always a however, no?), I think Bryan is right in that this may have killed the regulator... I'm seeing voltages of over 15.9 when reving the bike (hard to correlate voltage to rpm as it was quite jumpy). I'll install the original regulator and rectifier and see if that resolves the issue.

In the meantime, I measured the voltage between the black wire that goes into the r/r and the battery and see two things:
1. When disconnecting the r/r before measuring, I get 11.6v on the wire vs 12.35 at the battery (switch open, hl on)
2. When measuring the black wire without disconnecting from the r/r, it's even lower, 11.3v vs 12.35v at the battery.

For good measure, I redid the reverse diode test on the regulator, it definitely failing (consistent readings across yellows with black probe connected to red wire, but no beep). I suppose resistance is too high?

To conclude, this could be 3 separate issues...
1. Burnt connector (fixed in the meantime)
2. Failed regulator (will test by swapping with my original r/r
3. Wiring issue - what am I looking for with regard to the loss of voltage going into the regulator?

Thanks again,
J

[bryanj]

 The voltage on the black at reg is too low hence the overcharge voltage.
The black at reg needs to be within o.5v of battery voltage with lights off and kill switch off(no power drain) if more than that suspect bad comtacts at
Fuse
Ingmition switch in
Ignition switch out
Multi black connector in headlamp

[Jonm99]

Swapped out thr r/r for the original regulator and rectifier (separate units) and I'm getting similar results. I'm fairly sure the original units were working before I replaced them with the new one... On the other hand, both the old and the new are failing diode tests. So either both are bad, or I'm not properly testing them (

As for the discrepancy in voltage BTW battery and black wire going into r/r, on further testing, I see that I'm within 5%  or so BTW the 2 when headlight is off and 1.2v difference (less at the r/r) with the head light is on. Not sure what to make of that and whether or not it's actually related to the overcharging problem.

In a nutshell... I'm at a loss. Should I just bite the bullet here on a new r/r under the assumption that both units are bad?

[Stev-o]

OMP has a very good write up on testing electrical parts and sells good products...

http://www.oregonmotorcycleparts.com/FAQ.html

[Jerry Rxman Griffin aka MuthaF'er]

What is the output voltage at the ignition switch with a fully charged battery?

[bryanj]

With kill switch and lights off should be same as battery

[Jonm99]

12.65v at the battery (switch on, hl off, kill switch off), 12.18v at switch out (black). I will just note that I am disconnecting power to the hl from a relay I installed near the fuse box that mimics the temporary power cut off upon start. Not sure if that matters any.

[Jonm99]

More testing...

I jumped the black switch wire directly to battery positive and it seems to resolve the over charging problem (I don't see charging above 14.8v), so that seems to rule out the r/r as the suspect. However, after going through all the connectors, I now have no drop of voltage at the black switch cable (vs battery poles), but only as long as I'm disconnected from the r/r. The moment I connect the two I get a drop of 0.9v. To be sure its not an issue with the connector, I hooked up an extension wire to the black wire connector (harness side) and tested voltage coming out of it...no drop.

Essentially, the issue is somehow with the regulator drawing from the system as soon as its connected (but not if jumped directly to the battery?). Does that make sense? Any suggestions as to where to go from here?

[bryanj]

Possible high resistance(dirty) connections in ignition switch.
There may be a small volt drop with reg connected as it will be powering the field coil and any current draw will drop battery voltage, bad connectors will exacerbate the problem

[Jonm99]

Well, I think I found the source of the issue, just not sure what it means....

There is no problem on the power switch line, but rather on the white wire that goes from r/r to alternator. If I disconnect it, drop of power on the black line is 0.1v, and if I disconnect ground as well, no drop on the black line. There is only 11.97v coming out of the r/r on the white line with 12.6 or so on the black line (and at the battery posts). I thought maybe the connector coming out of the r/r might be the issue, so I spliced in my multimeter just before the connector, same voltage. I also checked the red power wire going into the r/r and I have 12.6v (makes sense as that is the output on black from r/r). So the drop is happening in the r/r? Is this nevertheless an indication of a r/r gone bad?

[bryanj]

The voltage on the white wire should vary with battery voltage, if battery is low full battery voltage should be on white wire decreasing as battery voltage increases, if this isn't hapining properly the regulator side is either not set up properly from new or is failing

[Jonm99]

Thanks Bryan, makes sense that I would see less than full voltage on white (output) with a full battery. The question is why would that have any impact on the black (input) voltage? In other words, as soon as I connect white, I get a drop in black... I just can't understand why the r/r (seems to) work when input is jumped to battery plus despite the fact that I have no voltage loss on the switch line while I've ruled out the connector (switch line to r/r) by creating a temporary extension (with a new connector) with the same result.

Can resistance someplace in the harness be low enough to not impact the voltage reading I'm getting at the black connector to r/r but high enough that when it has the load of the r/r it drops significantly? Not sure if that even makes any sense...