Bosch Charging Light

[hudson]

Hey guys - quick question:

(1971 75/5)

What would most likely be the culprit if the charging bulb on dash does not go out until 2k rpms? Normally this should go out around 1500 rpms or so. When rpms are 3000-3500, battery is charging at 13.5v. 4K + it is low 14v.

Running on idle for a few minutes (fans blowing on cylinders), the battery voltage drops to 12v (or 11.9) and seems to level out there, and the light comes back on. Give it throttle and it behaves like mentioned above with light going off.

Thanks,

Hudson

[PITAPan]

Hey guys - quick question:

(1971 75/5)

What would most likely be the culprit if the charging bulb on dash does not go out until 2k rpms? Normally this should go out around 1500 rpms or so. When rpms are 3000-3500, battery is charging at 13.5v. 4K + it is low 14v.

Running on idle for a few minutes (fans blowing on cylinders), the battery voltage drops to 12v (or 11.9) and seems to level out there. Give it throttle and it behaves like mentioned above.

Thanks,

Hudson

The bulb indicates the difference between charging voltage (at the diode board) and battery voltage. When charging rises above battery bulb goes out. See wiring diagram.

Charge light should be out at idle (950-1000rpm). Voltage regulator has energized the rotor, alternator is making power and voltage rises above battery. Light out. With the stock lead acid battery on a SWB this is fast---but it you use one of those new fanglwed batteries, or if you kick started it, the behavior may be different simply because the battery voltage is higher. So higher rpm to get the charging voltage higher, etc.

You need to say where you are measuring battery voltage. You want to measure it off the lead first, then off the connectors. Should be same. Then between the transmission case and the battery lead +. You are checking some connections here. If you measure in the headlight bucket it is normal for it to measure a couple tenths low---but that is where the bulb is.

Use your tach/dwell meter to check your tach. Just for grins and giggles. See how off it is. might be some data you want.

You have a 43 year old motorcycle. Most likely? Bad connections come to mind. Clean them all. Unless recently done it needs doing. Plug to voltage reg., everything on the diode board and alternator. Eyeball up the brushes and slip rings while you are there--Lots of black on the slip rings and short brushes (weak spring pressure) will pass less juice, hurting you. Clean slip rings with a pencil eraser. Replace brushes. Connection of brushes to alternator will not need attention unless green. Check and clean all grounds esp @ the tranny (beware the bolt, east to strip!), etc. . Normally I don't advocate a shotgun approach to anything---except old wiring (I'll think of something else no doubt). What you are really doing is baselining. Gets rid of some gremlins---including ones waiting to bite in the spring. A variety of metal (brass and steel) brushes, including plumbing and gun brushes. I Use an air eraser a lot running baking soda, or other soft media. Totally worth the money but you really have to think about what you are doing if working on the bike itself. Experiment well away from the bike to see how that dust flies.

Check the bulb socket. 12VDC just isn't much for punching through even a thin oxide film.

When you are confident of the connectivity do a shorts check. You could be bleeding some current off somewhere, depressing the charging.

Check the voltage regulator output. It's all about cutting off the juice rather than cutting it on. If it's cutting off at higher RPM it's likely good. It's just passing battery voltage up to it's cutoff voltages, it can't make a higher voltage than is running around the system---but if that voltage isn't getting to the rotor you got a problem.

If the diode board is leaking then the alternator output can be depressed. So you look up how to do the diode board check and run that while you have it in your hands cleaning the contacts on the back.

The alternator is low on the list (actually the diode board is too). Problems here will depress the system output across the board. So you would need much higher RPM to get good output and it sounds like you are OK @ 3000.

Touch your DVM leads together when set to DC volts to check for error.

You want a trickle charger (a good one) anyway. Use this to evaluate the state of the battery.

If the light is on at idle the battery is being drained so the time you put into this one will be well spent. If you were in Upper Mooseglue Minnesota and needed to get home to Florida, you could ride on being careful to keep the light out as much as possible.

Good news: The light is working.

[Kurt in S.A.]

The bulb indicates the difference between charging voltage (at the diode board) and battery voltage. When charging rises above battery bulb goes out.

The first sentence seems right, but sounds like the second sentence contradicts the first. My understanding is that the bulb goes out, ie no current flow, when the voltage on either side is the same. If there is a difference, then current flows and the bulb lights up.

Kurt in S.A.

[PITAPan]

The bulb indicates the difference between charging voltage (at the diode board) and battery voltage. When charging rises above battery bulb goes out.

The first sentence seems right, but sounds like the second sentence contradicts the first. My understanding is that the bulb goes out, ie no current flow, when the voltage on either side is the same. If there is a difference, then current flows and the bulb lights up.

Kurt in S.A.

I don't think so but something non-obvious happening. I'll have to look into it later--need to work on truck while there is some sort of vauge warmth out.

Edit: Yes, to be technically precise you have to say 'equal to or greater than..'.

I don't tend to think of it this way because the equals condition is very transient. I poked around Buchanan (http://www.buchanan1.net/motorcycles.shtml) in the charging system piece for nfo on a dim bulb condition. Thought there might be a condition where the bulb was actually flickering on either side of equal. Nope--localized voltage drop.

I had to check a few things on mechanical regulator operation. It indeed delivers full power, but the 'low' relay sends it in pulses so effectively you less coming out than goes in.

[SteveD]

What would most likely be the culprit if the charging bulb on dash does not go out until 2k rpms? Normally this should go out around 1500 rpms or so. When rpms are 3000-3500, battery is charging at 13.5v. 4K + it is low 14v.

My R100RS did that. Two weeks later the rotor was kaput and the light wouldn't come on at all. I'm guessing assessing the rotor whilst it's "in transit" might be difficult. A new rotor solved the problem of course.

[PITAPan]

What would most likely be the culprit if the charging bulb on dash does not go out until 2k rpms? Normally this should go out around 1500 rpms or so. When rpms are 3000-3500, battery is charging at 13.5v. 4K + it is low 14v.

My R100RS did that. Two weeks later the rotor was kaput and the light wouldn't come on at all. I'm guessing assessing the rotor whilst it's "in transit" might be difficult. A new rotor solved the problem of course.

You would test that by killing the charging system (pull the regulator maybe) and running the bike dead loss in the driveway on the battery. You check for continuity across the rotor brushes as the RPMs rise with a meter. This will reveal an intermittent condition that comes and goes with speed as well as one that happens because it feels like it.

But if the rotor is open at low RPM it should also be open static. So you can just check for that.

Because you are getting charging it isn't on the radar and you don't go looking. But sure wouldn't hurt.

[hudson]

I tested the rotor across slip rings on the current rotor - good, but replaced with a new one which tested good as well. I did pull the regulator and the charging resumed. I replaced the regulator & diode board, put new brushes on as well (although the previous ones did not look to be worn to bad) and checked the snail springs that secure the brushes on the slip rings.

I also tested the stator to ground and the 3 yellow prongs all read the same - in the 6 ohm area if I remember correctly. Seems intact.

But again, 2500 rpms before light goes out. then it charges normally from there. At idle it takes a while for battery to lower to 12v, and then light comes back on until the throttle is punched increasing rpms again. The bulb really doesn't flicker at all - on solid and then off solid. So the system seems to be working properly with the exception of higher rpms to trigger the charging.

Still scratching my head...

[Ken in Oklahoma]

I tested the rotor across slip rings on the current rotor - good, but replaced with a new one which tested good as well. I did pull the regulator and the charging resumed. I replaced the regulator & diode board, put new brushes on as well (although the previous ones did not look to be worn to bad) and checked the snail springs that secure the brushes on the slip rings.

I also tested the stator to ground and the 3 yellow prongs all read the same - in the 6 ohm area if I remember correctly. Seems intact.

But again, 2500 rpms before light goes out. then it charges normally from there. At idle it takes a while for battery to lower to 12v, and then light comes back on until the throttle is punched increasing rpms again. The bulb really doesn't flicker at all - on solid and then off solid. So the system seems to be working properly with the exception of higher rpms to trigger the charging.

Those sure are strange symptoms. I don't have any bright ideas, but I have a couple "food for thought" ideas.

You said you pulled the regulator and the charging resumed. That is a total puzzlement. The regulator "regulates" the current flow to the rotor. Without a regulator in the circuit there should be no charging at all, at any RPM. Have you done the test where you pull the regulator and at the regulator connector you jumper the 12V from the battery directly to the field (meaning the alternator rotor)? That would normally cause the alternator to attempt to run "full bore" at all RPMs. Of course at low RPM's the alternator can't run deliver the current at full bore levels. But by monitoring the voltage AT THE BATTERY you could learn a lot about the health of the charging system without having to concern yourself about the red charging light.

My other thought is sort of similar. You are doing your testing with the motorcycle on the stand and necessarily have to worry about overheating the motor. I assume you're using a digital multimeter to do your testing. My thought would be to extend the meter leads and perhaps tape the DMM to the tank so you can see what's happening charging wise in "real world" riding.

Do I understand correctly that in the process of troubleshooting the problem that you have actually exchanged the voltage regulator, the rotor, and the diode board? And by checking the resistance of the stator windings you are fairly sure it's OK? I too would tend to think so. If the stator really is OK (and there is a chance that it isn't) that would seem to leave only wiring integrity and connections as the possible culprits. That would leave you with the tasks of cleaning all electrical connections, whether they need it or not, and doing what you can to check the integrity of the interconnecting wires. There always is the slight possibility of corrosion under the wire jackets which you can't see. To figure that out you would pretty much have to jump around the wires and see if the problem is still there. And while you're doing the connection checking don't forget any ground connections.

For connection suspects the high current connections would be my prime suspects. That would be the stator connections and the diode board connections, the connections at the battery terminal, and the battery cable connections, including that hard to get at one under the starter motor cover.

Another thought is that while you are doing the various tests you might want to switch the headlight off and on and see if there is a significant variance. The headlight is a significant load. The horn too would be a significant load.

Electrical problem can be real bitches! I'm not reading that you're doing a lot of scatter shooting, but a methodical approach is certainly best. My frame of mind when checking the various connections would be what is supposed to be happening at that connection, and is it happening?

As food for thought goes, mine is a little on the bitter side.


Ken

[Major Softie]

As food for thought goes, mine is a little on the bitter side.
Ken

You say that like it's a bad thing...

[chasbmw]

When you fitted the stator back on the bike, did you remove the springs that bear onto the brushes?

It's very easy to fit them so that they don't apply enough pressure to the brushes and get similar symptoms.