Current (Amps) Question

[gbw]

I'm troubleshooting the charging system on my 77 GS750B.

Everything tests good except I never get above 12.8VDC across the battery at 5k rpm. So Something is going on, but the R/R and Stator test good.

When I test current draw (engine NOT running) I get the following:

With ignition off: 0A
*So there is no current being drawn with ignition off

With ignition on and headlight off: 5A

With ignition on and headlight on low beam: 8A

With ignition on and headlight on high beam: 10A

So my question: Should the bike, when basically just powering the coils, idiot lights, and gear position indicator, draw 5A?

My guess is that 5A is about right. If the coils are around 3 Ohms, with about 12V going to them, you would expect at least 4A. Ohm's law says V=IR, so I=12V/3 Ohms=4A.

Am I thinking of this correctly or is 5A out of line for current draw without the headlight on?

Thanks,
Greg

[tom203]

Mr. Ohm would be happy! Coils are powered alternately, so 4 amp seems good. But just getting 12.8 volts at battery at high rpm is borderline- it takes about 13.2 volts from charging system to overcome the internal resistance of a 12 volt battery and push juice back at a good clip. Have you cleaned the connections from your regulator to the battery and the grounds? Have you upgraded to a combo R/R from the stock seperate units?

[gbw]

Thanks Tom. Yeah, I've done all that stuff. I'll start a separate thread on what I've done and my data from the Stator Papers. Just wanted to check my assumptions about the current draw.

[duaneage]

Mine pulls 9.8 Amps, the main fuse is 15 amps on mine, so you're right in the ballpark.

[salty_monk]

Are you sure you don't have a deal leg in the stator? My 750 produced similar results when it did....

Also if yours sends one leg of the stator up to the headlight make sure you disconnect that & take that loop out of the circuit.

If you need parts for testing I have a "known good" RR from a 80 1000G that I can send you. You could also pick up a kawasaki stator for about $15 on Ebay (I have one of those on the shelf that bench tests good that you could have for test purposes too if necessary).

I have heard of charging parts testing good when cold only to fail when hot...

Dan

[Killer2600]

Mr. Ohm would be happy! Coils are powered alternately, so 4 amp seems good. But just getting 12.8 volts at battery at high rpm is borderline- it takes about 13.2 volts from charging system to overcome the internal resistance of a 12 volt battery and push juice back at a good clip. Have you cleaned the connections from your regulator to the battery and the grounds? Have you upgraded to a combo R/R from the stock seperate units?

Actually the coils are powered simutaneously, they fire alternately.

[rustybronco]

But no current is flowing when one set of points is open.

[Steve]

When I test current draw (engine NOT running) I get the following:

With ignition off: 0A
*So there is no current being drawn with ignition off

Sounds good, so far.

With ignition on and headlight off: 5A

I would have suspected just a bit more, but am not home to measure mine for comparison.

With ignition on and headlight on low beam: 8A

Unusual to have a headlight that only draws 3 amps. Should be closer to 4.5 or so (55/12).

With ignition on and headlight on high beam: 10A

This is better, I expect about 5 amps (65/12).

So my question: Should the bike, when basically just powering the coils, idiot lights, and gear position indicator, draw 5A?

Idiot lights and gear indicator are negligible, I would still expect more from TWO coils.

My guess is that 5A is about right. If the coils are around 3 Ohms, with about 12V going to them, you would expect at least 4A. Ohm's law says V=IR, so I=12V/3 Ohms=4A.

OK, you are about half right.
Each coil will draw about 4 amps, for most of the time, both of them will be conducting current, so I would expect closer to 8 amps.

Posplayr might have a better idea, or at least the equipment, to see how long the coil current is shut off when it's time to fire the plugs, but I don't think it's very long, meaning that for most of the time, both coils should be drawing current.

Mr. Ohm would be happy! Coils are powered alternately, so 4 amp seems good.

I would have to see actual duty cycles for the coils, but I really suspect that both of them are powered most of the time. They are interrupted alternately, but for only a very small percentage of the time that they are powered.

[salty_monk]

I have a spare Mosfet RR if you want to try it...

[tom203]

Actually the coils are powered simutaneously, they fire alternately.

Being an inductor, the coil resists a change in current, so it doesn't draw 4 amps instantly, or fall off from 4 amps when the points are open. In the steady state condition with points always closed, it draws a steady 4 amps, but with points opening and closing, life is different.

[Steve]

Being an inductor, the coil resists a change in current, so it doesn't draw 4 amps instantly, or fall off from 4 amps when the points are open. In the steady state condition with points always closed, it draws a steady 4 amps, but with points opening and closing, life is different.

True enough, but it's going to be the avaerage draw that will really matter.

Also, my point was that most of the time, current WILL be flowing steadily through the coils. If your bike has points, current will stop flowing when the points open, then start again when they close. Assuming a dwell angle (open time) of about 60 degrees, the points will be closed for 720-60, or 660 degrees of rotation. This is about 92% of the time. With electronic ignition, the "open time" of the electronic "points" is much less, meaning that current will be flowing steadily even more of the time.

I don't have an oscilloscope handy to be able to see the actual waveform, so will have to leave that to those who do.

Unfortunately, I also don't have a bike available to put my clamp meter around a coil lead to see what it is drawing.


.

[rustybronco]

The Points open and close twice per each revolution of the crankshaft. I.e. two sets of points each opening once per.

[tom203]

True enough, but it's going to be the avaerage draw that will really matter.


I don't have an oscilloscope handy to be able to see the actual waveform, so will have to leave that to those who do.

Unfortunately, I also don't have a bike available to put my clamp meter around a coil lead to see what it is drawing.


.

A clamp meter or simple multimeter would give erratic results unless it was designed for a pulsating (inductive) current load. An oscilloscope would be dandy, but do we really need to know the precise average current? it's less than 10 amps or fuse would blow. On bikes with ignitors, it's still less than 10 amps ( ignitor is soaking up some additional juice). One hopes that suzuki left some margin for fusing- my wild guess is ignition system (ignitor type) draws 6 amps under normal operating conditions. Why 6 amps? the starter solenoid is on same fuse and draws 3 amps when button is pushed- lets hope we all have good fuses!

[Killer2600]

Being an inductor, the coil resists a change in current, so it doesn't draw 4 amps instantly, or fall off from 4 amps when the points are open. In the steady state condition with points always closed, it draws a steady 4 amps, but with points opening and closing, life is different.

Well as I am assuming these are KOEO (key on engine off) current drain off the battery tests. So coil state remains constant and for electronic ignition will almost always charging.

[tom203]

Well as I am assuming these are KOEO (key on engine off) current drain off the battery tests. So coil state remains constant and for electronic ignition will almost always charging.

I'm not sure what you're driving at. I would suspect that " KOEO" would likely mean one coil would be drawing current (getting hot), BUT I have never investigated this- just turning on ignition causes noticeable drop in battery voltage even with headlight off.