Topic: 1976 CB750F charging/headlight issues

[operasinger750]

I'm having trouble keeping a charge with normal riding on my '76 CB750F.  I can ride the bike to and from work for a couple of days, but after that it's dicey.  I've tried to be very conscious of engine rpms-I try to keep the bike revved as much as possible, but I still get the same result.  I've been charging most every night to keep it charged, but I'm also noticing fairly significant loss of performance as the battery goes from fully charged to not-so-fully charged.  The bike has an elctronic ignition and an H4 headlight conversion (I think-added by PO).  I suspect the headlight.

To try to get to the bottom of this, first I adjusted the regulator points gaps to within spec (this helped a bit with charging voltage). Then I went to do some multimeter testing: With the headlight fuse pulled, I'm getting great voltage to the battery (over 12V at idle and over 15V at 4K rpm), but when the headlight is on as in normal driving, I'm not even getting over 13V at 4K rpm at either high or low beam.  The bulb is a 60/55 watt bulb.

Help!  Where do I start?  I thought maybe I should go to a smaller h4 bulb, but I don't really want to lose any headlight power.  On the other hand, the bike should be charging!  I've read a bunch of threads on this, but didn't find what I needed, exactly.  The issue seems to be with this conversion, which I don't really understand.  Thanks, all.

[operasinger750]

I don't know the first thing about the stator.  Is there a way to test it?

I forgot to mention that the headlight is pulling about 4 to 4.5 amps from the fusebox.

[david 750f]

Start with the basics.

What condition is the battery in? What voltage do you get at the battery after charging? (and after the battery sits overnight).

Check the voltage at the headlight with the key on but the bike not running. Compare that to battery voltage. You may be getting a voltage drop due to dirty grounds, connectors etc.

[Steve F]

Have you checked the electrolyte level in the battery?  It MUST be over the plates to charge right.

[Markcb750]

Sure looks like a bad battery to me, at those voltages you should be fine. 

The extra draw from the headlight is not good but with it holding 13V with the light on it should be charging, albeit slowly if you keep the rpm up.

I would ditch the light, but I think the battery is bad. (I assume you keep the level up...)

[Spanner 1]

Have a careful look at the 6-way connector to the alt. ( yel/yel/yel/white /green ) also a bad battery main ground could cause this. Ever cleaned all your connections on your 41 yr. old bike ?
If a clean-up fixes the charging problem, don't forget to adj. the reg. back to reduce peak charging voltage.... good luck.

[operasinger750]

Thanks for the ideas, all.  I neglected to mention that the battery is essentially new.  I put it in in October, shortly after I bought the bike.  It's had this type of charging issue since I bought it.  I think I'll start this afternoon by cleaning as many connectors as I can find (especially the headlight connectors), then I'll recheck my charging voltage and see where it stand at that point.  I guess emory cloth is the best to use for this purpose, rather than sandpaper.  Where would a guy find that?

The issue I perceive with the two voltages-one with the headlight fuse pulled, the other with the headlight on-is that someone has cranked the regulator up to compensate for all the draw through probably corroded connections, such that when I eliminate the headlight draw, the voltage is a bit too high.  I'm guessing I'll turn the voltage reg down after I do some cleaning....

As far as running 13V at 4K rpm, I sense that without 14.4 going to the battery and spark plugs, I'm not going to get peak performance with my ignition.  This bugs me more than plugging the thing in every night.   Especially with this thing running aftermarket exhaust and electronic ignition, I want power baby! It runs its best right after a full charge.  The trip home is usually somewhat dimished in peformance.

Anyway, I'll spend a couple of hours and clean some connections and see what my readings are afterwards.  I'll try to decipher Rotortiller's uploaded diagram and check the alternator too.  Such a Newb here...  Thanks for the help.

[TwoTired]

When new, the bike had a 210 Watt alternator.  (At idle speed it's about 1/4-1/3 of full output)
When new, the bike drew about 120-130 watts when the key switch was on.
This left about 80 watts for battery charging when the engine was revved.  (New condition)

But, things have changed.
Stock ignition draws about 35 watts.
Stock coils and Dyna ignition draws about 56 watts.
3 ohm coils and Dyna ignition draws about 93 watts.
Depending on what coils you have, the ignition change over could have effectively removed 58 watts from your charging system.

Stock headlight was a 50-40 Watt unit.
Your headlight is a 65-55 watt unit. Removing, at least another 15 Watts from the charging system.

These two (or three) changes combined, would remove 73 watts from the original 80 watts available to charge the battery.
Not really surprising that there could be "trouble keeping the battery charged.

But, your bike is no longer new.  Switches, connectors, fuse holders etc. have all oxidized where there was once pristine electrical contact.  Oxidation means resistance, and power lost at those points.  Consider, if 100 connections increase just .1 ohms, that 10 ohms or about 16 Watts wasted as heat.  In effect, you can remove that from the charging system output capability as well.
Don't forget that switch contacts can also degrade.

The voltage regulator does not regulate system voltage directly.  It's real function is to never allow the battery to be overcharged.  It is the battery status that dominates the system voltage.  Mucking with the Vreg adjustments won't help "bring up" the system voltage.  But, it does risk overcharging the battery if it can't do it's job of limiting battery voltage to 14.5 V maximum.  Be aware, that the alternator itself limits the max output it can make by design.  So, you can't ever make a stock one put out 300-400 Watts.

Then there is the charging system health to consider.  If any of the six diodes in the rectifier are blown, the alternator effective output will be reduced at all RPMs.  Test these with an ohmmeter.  They should have low resistance in one probe polarity only.

The alternator stator rarely goes bad without some sort of physical trauma.  The resistance between yellows is very low.  The resistance between yellows and the engine case must be very very high.

The field coil resistance White to green S/B about 6.8 ohms.

Ask if you need more assistance.

Cheers,

[operasinger750]

Thanks for the regulator clarification, TT.  Spanner, I will be sure to turn the regulator down before I do any riding to avoid overcharging.  I do need help with the electrical procedures.  If anyone else can shed some light on some testing for a newb, I'd appreciate it.  Do I run the alternator test and other tests with the bike running (wow-this opera singer is really dumb)?  Sorry guys, I need it spelled out a little bit.

I would like to put a smaller headlamp in the bike, but I like the price of my H4 replacement bulbs.  The only ones I can find smaller than the 60/55 I have are 35/30.  Is that going to cut it?  I'm guessing that I should try to resolve other things first.

Battery voltage is 12.6 when fully charged.  I don't know if my coils are original.  I'll look into that.  I'm hitting those connectors (starting with battery, then everything else) this afternoon.  Is emory cloth the way to go?  Autozone has that, I assume.  In the meantime, I will also check the voltage at the headlight.   Thanks, all.

[flybox1]

35W HID kits for your H4 housing, are readily available for under $40, and are far brighter.

Please be advised that HID's in MC headlights are really F'n bright to oncoming traffic   adjust your beam accordingly. 

[operasinger750]

Thanks for that info.  I had been reading a lot about HID kits, but I assumed they were way more expensive.  I'll look into that.  I'll do my best not to blind anyone-that seems to be an issue, especially when adding HID to a halogen housing.  Do you have any recommendations as to which kit?

[operasinger750]

Hello all.  Today I did some cleaning and checking.  I cleaned any contacts visible at the headlight and I cleaned the contacts behind the left side cover as best I could.  I tested the battery voltage with the bike off: 12.6V.  Check.  Then I checked the voltage to the headlight: 11V when the machine is off.  Perhaps a little low, even after my cleaning.  I still don't seem to have any problem getting voltage to the battery when I'm running the bike at 4K rpm with the headlight off. Since I'm a real numbskull with doing electrical tests, I didn't have the faintest clue as to how to start testing the alternator or rectifier even with all the help here.  I'm debating just throwing in a smaller H4 bulb (I found a 35/30 halogen) and being done with the whole thing.  Any guidance is appreciated.

[Jerry Rxman Griffin aka MuthaF'er]

Just a thought - check the voltage coming out of your ignition switch and see what it's letting through. That is a 2 year only switch used on the 75/76 750F and a couple other models. They changed it fairly early for whatever reason. The main change was the electrical connector/switch end. We just cut off a faulty switch (different style) on a 550 on our ride and twisted the black and red wires together to hotwire it until it can be changed. The unintended result was a much brighter headlight.

[TwoTired]

ok. So it looks like you are able measure voltage.
Find the voltage drop between battery and headlight connector.
Put one voltmeter probe on the Battery POS terminal and the other voltmeter probe on the the blue or white headlight connection.
The reading will tell you how much voltage is being lost in transit.

Do another test with headlight on with probes on the Battery NEG terminal and the headlight Green terminal. 
Report the results.

Tell us the brand and model number of your meter.  Just like bikes, they are all a little different among brands.  Or, post a picture of the one you have, so we can read the markings on the front panel.

Have you checked the electrical FAQ yet?

You could use scotch brite pads to clean electrical connections.

Cheers,

[operasinger750]

Thanks, Jerry.  By ignition, do you mean where I put the key?  It looks like I have a replacement key fyi.  Can you treat me like a real dummy-where do I touch the contacts on the multimeter?  Many of the posts here went right over my head!  Thanks!

TT-Thanks for the clarification!  I will do both tests as you described.  It will have to be tomorrow since it's dark and pouring here in Chicago.  I have an Innova multimeter from Autozone-$15 digital type.  I've attached a picture.  Hopefully it loads.  I will re-visit the Electrics FAQ.  I tracked down some Emory cloth from Menards.  I think I have a scotch brite pad around here somewhere. 

[TwoTired]

Nice picture.  Can't read the markings though.  Guess I'll have to search for a better one when I have the time.

Cheers,

[operasinger750]

Sorry-It's got DC volts, Ohms, AC volts, DC amps and a diode test functions.

[TwoTired]

Unplug your rectifier.  Do 12 tests on the rectifier's plug.
Green to each yellow, reverse the probe contacts and repeat.  Record six readings.
Red to each Yellow, reverse the probe contacts and repeat. Record six more readings.
Report the readings.

Set meter to 200Ω scale, disconnect the White and Green wire from the V reg.  Measure across the white and Green wires.
Report the readings.

You still need to do the Vdrop to headlight test.

Cheers,

[operasinger750]

OK.  Break in the rain.  I went to do the tests and rubbed the two brain cells I have that possess my electrical knowledge together and I have some numbers.
I did this:
"Put one voltmeter probe on the Battery POS terminal and the other voltmeter probe on the the blue or white headlight connection.
The reading will tell you how much voltage is being lost in transit. "
My result: 12.5V (same as battery at the moment)

"Do another test with headlight on with probes on the Battery NEG terminal and the headlight Green terminal.
Report the results."
My result .03V
(does that sound right?)

Then I unplugged the rectifier and did the 12 tests:
green to each yellow:
-.13, -.07, -.18
reversed multimeter probes and got the opposite: .13, .07, .18

then I checked red to each yellow:
.00, .04, -.04
reversed multimeter probes: .00, -.03, .04

Finally, I did this:
Set meter to 200Ω scale, disconnect the White and Green wire from the V reg.  Measure across the white and Green wires.
My result: 8.0 ohms

Thanks for your patience with this.

[TwoTired]

OK.  Break in the rain.  I went to do the tests and rubbed the two brain cells I have that possess my electrical knowledge together and I have some numbers.
I did this:
"Put one voltmeter probe on the Battery POS terminal and the other voltmeter probe on the the blue or white headlight connection.
The reading will tell you how much voltage is being lost in transit. "
My result: 12.5V (same as battery at the moment)

This was with the headlight operating?

"Do another test with headlight on with probes on the Battery NEG terminal and the headlight Green terminal.
Report the results."
My result .03V
(does that sound right?)

This was with the headlight operating?

Then I unplugged the rectifier and did the 12 tests:
green to each yellow:
-.13, -.07, -.18
reversed multimeter probes and got the opposite: .13, .07, .18

then I checked red to each yellow:
.00, .04, -.04
reversed multimeter probes: .00, -.03, .04

Did you use the diode test function on the multimeter?
Have you read the manual that comes with the multimeter?

Finally, I did this:
Set meter to 200Ω scale, disconnect the White and Green wire from the V reg.  Measure across the white and Green wires.
My result: 8.0 ohms

That's a little high.  You may need to clean connectors in the path to the field coil.

Cheers,

[operasinger750]

OK, take two on the multimeter testing as described by TwoTired:
"Put one voltmeter probe on the Battery POS terminal and the other voltmeter probe on the the blue or white headlight connection.
The reading will tell you how much voltage is being lost in transit. "
Battery voltage at the time of test is 12.35V
I took one reading with the light plugged in and lit: 11.46V
...and one reading with the light pulled out but the circuit on: 11.77V

"Do another test with headlight on with probes on the Battery NEG terminal and the headlight Green terminal.
Report the results."
I did this similarly...
with bulb in and lit: .12V
with the bulb out but switch on: .03V

Then I unplugged the rectifier and did the 12 tests:
green to each yellow:
517, 514, 520
I got essentially no reading with the probes reversed

then I checked red to each yellow:
507, 510, 498
The display did not change with the probes reversed it read "1" with or without the probes touching the contacts.

I hope I mangled this test less than the first one.

Thanks all!

[TwoTired]

Looks like your rectifier is proven good.  Hooray!

You are losing a whole volt of potential between battery and headlight.  The POS power path is losing more than the ground return path to the NEG terminal.

So, take a look at the wire diagram for you bike and trace the path from the headlight to the POS terminal.  Some or all of the connections, switches, or fuses terminals are making resistance.  Kinda your choice now.  You can go after the worst offenders by putting voltmeter probes on either side of the devices in the path.  Or, can can simply clean and tighten each one to eliminate or greatly reduce the voltage loss along the path.  Your headlight will get brighter, by the way.

I'll take another guess that you will find the key switch contacts nasty.  Some can be rebuilt cleaned and restored.  Some get melted inside and have to be replaced.

Further I can probably infer that the alternator is being excited 11.5V instead of true battery voltage, so it can't be all that it can be when fed weaker voltage.

Ever figure out the coils you have?

Cheers,

[Don R]

I read a report that the Hondaman replacement fuse panel has helped reduce voltage loss to the headlight. The fuse panel connections have been known to corrode or loosen. Just another idea. Maybe test volts at the in and out of the fuse panel? Or disconnect and check resistance. An easy check is feel to see it the fuse block is warm when under load.

[operasinger750]

Don-a few weeks back, I started from scratch with my fuse box.  I was blowing main fuses, so I ended up re-soldering the fuse box connections and cleaning all of the contacts.  No blown fuses since then. This was all before I did my check this week.  I just bought the multimeter so I'll go back now and check for resistance as you described.  Thanks!

TT-Thanks for your help.  I'm not going to be able to get to the bike now until Monday at the earliest, so I thought I would summarize my plan of attack: Check the key switch for corrosion-clean if possible, Consult the diagram for headlight and clean contacts along the path-especially on the positive side, and finally check the type of coil.

I'll post again when I have some results next week. 

[TwoTired]

His meter isn't capable of measuring more than 10 Amps.

The 750 Charging system is capable of more than that. (14-16A)
The stock bike draws 10-11A, without the Dyna or headlight "upgrade" current suckers.