Topic: CB750 Charging Issues: 13V with no lights is not good enough

[banzaibob]

1974 CB750, Cycle X 849cc kit, f8 cam, RC Engineering pipe, Dyna Ignition, stock coils, brand new gel battery, NO performance issues, NO handling issues. 

Couple of weeks ago I took it out for an afternoon ride, about 150 miles at very fast average highway speeds.  I have a headlight on/off switch and ran in the afternoon with it off.  Parked it 25 miles from home for a couple of hours.  By the time I rode home it was dark, headlight on.  About halfway home it started to run like it was out of gas.  Reserve on, no change.  I limped it home and did a voltage check.  Sure enough, voltage at rest was 10.5v. 

I charged it overnight, let the battery rest for an hour and did a charging test with the headlight off.  As the rpm's climbed it revealed a very anemic charge rate.  Holding the rpm's at 4000 consistently, it would start reading at around 12.5v and slowly rise to about 13.2v after a minute.  Turning on the headlight, it would not get above 12.5v. 

I checked all of the components.  Alternator checked out.  Resistance between the legs of the stator was 0.7 ohms.  Field coil resistance, 7.2 ohms.  I hooked up some jumpers to a variable power supply I have and hooked it up to the field coil and energized it.  As I held the rpm's at 4000, I did an AC voltage check between the legs of the stator.  I adjusted the power supply until the AC voltage across each leg was about 75-80 volts.  I then shut it down and measured the DC voltage at the power supply....12.7 volts, I can therefore conclude that the stator and field coil are good. 

I had an old used Regulator that I hooked up.  Sure enough, it appeared to fix the problem and at the battery the voltage was 14v with the light off.  I bolted everything in and let it cool down a bit.  After an hour or so I re-checked and it appears to be back to it's old ways.  It is MARGINALLY better with a final charge of 13.4 volts (anemically climbing and with the headlight off).  But switch on the headlight and I'm lucky to get 12.7 volts.  I even hooked up a different rectifier...no change. 

I tested the headlight circuit, there is no evidence of a parasitic current draw.  All visible contacts are clean. 


Question 1)  Why would the spare regulator work fine one minute and after a rest work sort of like the one that I replaced?  There are no grounding issues with the regulator as it rides on stock rubber shock mounts.  Green wire is grounded,  Black wire is 12 volts, hot. 

Question 2)  I had a solid state regulator/rectifier out of a '79-'82 CB750K at one time but I could not keep it from overcharging batteries.  It would sometimes get as high as 17 volts.  Needless to say I went through a lot of batteries.  My conclusion was that there were internal values in the solid state unit that I could not change.  Is there a solid state unit (different make/model?) that may be better suited to an SOHC CB750?

Question 3)  I can understand the stock regulator getting old.  But the adjustments on those always seemed sort of hit or miss.  Is there a decent way to adjust or test these regulators on the bench? 

[links56]

What is your current draw with all the lights on? It almost sounds like you are using more wattage then the alternator can supply and it is slowly draining your battery because it can't keep it charged. It is normal for the volts to be lower when a load from the lights is present. You have the black wire hooked to a switched hot from the ignition? The regulator takes voltage from the battery and then meters how much to pass through to the field coil so it can adjust the alternator output, if the battery is low it can't put out enough to excite the alternator.

[Grinnin]

You started with a new gel battery and told about charging it in the middle of your tests.  Sounds good so far.

You didn't report the battery resting voltage after charging or how much voltage sag the battery has when starting.  Even if you usually kick-start, the amount of sag is a good indicator of battery health.

A battery can be bad from the factory.  It could look like the spare regulator works one minute and not the next.

When I read your description, the crisis occurs with the lights on, but the problem is there all the time.  True?

[Bodi]

The 750 alternator is the best of the bunch: it should handle a Dyna with no problem. If you add a high wattage headlight or driving lights that may be too much load though.
You need to keep revs up when riding: even with a totally stock bike, city trips usually discharge the battery.
Gel or AGM batteries have a slightly different charge profile and charging/resting voltages compared to a wet cell battery. This should not be enough to matter.
With a fully charged battery try bypassing the regulator: unplug then connect the black and white regulator wires together. This forces maximum possible alternator output. Switch off headlight, switch on ignition and kick start (reduce discharging before it's running). Measure battery voltage at 3000+ rpm. It should be rather high, it may take a minute to rise as the battery accepts charge but should get to at least 16V. Don't leave it doing that for more than a few minutes or the battery will be damaged.
If it does go high, check with headlight on. If it drops you may just have too much load (unlikely). Generally, if this test gives a high voltage then the charging problem is with your regulator.
If the voltage doesn't go high... something else is wrong.
Most likely causes:
#1) harness voltage low because of corroded connectors and switch contacts. Measuring from the battery "+" terminal to the regulator black wire you should read less than one volt with ignition and headlight on, running or not.
#2) poor ground connection. Measuring from battery "-" to any green wire should be less than 0.5V with ignition and headlight on.
#3) poor connection in alternator circuit - the field coil or stator coil wires have bullet connectors etc., when these get corroded the circuit can read OK with an Ohmmeter but it causes reduced output when running.

[Jerry Rxman Griffin aka MuthaF'er]

Check the in and out voltage on whatever ignition switch you are using.

[banzaibob]

Using the advice and wisdom gleaned from this thread, this morning I conducted some tests, etc. 

The battery is a TMS gel type (Orange case).  The voltage at rest from sitting all night was 12.72v.  I then unhooked the voltage regulator and jumped the black wire to the white wire energizing the field coil.  At idle, the voltage read 12.56 volts.  Revving the engine to 4000 rpm and holding it there, the voltage slowly climbed for about 2 minutes until it topped out at about 14 volts (Yes, I had a fan blowing directly on the front of the engine). 

I then continued holding the revs at 4000 rpm and switched on the headlight.  The voltage steadily and slowly dropped until after about 90 seconds it read 12.85 volts.  At idle, it drops off to 12.4 volts and continues to slowly fall.   

The headlight is a stock headlight, stock wattage, probably 40 years old.  Like I said earlier, I cannot find a parasitic current draw in the lighting circuit.  That does not mean it is not there. 

My conclusions at this point:

The charging system does not have a complete failure.  Except for when the headlight is on, it charges fully yet slowly. 

Do headlights draw more current with age?  This seems counter-intuitive. 

I could ride indefinitely in the daytime.  However for the bike to be a good performance/tourer this problem simply has to be positively resolved.   

[links56]

You have other lights on the bike also? It seems evident to me that the headlight is too much load for it.

[Jerry Rxman Griffin aka MuthaF'er]

Get your battery load tested    Seems your system is mostly good.

[banzaibob]

You have other lights on the bike also? It seems evident to me that the headlight is too much load for it.

It is a standard garden variety 50/40 watt hi-low sealed beam bulb.  Unless it is drawing additional current with age I cannot wrap my mind around it causing a problem (bear in mind, I could DEFINITELY be wrong).  This would also apply to all of the other bulbs in the system.  They are standard Honda issue.  This may be opening up an entirely different subject but are they low wattage headlight bulbs out there that offer the same brightness?  Regardless, that doesn't seem like a permanent solution.

[Bodi]

did you measure the voltage at the regulator black wire?

[mystic_1]

14VDC with the regulator bypassed @4000RPM seems kind of low to me.

Perhaps you have some connector terminal oxidation causing resistance in your harness between the alternator and the rectifier? 

Also, I know you swapped the rectifier but have you tested them?  A faulty rectifier could cause a low-voltage situation.

mystic_1

[banzaibob]

Come to think of it....14v with a maxxed out field coil really isn't that good.  This is what I know; when I supplied the field coil with independent 12.7 volts, the AC current across all 3 legs was pretty even at 75-80 volts AC.......therefore alternator is good.  (right?)

As far as cleaning plugs and contacts I have yet to do that.  Quite frankly, I've seen much worse than what I have.  The only way to conclusively rule that out is to go through every junction and plug within the entire charging system.  Sounds like tomorrow morning's project. 

[HondaMan]

Here's what I can tell you from bikes I see around here often:
1. Check the condition of the 3 bullet connectors that connect the alternator to the sub-harness that in turn plugs into the big white 8-contact plug at the top left rear of the engine. Many of these are quite cooked now, and on the pre-1976 bikes the connectors were zinc-plated steel. Today's versions (from vintageconnections.com) are brass and will last forever. The zinc ones are quite past their life and are now shedding their plating, which makes them heat up and conduct poorly when hot.

2. The Dyna S ignition, with the headlight on, are almost max current for the system. It draws about 1.2 amps extra current (with stock OEM coils), so any riding below 4000 RPM will show falling voltage until it reaches the point where the lower voltage produces less coil current, and the system equalizes. One local rider who has a beautiful [Antares Red] 750F1 installed a Dyna S 3 years ago: even in trips around the mountains and back to Denver, it had a dim headlight during the return leg (slow traffic, 40 MPH for 40 miles) of every weekend trip, and the next day the electric start would not work when he rode to work. After riding this way for 2 years, he installed a Transistor Ignition instead (all else being equal), and the bike works great again.

So...at the very least, I'd suggest doing these 3 things: 1. Check/clean/replace the 3 bullet connectors for the alternator, 2. remove and clean the engine's main ground (which must be trapped between the frame - bare- and the left top rear engine bolster, and 3. disconnect the main Green ground and coil mounts, clean that bare spot on the frame, clean all and reassemble.

[mystic_1]

I agree, it sounds like your alternator itself is working properly, 70-80VAC going to the rectifier seems pretty healthy.  I assume that you tested that at the block connector coming from the stator coil, you could also test the AC voltage at the block connector for the rectifier to see if you're getting any voltage drop within the harness. 

Bodi also recommended some good tests to detect voltage drop in other parts of the harness.  All it takes is one bad connection along the way to throw things off.

IMHO cleaning up all the contacts would be a good time investment even if it doesn't prove to be the cause of your problem.

mystic_1

[links56]

I knew I had seen the breakdown for wattage draw on a stock system on a few posts here so I was looking and found one, it just so happens to be your post from a couple years ago.

http://forums.sohc4.net/index.php?topic=125271.0

This was posted by forum member TwoTired:

Unless you've added electrical load to the bike, bigger headlight, dyna ignition, low ohm coils, etc., do resist using a 20A fuse.

Here is one version of the 750 budget I found.  It was in word, so the formatting is gone with the conversion to text.

Stock CB 750 electrical budget.
Alternator output - 210 watts at 5000 RPM

50  W -High beam   Headlight
3W -High beam indicator
7W    -tail light
23W    -Stop light
14 W  - 7 W x 2 Front run lights
12W     - 3W X 4  Instrument lights
49W   -23 W X2 turn signals plus 3 w indicator
24W -maximum for ignition  (when points closed)
20.4W – Alternator field coil (whenever battery is at low state of charge)
202.4W -total  maximum drawn from battery

600W  -Starting motor

Stock CB 750 electrical budget   /  minimum lighting
3.3 A  / 40 W -low beam Headlight
0.58A  / 7W    -tail light
1.91A  / 23W  -Stop light  (brakes applied)
1.17A  / 14 W  -7W   x 2 Front run lights
1.0 A  / 12W   - 3W X 4 Instrument lights
1.2 A  / 15W  - ignition estimate  (while idling, using points)
1.7A   / 20.4W – Alternator field coil (While idling)
0.0A   / 0 W / Turn sigs?
__________ Dyna ignition module ?
 9.78A  /117.4W -total  (normally drawn from system)
  0.05A /2.5W  - wiring, connector, and switch contact resistance losses
(Assumes 0.5 v voltage loss in wiring at 10 A draw)

approx.  1/3 of peak output is available at idle  5.42 A (or 70 watt) delivery from alternator at idle.

Peak Alternator output – 17.5A  (210 watts at 5000 RPM)

Seeing as how you have added the dyna ignition which draws even more current you are really riding at the edge of your maximum wattage output. Unless you are constantly running the bike at 5000 plus rpms you will be discharging the battery when you have everything on. You could replace some of the stock bulbs with LED ones to reduce some of the load to offset the extra draw for the dyna ignition.

[banzaibob]

By the way, that bike being discussed in that thread was my son's bike.  The culprit was eventually found to be a very small amount of insulation rubbed off on the turn signal circuit. 

[links56]

I was just posting it for the stock wattage breakdown.

[banzaibob]

Current draw with ignition switch on, headlight on, Dyna/ignition on and engine NOT running is about 9.6 amps.  Switching off the headlight, 6.5 amps.  Switching off the ignition/kill switch, 2.8 amps.  Headlight/tail light/instrument lights is drawing about 3 amps and Dyna Ignition is drawing about 3.8 amps. 

[banzaibob]

Here is where I am at:  Since it appears that there is no major component that is misbehaving I figured that it is voltage/current loss because of the connections, plugs, etc. 

I went through every connection in the charging system and cleaned the plugs by first soaking them in distilled vinegar, vigorous brushing, contact cleaner and compressed air.  I also replaced the 8 pin plug from the engine to the harness.  It had some cracks and obvious thermal damage.  I also cleaned the contacts in the regulator. 

Upon firing it up and after warm up, holding it at 4000 rpm, it quickly shot up to a healthy 14.6 volts.  I turned on the headlight and it just as quickly dropped  to 13.7 volts.  I waited for things to cool a little and tried duplicating my results after everything was buttoned up and bolted back together.  Battery at rest, 12.68 volts.  Turning on the ignition key w/o it running, 12.23 volts.  Right after starting with no revving of the engine it read 12.5 volts.  Bringing it up to 4000 rpm again and turning on the headlight produced the same results; 14.56 volts, no headlight, 13.6 volts with. 

I think we might be done here fellas.  Many thanks for all of your insights and help!!