Topic: CB750F Charging Voltage Problem

[TurboD]

Guys I have a 77 CB750F that I recently put together and running from a basket case, so I don't know much as far as history.

It seems to appear as though the charging system is not keeping the battery charged well enough. I have checked the alternator and field coil, both seem to be in spec. I cleaned and checked the regulator. As well as cleaning all the connectors and fuses.

Here is where I am a little lost. With a fully charged battery (12.7v) , I can start the engine and at idle I have the base battery voltage, when I rev the engine to say 4000 and above I only see 13.3- 13.4 volts. My manual is saying I should be in the area of 14.5 volts.

What do I look at to raise the voltage up??

[HondaMan]

Guys I have a 77 CB750F that I recently put together and running from a basket case, so I don't know much as far as history.

It seems to appear as though the charging system is not keeping the battery charged well enough. I have checked the alternator and field coil, both seem to be in spec. I cleaned and checked the regulator. As well as cleaning all the connectors and fuses.

Here is where I am a little lost. With a fully charged battery (12.7v) , I can start the engine and at idle I have the base battery voltage, when I rev the engine to say 4000 and above I only see 13.3- 13.4 volts. My manual is saying I should be in the area of 14.5 volts.

What do I look at to raise the voltage up??

Your manual is wrong, I'm afraid...the highest I have ever seen on the '77-'78 SOHC4 750 is 13.8 volts at 3500 RPM with a Maintenance-free or gel-cell battery, using a stock regulator. And, for good reason...

These bikes have a "voltage limiter" system, a forerunner of the "voltage regulator" systems that came later. "Voltage limiters" do just that: when it senses the voltage rising above 13.8 volts, it shuts off the field coil current. This system was developed, though, for the old-style lead-acid battery: if you use a modern gel-cell or maintenance-free battery, you can touch up the "regulator" (i.e., the Limiter) to let it run a little higher before dropping back the field coil power. This is done by adjusting the armature inside the so-called "regulator unit" to increase the spring pressure a small amount (they have a tiny lockscrew inside the cover) against the upper contact. You'll see how it works when you look inside: the little moving arm (armature contact) rests against the upper contact for 100% charge (about 13 amps). When the voltage rises to about 13.4-13.5 volts, it pulls the moving contact in between the upper and lower contacts: this is the limited-charge mode that then 'limits' the field coil's current through the resistor on the back of the unit. This action decreases the charge rate to about 8 amps. If the voltage reaches above 13.8-13.9 volts, the armature pulls all the way down and shorts out the field coil current altogether, so as to not overcharge the battery. This almost never happens on these bikes unless the battery has dried completely out (wet cells), though. I have seen it happen on my own 750, in the days when I toured at triple-digit speeds all day long, so it does try hard to "keep up" with it. After that trip across Nebraska, I carried a small bottle of water in my pack - for the battery...

The purpose of this method was to prevent the batteries from boiling themselves dry on long trips. This was a constant problem with the early bikes like the CB350/450 twins, and the 750's regulator (limiter) system was their answer, circa 1968.

[Jerry Rxman Griffin aka MuthaF'er]

Hondaman, I don't get it. If you cross reference the part number for the 1977 CB750F voltage regulator you find that it is the same on all these bikes:

31400-300-035  REGULATOR ASSY. (HITACHI)

PLEASE SELECT THE MODEL YOU WOULD LIKE TO VIEWX.CB500 A MOTORCYCLE, JPN, VIN# CB500-1000001
CB500K1 A MOTORCYCLE, JPN, VIN# CB500-2000001
CB500K2 A MOTORCYCLE, JPN, VIN# CB500-2100001
CB550 A MOTORCYCLE, JPN, VIN# CB550K-1000001
CB550F A MOTORCYCLE, JPN, VIN# CB550F-1109886
CB550F A (76) MOTORCYCLE, JPN, VIN# CB550F-2000003 TO CB550F-2010866
CB550F A (77) MOTORCYCLE, JPN, VIN# CB550F-2100001 TO CB550F-2114871
CB550K A (76) MOTORCYCLE, JPN, VIN# CB550K-1230001
CB550K A (77) MOTORCYCLE, JPN, VIN# CB550K-2000007 TO CB550K-2015308
CB550K A (78) MOTORCYCLE, JPN, VIN# CB550K-2100001 TO CB550K-2122082
CB550K1 A MOTORCYCLE, JPN, VIN# CB550K-1200001
CB750A A (76) MOTORCYCLE, JPN, VIN# CB750A-7000001 TO CB750A-7007331
CB750A A (77) MOTORCYCLE, JPN, VIN# CB750A-7100001 TO CB750A-7110560
CB750A A (78) MOTORCYCLE, JPN, VIN# CB750A-7200001
CB750F A MOTORCYCLE, JPN, VIN# CB750F-1000002 TO CB750F-1015054
CB750F A (76) MOTORCYCLE, JPN, VIN# CB750F-2000003 TO CB750F-2022317
CB750F A (77) MOTORCYCLE, JPN, VIN# CB750F-2100011 TO CB750F-2111489
CB750F A (78) MOTORCYCLE, JPN, VIN# CB750F-2200001
CB750K A MOTORCYCLE, JPN, VIN# CB750-1000001
CB750K A (76) MOTORCYCLE, JPN, VIN# CB750-2540001 TO CB750-2575894
CB750K A (77) MOTORCYCLE, JPN, VIN# CB750-2700009 TO CB750-2729534
CB750K A (78) MOTORCYCLE, JPN, VIN# CB750-2800001
CB750K1 A MOTORCYCLE, JPN, VIN# CB750-1044650
CB750K2 A MOTORCYCLE, JPN, VIN# CB750-2000001
CB750K3 A MOTORCYCLE, JPN, VIN# CB750-2200001
CB750K4 A MOTORCYCLE, JPN, VIN# CB750-2300001
CB750K5 A MOTORCYCLE, JPN, VIN# CB750-2500001

Are you saying that the later model F's used a different type regulator vs this cross reference? My manual states "When the battery voltage exceeds approximately 14.5 V the armature coil pulls the armature away from the upper contacts and closes the lower contacts to insert a resistance (10 ohm resistor) in the generator field coil circuit ; as the result of the resistance, the current to the field coil is reduced to 0.7 A and consequently a lower voltage is produced by the generator, limiting the amount of charge to the battery".

Please explain this reasoning. I am curious to learn something new.

Thanks.

[iron_worker]

I'm going to have to agree. AFAIK, the charging voltage limit should be set around 14.5V.

IW

[TurboD]

Update..

Now this morning I go out and test the bike and it appears normal. I tested the voltage before starting, it was normal. I tested the voltage at idle and it stayed pretty much the same, revved the engine and still seen the mid 13 volts. Then I rode it about 3 mile and checked it when I got back, idle voltage is good and now when revved it shows mid 14 volts as it should. I don't know what happened but for now it appears ok.

[TwoTired]

Here is where I am a little lost. With a fully charged battery (12.7v) , I can start the engine and at idle I have the base battery voltage, when I rev the engine to say 4000 and above I only see 13.3- 13.4 volts. My manual is saying I should be in the area of 14.5 volts.

What do I look at to raise the voltage up??

You have to wait f r the battery to accept full charge.  Recall the beginning of the manual where it say to use a fully charged battery for the test?

The most desirable feature of the battery is that it neither drops voltage quickly (a good thing) nor raises it's surcharge voltage quickly (any thing above 13V is a surcharge).
The lead-acid battery can sustain a charge input of up to 14.7V without damage.  The SOHC4's voltage regulator indeed only limits the peak voltage that can be sent to the battery.  It does not "regulate" the voltage outside of what the battery's normal voltage ratings should be.

The SOHC4's "VReg" is a three state unit allowing full power from the alternator, half power, or no power based on what the battery's voltage status is.
To find the up limit, the battery must be at full stored power.


At idle, the SOHC4's electrical system draws more power than is available from the alternator.  The excess power required is taken from the battery, which therefore lowers the battery's charge state to below full; 12.5-ish volts or lower depending on load factors.   Rev the alternator and it makes enough power to feed the bike AND charge the battery.  But, as stated, the battery's state change is not and should not be instantaneous.  Charging takes time.

There is also the possibility that the Vreg is not getting true info about what the battery's state actually is. The "VReg's" terminals must be at the same potential as at the batteries terminals for it to do it's job correctly.  Switch and contact resistance in the connection's path may cause false voltage level reporting.