Topic: Voltage WAY too high (17v)

[Round Dice]

This is my first post so I hope it goes well.

This is for my 1978 cb750k.

Background:

Bought motorcycle and voltage was low, 10-13.  I played with the regulator and couldn't really get anything better so I just road that way for about one year.  I wasn't that bad because it was mostly day riding about 20 miles a day.

I installed a solid state rect/reg off a 1981 cb750 hoping that would bring up the voltage.  No luck.  The voltage remained low but I still road it another year.   

So after two years my voltage drops to 8ish!  I open up the alternator casing to test the coils and the first thing I notice is that one of the three phase wires has been chewed up by the starter motor gear (the small one).  So I repaired the wire and removed the gear (I never use that thing, I'm a kicker).   

Problem:

Now my voltage is outrageously high, 12.5@1G, 17@3/4G, and I've seen it in the low 20s when I flick it (fried my LEDs).

I figured it had something to do with the 1981 solid state rect/reg so I put the old OEM stuff back in and it is still mega high.   

I'm at a total loss.  Any suggestions?

[fmctm1sw]

Did you have a manual?  Plus, this should help you get on track:  http://www.electrosport.com/media/pdf/fault-finding-diagram.pdf

Also, I'd repair instead of removing stuff or replacing with parts off other machines.  You might be introducing more variables than you need.

[Round Dice]

I do have a manual but that is a pretty sweet flow chart, thanks.

[DJ_AX]

The voltage regulator has a screw to adjust the amount of voltage put out.
I'm guessing you cranked it up trying to make up for the power loss in the short.
And when the short was fixed ... we'll you'll need to adjust it back down.

Same thing just happened to me with a short in the light switch.

Just an adjustment .. no new parts needed.

[DJ_AX]

here.. 

[elbowdeep]

low speed charging seems ok, but its supposed to switch to a "high speed charging circuit" or something.  That way it charges (or maintains at least) at low speed and switches to a lower volt charging setting at 3500 ish (my memory is a little hazy on this).  In other words, its not making the "switch" to the high speed charging circuit

[scottly]

You had to modify the field wiring to use the late model regulator properly. Did you undo the mods when you went back to the mechanical reg, or were they never done in the first place?

[iron_worker]

Ya I was thinking it kind of sounds like you're running straight 12v to the field.

IW

[Round Dice]

Thanks for the input guys.  I did have to modify the connectors for the '81 rect/reg but I am not familiar with any standard modification for this conversion.  I just did the best I could.  It is likely that I had 12v across the field coil, I never checked that.  What exactly is the modification to the field coil wiring that you're refering to?

[TwoTired]

I thought the field coil had one leg of its wind connected to engine case/frame ground as part of the construction of the coil itself.

The SOHC4 charge system only needs control of one wire (white) from the regulator.

Below is the schematic for a CB650 regulator.  And I am led to believe the 81 Cb750 regulator is similar as it also has a slip ring rotor.

The CB650 regulator needs control of both wires of the field coil (rotor).  With either coil "leg" grounded, the regulator cannot control properly, and I believe there is some risk that such a regulator can be damaged by a grounded connection.

If the SOHC4 750 could have it's field coil modified to free the grounded leg, then the 650 or DOHC 750 regulator could work.

[Round Dice]

Wow, I've seen quite a few wiring diagrams trying to describe the DOHC rect/reg to SOHC thing but just explaining the need for field coil control makes all the sense in the world.  I bet that was my problem with the '81 rect/reg I was trying to use.  Thanks guys.  I'll let you know how it goes. 

Another question: If the field coil is wired for green and white wire control as in the DOHC case, does it matter if you switch the wires to run the current through the coil in the other direction?  I want to say it doesn't matter. 

[TwoTired]

I thought the DOHC slip ring control wires were White and Black?

But, if they are isolated and under full control of the regulator, I don't think polarity will matter.

Green is Honda code for frame ground and connected eventually to Battery NEG terminal.

[Round Dice]

Right, sorry, I crossed the terminology there.  DOHC is white and black were SOHC is white and green I think.  But in some of the diagrams I've seen the DOHC white is connected to SOHC green, and the DOHC black is connected to the SOHC white.  I just wanted to make sure that was right and it does make sense in my head.  Thanks for seconding my opinion that in the case that the two connection were reversed, nothing would be damaged and it might even work.  Maybe I'll be brave enough to try it and let you know. 

[TwoTired]

I'm going under the assumption that the DOHC regulator is constructed similarly to the Cb650 regulator I posted earlier.
Note one leg of the field coil is connected to Battery POS  (I assume that is the Black field wire as that fits with Honda convention, but I can't say with certainty due to lack of intimacy with DOHC wire diagrams).  This leaves the white as control of the field where the regulator controls current to a battery NEG connection.

The SOHC4 750 controls +12v to the White wire and green is always connected to the Battery POS terminal.

While the two schemes both control current through the field, they do it it in different ways and with different (opposite) polarities.  While the field coil doesn't particularly care about voltage polarities per se, the respective regulators do, and the SOHC4 750 field coil only allows the option of controlling the white wire with Positive polarity, which the DOHC regulator can't do (assuming the DOHC regulator is constructed in the same manner as the CB650 regulator).

This appears to be a square peg and round hole scenario.

[Round Dice]

I think I understand your assumption but I think it is different in the DOHC rect/reg case.  I'm just going by these two wiring diagrams and my intuition.  I understand what you mean about the black wire being connected directly to the POS battery terminal  but the DOHC r/r isn't designed that way from what I can tell.  Maybe it does that internally or may it actually uses both ends of the coil for control.  Either way I'm pretty sure I did what you're suggesting the first time around and it messed me up. 

[scottly]

The SOHC grn field wire is grounded through the main harness. It needs to be isolated from ground and connected directly to the reg. The best place to do this, without cutting any wires, would be at the bullet connector under the shifter cover.

[TwoTired]

Take an ohmmeter and measure the DOHC reg/rect. between the two black wires.  If there is little to no resistance measured, then it works the same as 650 regulator.
If so, then connecting the green SOHC wire to the white regulator terminal effectively shorts out the reg's drive transistor, and renders the regulator unable to control current through the field coil.  If you have then connected the black wire the SOHC4 white field coil connection, then you have applied 12V across the field coil with NO regulation.  Wiring in such a way, is effectively telling the alternator to put out all the power it can all the time, and that invites battery overcharging when the engine sustains RPM's over 3000-ish.

To be clear.  I'm not suggesting you use the DOHC reg/rect on the SOHC4, as I don't believe the devices in question are compatible.  Barring some overlooked detail, it's likely the root cause of your battery voltage too high when this thread began.

But, there is no reason to believe a 78 CB750 can't properly charge a battery if all the components and connections are good.

However, if you randomly tweaked the original rectifier to set a run voltage, you fked up.  As the system doesn't work the way you think it does.  The vreg lets the alternator be all it can be up until the battery reaches full charge (14.5V). Then, it reduces field current and therefore the Alt. output so as to keep the battery near full charge.  The Vreg can only diminish alt. max output, it cannot boost it.

To set the original vreg on the bike (vs, on the bench) you need to set up a working condition whereby the vreg is supposed take action, eg, battery at full charge and alt spinning at optimum power making RPM.  Then the Vreg should limit the battery voltage to 14.5V.

At idle, the alternator can't make enough power to drive all the electrical loads and charge the battery, so tweaking the adjuster won't do anything but make the Vreg adjustment wrong, as the battery is in depletion mode and the voltage must fall.  The Vreg has no way to control voltage when the alternator isn't making enough power for the bike's systems.  At idle, the alternator makes about 1/3 of it's max capability.

Cheers,

[Round Dice]

Scottly,

That's what I was planning on so thanks for clarifying.  But thats what you were referring earlier, right?

You had to modify the field wiring to use the late model regulator properly. Did you undo the mods when you went back to the mechanical reg, or were they never done in the first place?

Ya I didn't do that the first time.

TwoTired,

Great advise, clear explanation, thanks for the help.

[Round Dice]

Turned out that my ground (green) wire for the regulator was bad.  So yes, the field coil was constantly getting 12v.  So now I have the SOHC reg and rect working.  Thanks everyone.  I'm gonna try the DOHC again next weekend now that I understand the field coil ground wire problem. 

[Spanner 1]

Stay with your original SOHC reg/rect. now that your charging correctly ( and yet again, neither the reg. or rect. was bad to begin with ! ).... the DOHC r/r will boil your battery and much shorten it's life.. IMHO.

[Jerry Rxman Griffin aka MuthaF'er]

I'd like to see you make the DOHC solid state reg/rect work. It'd make a good inexpensive upgrade. Please keep us informed and maybe a brief post how to do it. Then you could lock the post so it'd stay clean. PM's work when someone can't add to a posting.

[Spanner 1]

With respect, a DOHC reg./rect. installed on your SOHC bike is not ' an upgrade'  ... the stock electromechanical regulator cannot be beat in combo with the low tech lead-acid battery these bikes use.... I would say that if you install electronic ignition then you better install a solid state R/R as the ign. would then need a rock solid voltage at all times.....  ... downside is reduced battery life due to constant voltage applied by the charging system once the bike is running .
The DOHC R/R will work as pictured above , simply cut the Green wire and connect the White as shown.... even tho' this end of the field coil is Green, it goes directly to one end of the coil and does not connect to ground in the stator case... so your good. You could do that at the 6-way connector ( yellow x3, white, green, purple/red ) connect to the green that goes into the stator, let the other green end 'hang'....( won't hurt anything )......

[scottly]

I'd like to see you make the DOHC solid state reg/rect work. It'd make a good inexpensive upgrade.

No doubt in my mind the DOHC will work, but is it less expensive than pinhead's Ford reg/ Radio shack rect? Mine cost me about $30...