Topic: Voltage drop at Key Switch

[Staypuff 68]

Working on a 78 CB750 A that has some charging issues.  I have 12 volts at the key switch on the red wire in the off position, once I turn the key to the on position the volts drop to 10 and stay that way even when the bike is idling.  I get 10 volts at the black wire to the regulator, 10 volts to the field coil and 7-9 volts (AC) out of the alternator.

I have a freshly charged gel battery that I am taking to get load tested, but I tried hooking up jumper cables to a know good car battery in a running car the results where the same.

I have cleaned all of the ground I can find, not sure where the green wire chassis ground is at. 

Any help would be great, thanks.

[STLrocker]

does the voltage go up if you rev the engine? you should get 14.5v at 3000rpm checked at the battery terminals.

you can also check for amperage. to do this, disconnect the smaller wires from the starter solenoid. they are probably red and red/white. one goes to the main fuse the other to the rectifier and are likely crimped into one terminal. connect your meter between the solenoid terminal and the wires you just disconnected and measure amperage you should have 8.7a at 3000 rpm.

charging voltage and amperage isnt usually measured at idle on these systems. im not saying you dont have a problem, just trying to get a base line to start figuring it out from.

you can find the info here:

https://www.classiccycles.org/media//DIR_1653304/DIR_1653404/DIR_1840056/d28e7d12b0a2c4d3ffff875bffffe417.pdf

[Jerry Rxman Griffin aka MuthaF'er]

You may very possibly need to replace the switch. They carry lots of the power supply through there and can be a significant voltage drop problem. Corrosion, lack of spring pressure, wear in the ball system, cracked plastic, etc.

[scottly]

I have 12 volts at the key switch on the red wire in the off position, once I turn the key to the on position the volts drop to 10 and stay that way even when the bike is idling.

If you are reading 10 volts on the red wire with the key on, the most likely suspect would be the fuse box/clips/connections for the 15 amp main fuse.

[TwoTired]

Working on a 78 CB750 A that has some charging issues.  I have 12 volts at the key switch on the red wire in the off position, once I turn the key to the on position the volts drop to 10 and stay that way even when the bike is idling.  I get 10 volts at the black wire to the regulator, 10 volts to the field coil...

You just proved the key switch faulty.  They can usually be rebuilt to restore the internal contacts integrity...  unless the contact holders inside have melted.   There have been a few threads posted outlining the procedure.  Search is your friend.

Do be more precise in your measurement reporting by including at least one digit right of the decimal.

You can do the same voltage drop measurement for each component in the voltage delivery pathway, fuses, connectors, etc.

Cheers,

[scottly]

You just proved the key switch faulty.

No, if the voltage on the red wire, which supplies power to the switch, drops to 10v with the key on, the problem is between the ignition switch and the battery positive terminal. If there is 10V going into the switch, and 10V coming out to the black reg wire, it proves that the key switch is operating properly.

[MetroRedneck]

I have 12 volts at the key switch on the red wire in the off position, once I turn the key to the on position the volts drop to 10 and stay that way even when the bike is idling.

If you are reading 10 volts on the red wire with the key on, the most likely suspect would be the fuse box/clips/connections for the 15 amp main fuse.

X2 - Mine have glowed red (not really but gotten really hot) before.

[Jerry Rxman Griffin aka MuthaF'er]

As far as the fuse box goes, take it off and check the back side of it for loose prongs, etc. They have a tendency to overheat and melt the plastic.

As far as the switch goes, look for a voltage drop across the switch ie input and output.

[Spanner 1]

I would only at the risk of peril dismiss TT's answer Scottly !
Could be crappy contacts in the ignition switch unable to serve the load  ( '78 CB = lights on with ign, 'on' ). Could be a crappy main ground connection, could be a crappy fuse box connection or even crappy ( non shiny ) battery connections or all of the above ! My vote is all or most of the above.

[scottly]

I would only at the risk of peril dismiss TT's answer Scottly !
Could be crappy contacts in the ignition switch unable to serve the load  ( '78 CB = lights on with ign, 'on' ).

No, there was no voltage drop across the ignition switch reported by the OP, Ray.
Where is that guy???

[Spanner 1]

Huh ?... If all before the ign. switch is 'solid' inc. ground, why couldn't it be the ign. switch. He reports a 2v drop with the ign. turned on, could be crappy switch as suggested by TT, or all or some of the  other suggestions. Agreed, no report yet of voltage drop across the switch itself but doesn't rule it out ! But, hey, any voltage drop with ign. 'on' would not  definitely prove the switch as bad either, now see your point

[PeWe]

Try a jumper over the switch, hot wire to verify switch connections.
Turning on switch can also reveal high load after the switch.

[scottly]

Per, for the third time, there was no reported voltage drop across the ignition switch.

[Spanner 1]

Not much to 'partially' short out after the ign. switch PeWe, can you think of anything to produce a 2v drop ? I'd always go straight to the 40yr old fuse block and get rid of it !. Just do it even if it's not the problem now it will be in the future ! Voltage drop is almost always lack of electrical path on the battery side of the ign. switch IMO and from reading here for years and years.

[PeWe]

It does not matter. I should start with hot wiring the switch when it  has 12V when off, voltage dip when on.
Let’s see what the fault tracing will end up with.
Disconnect circuits to isolate the faulty area is std fault tracing when something cause too high load.. That has worked for me, private and professionally. 

[Staypuff 68]

First thanks to all that took time to respond, electrical faults are the worst.

After taking the fuse box out and cleaning the terminals with some sandpaper and using a fully charged and load tested battery here are the volts:

At the battery with the key off- 12.60; key on- 12.30; bike running- 12.30 (constant no change with revving the engine)

At the key switch in the off position- 12.50; with the key in the on position- 11.30; cranking 10.80; bike running 11.20 (constant no change with revving the engine)

The fuse box at the main fuse looked a little melted and the terminal were a little loose with the fuse removed

Is there a main "green wire" ground somewhere on the frame?  I cleaned up the frame/battery/engine ground but I haven't been able to find the one for the wiring harness, if there is one.

[scottly]

At the key switch in the off position- 12.50; with the key in the on position- 11.30; cranking 10.80; bike running 11.20 (constant no change with revving the engine)

The red wire at the key switch, or the black wire???

[Staypuff 68]

The voltage reading were done at the red and black wire while the key was on and while the bike was running.  There was no difference on between the two.

I think i might have figured it out, at least a little.  Looking at the info the SLTrocker provided I did a continuity check on the regulator and I had continuity between all three terminals, ignition, field and ground.  With the bike running I was getting 11 volts at the ground terminal.  I might not be the sharpest knife on the christmas tree but I am pretty sure that isn't right.

[scottly]

The voltage reading were done at the red and black wire while the key was on and while the bike was running.  There was no difference on between the two.

If you have the voltage drop at the red wire, it means you have a high resistance somewhere between the key switch and the battery + terminal, and there is no point in looking elsewhere until that is corrected.
Test the voltage drop directly, by connecting the positive meter probe to the battery + terminal, and use the other to test the voltage at the key switch red wire with the key on, motor not running. The voltage drop should read about 1.2V, based on your prior readings; if so move the black probe to the clips on each side of the main fuse. Note your readings and report back.   

[scottly]

With the bike running I was getting 11 volts at the ground terminal.  I might not be the sharpest knife on the christmas tree but I am pretty sure that isn't right.

Sorry, I missed this. The voltage reading from the ground terminal on the regulator to the battery negative terminal should be zero, or very close to it, if the green wire is attached. The connection from the main harness green ground wire to the frame on your bike is probably at the coil mount, under the gas tank, but if it was not connected, there would be other issues besides charging..
EDIT: The stock regulator defaults to a full charge condition if not grounded. Symptoms of this would be over-charging, not under-charging.
Edit#2 The voltage drop at the key switch this thread started with would cause an under-charging condition, but it seems you have a zero charging condition??

[Staypuff 68]

After all of the voltage readings I would have to agree that it seem to be an undercharging issue.

I have checked the am getting AC voltage from the alternator, 9.0 volts AC per leg.  This led me to the field wire to see if I was getting enough volts to power the rotor and I was getting 9.0 volts DC.  I followed that to the regulator, again 9.0 volts which led me to the key switch and the 2 volt drop when the key is in the on position.  I was looking for the main wiring harness ground to make sure that it was clean and secure.

  NOTE: all of these reading were taking before the battery was fully charged and the fuse panel cleaned.

[TwoTired]

The AC voltage is measured between each of the yellows. It does not need any ground reference to do its job.  The rectifier translates the ac to dc and gets a ground reference there.

Extremely rare for the stator to go bad (connections- yes, coils-no.).  But, it cannot output unless provided with a magnetic field, which is the job of the field coil and rotor creating an electromagnet.  To that end, the field coil needs to be provided a voltage to make it all happen.  Look to the white wire which passes voltage from regulator to the field coil.  Unless the battery is above 13.8V, this white wire should have full battery voltage on it.

Check and report.

Cheers,

P.S. I misinterpreted your voltage drop report earlier.  I though you were reporting voltage drop across the key switch rather than a general drop of all voltage systemic with the key switch on, which is pretty much normal without an operating charging system.

[Staypuff 68]

Just checked, 12.20 at the battery; 11.63 at the black ignition wire at the regulator; 1.5 at the white wire.
 I rechecked the continuity at the terminals of the regulator and again I am getting continuity between all three and the frame of the regulator. 
So now I'm thinking the regulator needs to be changed, after all it is only 40 years old.

[scottly]

Just checked, 12.20 at the battery; 11.63 at the black ignition wire at the regulator; 1.5 at the white wire.
 I rechecked the continuity at the terminals of the regulator and again I am getting continuity between all three and the frame of the regulator. 
So now I'm thinking the regulator needs to be changed, after all it is only 40 years old.

1.5V on the white wire??  Connect the white field wire to the battery plus terminal with a jumper, and start the bike and check the voltage. Remove the jumper after the test.
BTW, it's normal to have continuity between all three regulator terminals with the stock reg.

[TwoTired]

Just checked, 12.20 at the battery; 11.63 at the black ignition wire at the regulator; 1.5 at the white wire.
 I rechecked the continuity at the terminals of the regulator and again I am getting continuity between all three and the frame of the regulator. 
So now I'm thinking the regulator needs to be changed, after all it is only 40 years old.

At what RPM?  At idle speed, the White wire should read same as black wire.  Actually, anytime the black wire terminal reads under 13V-ish.

The Vreg has a 10 ohm resistor inside. You can get continuity through that.  It's normal.

It does sound like the internal contacts need cleaning.  Lack of thorough test details make us unsure.  And the .6V drop from battery to black doesn't exactly help.  Was this test done with the lights on?  The Vdrop will be worse when it is.

Cheers,

[Staypuff 68]

The voltage at the battery was 12.63 to begin with, key off. 
With the jumper wire to the white field wire  the volts at the battery was 12.23. 
Key on 11.90 at the battery and on the white wire.
Engine running the volts stayed the same.


Thats is when I saw the smoke and smelled burning plastic and the regulator was VERY hot to the touch.  So it looks like I will have to get a new one.

The headlight is LED, I have checked the voltage with and without it connected and there was no change to the voltage. The RPM is a best guess for now until I can get a tachometer hooked up, the bike doesn't have one.

[scottly]

Thats is when I saw the smoke and smelled burning plastic and the regulator was VERY hot to the touch.

What?? Where did the smoke come from?? Where was the regulator VERY hot to the touch? If it was the resistor on the back of the reg, it's normal for it to get hot, but not so hot plastic burns or smokes.

[Jerry Rxman Griffin aka MuthaF'er]

I've smoked a rectifier before but never a regulator.

[Staypuff 68]

The regulator was very hot to the touch, the smoke and smell came from the coil on the back of the regulator.

[TwoTired]

That wire wound resistor is something like a 30 watt dissipator, or more.  And, in normal operation dissipates 15 W max.

Have you measured the field coil resistance?  S/B about 5 ohms (550)  0r 6.8 ohms if 750.  If that white wire has lower resistance than that, it would explain the overheat,  (or if the white branch is shorted to ground).

I think you have have wiring issues.

Cheers,