Topic: Can't measure voltage at idle - CB750

[Ace]

Hi team,

Electrical gremlins have struck again on my CB750 F2 'devils bike'.

History - melted earth wire after starting, lots of smoke.  Got second hand wiring harness as you can't by new ones for the F2.  Replaced the rectifier and I think regulator due to melted wiring.  Stator and field coils tested within tolerance.  Cleaned terminals of harness, earth strap at the frame and added an extra earth wire from where the rectifier bolts onto the battery box to where the tool kit carrier bolts on to frame under the seat, read it in HMs book however he said to near the seat like on the K1 bolt hole near the latch. 

Fully charged battery, started up and tried to measure voltage with DMM across the battery.  The reading on the DMM wasn't stable and I was unable to obtain a reading.  I don't have an analog meter to try the test.  I then changed where I took the voltage, still on positive terminal but the black wire from DMM to various points on the bike, still the same unable to get a stable reading as it was changing so quickly I think the DMM was having trouble trying to keep up with it.  Voltage with ignition on and not running is 12.52 - Dyna S ignition, green 3 ohm coils, headlight on low beam - pretty much standard voltage from tests years ago if I remember.  Start the bike and up the rpm to 1900-2000 and I can then get a stable reading 12.79 volts.

Several trips to work and back about 30 mile round trip, one morning press the starter and nothing, lights on so kick start the bike (one thing I do like about these old machines) and away we go.  Voltage up to 13 volts.  Get home bike running so get the DMM and measuring 10.2 volts and on the decline.  Crap.  Cause stator coil wire nearly snapped off.  Changed stator out completely with new one, field coil tested good.  Put it back together and now same issue at idle, can't get a stable voltage reading at battery and now rev it up to 4000rpm and watch the voltage climb to 17 volts.  Gee just as well I was patient and tested it all.  Too early to start it up at the moment, I've just changed voltage regulator.  If this doesn't fix the problem, what else am I looking at as the problem with voltage at idle.  I never had this problem before the earth wire melted (the one that attaches to the frame under the coils, common fault I've found from looking on this forum).  Oh and I did a diode test on the regulator, a diode or 3 bad and on their way out as getting reading on the earth side doing the test...have I just unknowing discovered the problem? Just remembered that then.  I'm running out of spares and I don't think I've got a spare regulator actually I'll take the one off the K1, I know it has a dodgy diode as well.  40 plus years, somethings bound to give and it's not me haha.

I do apologise for the long comments, however I'm trying to give you all the information straight up, and I'm sure I've forgotten something.  Thanks.

[BobbyR]

I am sure you are awre that at idle with the 750s anyway at idle you are essentially running off the battery. You really don;t get much till you get 2K and above. There are several threds on this. I guess Honda did not think we had stop lights and traffic tie ups.

[Ace]

The problem is I can't get a stable reading at idle when I used to.  I know the idle voltge isn't the best but at least I could determine if it was 12.60, 12.52, 12.32 etc.  Two different DMMs now can't get that reading for some strange reason....

[TwoTired]

If the rectifier is bad, then AC voltage may be getting to the battery, (not good).
Alternating current has voltage swings both positive and negative.  If the meter is set to read DC, the fluctuating voltage will make the display unstable.

If you measured the rectifier as bad, I don't understand why you would try to use it anyway.  It's kind of like driving on a flat tire.  You can do it, but the ride won't be very smooth.

A bad (shorted) rectifier can easily smoke up the green earth wire as well as the positive red wire.  The rectifier is easily destroyed when the polarity of the battery leads are reversed.  That is why the failure is common, because people don't pay attention to proper polarity during jump or battery connection operations.

Cheers,

[Ace]

With my $20 DMM, both rectifiers tested ok.  The other DMM I had was from work cost over$100.  Cleaned the rectifiers as per another post and with my cheap DMM both tested good.  Adjusted the regulator gap to 0.6mm - 0.026 inches in an attempt to lower the voltage other gap set at 0.35mm - 0.014 inches.  Good news, steady voltage at idle if I put the DMM towards the back of the bike, test leads still at the battery.  Voltage at 4000rpm climbs slowly to 13.5 then climbs to 16 volts and then I go back to idle then turn it off.

I noticed the arm of the regulator doesn't move at any voltage on two regulators...

[Ace]

Just did regulator test on 3 regulators.  Took off bike, paper between top point gap to isolate.  Reading as follows in ohms:

             I      / II     /  III
E to I - 46.3  / 38    / 36.5

E to F - 36.3 / 48.1 / 46.1

I to F- 10.9  / 11    / 10.5

Are all 3 toast?

[TwoTired]

Oh and I did a diode test on the regulator, a diode or 3 bad and on their way out as getting reading on the earth side doing the test...have I just unknowing discovered the problem?

I'm confused, your initial post indicated the rectifier wasn't wonderful.
Now you say it is.  Are your meter reading now consistent, too?

The regulator connects the battery to the alternator field coil whenever the battery voltage drop below a certain value say ~13v - ish.  Then the alternator output power varies with the RPM.  It makes about 1/3 of max capacity at idle, and 210 watts at 5000 rpm.  Must have both full field voltage AND RPM to make 210 watts. "Normal" bike loading is 120 watts drawn from either the battery or the alternator or a combination, depending on which has more to offer at the time.

The upper regulator contacts will always be connected until the battery gets to full charge, then the upper contact will break and this lowers the voltage to alternator field, and makes the alternator output drop to about half power, in order to protect the battery from over charge.  If you have been mucking with the gap and the adjustment screw of both your regulators, they are unlikely to be in correct adjustment now, especially if you are getting to 16v at the battery.  The regulator is "THE DEVICE" that is supposed to prevent that from happening.

Cheers,

[TwoTired]

Just did regulator test on 3 regulators.  Took off bike, paper between top point gap to isolate.  Reading as follows in ohms:

             I      / II     /  III
E to I - 46.3  / 38    / 36.5

E to F - 36.3 / 48.1 / 46.1

I to F- 10.9  / 11    / 10.5

Are all 3 toast?

No, they are probably functional with correct adjustment settings.

What voltage appears on the white wire?  With a low battery charge state, the voltage should be the same as on the black wire.  When the battery is at 16V, the white wire should have zero V.

[Ace]

Oh and I did a diode test on the regulator, a diode or 3 bad and on their way out as getting reading on the earth side doing the test...have I just unknowing discovered the problem?

I'm confused, your initial post indicated the rectifier wasn't wonderful.
Now you say it is.  Are your meter reading now consistent, too?

The regulator connects the battery to the alternator field coil whenever the battery voltage drop below a certain value say ~13v - ish.  Then the alternator output power varies with the RPM.  It makes about 1/3 of max capacity at idle, and 210 watts at 5000 rpm.  Must have both full field voltage AND RPM to make 210 watts. "Normal" bike loading is 120 watts drawn from either the battery or the alternator or a combination, depending on which has more to offer at the time.

The upper regulator contacts will always be connected until the battery gets to full charge, then the upper contact will break and this lowers the voltage to alternator field, and makes the alternator output drop to about half power, in order to protect the battery from over charge.  If you have been mucking with the gap and the adjustment screw of both your regulators, they are unlikely to be in correct adjustment now, especially if you are getting to 16v at the battery.  The regulator is "THE DEVICE" that is supposed to prevent that from happening.

Cheers,

I'm confused as well.  Cleaned the regulators as per a post and both rectifiers tested within tolerance with my little DMM.  Ok so I'll live with that, voltage stable at idle but still going up to 16 volts plus.  Found another voltage regulator, actually took it off the K1 and it worked as it should with voltage up to 14.7v and then dropped to 13.3v.  Adjusted one of the other voltage regulators to same setting as K1 reg, voltage still climbing.  Changed setting to minimum clearances and also decided to back out the screw with the nut on it three turns.  Trial and error worked first time, voltage up to 14.6 v and then drops to 13.3 v.  Also broke the wire off my DMM.  Cheap only goes so far I guess and maybe that hasn't helped my cause.  I'll monitor the voltages for a while an hope all goes well.

I wonder if melting the earth wire has done any damage to the Dyna S?  Why do I say this, plus 2 and 3 slightly wet, not oil and up at 4000rpm as if there was a slight miss as you can hear the note out the exhaust change slightly.  Had this ever since I got it working again, but also changed carbs to Mikuni Smoothbores.  Fault finding one item at a time, oh what fun.  Thanks for you input TT.

[lucky]

That is why I like analog meters. The "DMM meters" keep changing the numbers and wont' settle down easily. Too sensitive.