Voltage regulator questions

[Enforced_Leo44]

Hi everyone,

I have a on-dash voltmeter and I noticed that it showed quite a low voltage when off-idle (around 13-13.3v), knowing that these voltmeters aren’t always accurate, I double checked with a multimeter at the battery poles and shows the same low voltage

So, first thing first since it’s the cheapest thing to replace in the charging system, I tried to exclude the voltage regulator from the equation, with a jumper wire between the D+ and DF connectors at the regulator plug.

Start the bike, revs just a little and sure enough the voltage climbs way higher than with the regulator connected, almost touching 15v

So, went ahead and bought a new regulator, the Wehrle electronic-style one (the one I had on the bike was electromechanical if that matters), plug everything up and start the bike aaand… still no more than 13.3v off-idle.

Does anybody have advices on where and what to test/look for next? I’m pretty lost as of now.

[Kurt in S.A.]

What model/year bike are you talking about? You should probably measure the resistance across the slip rings on the rotor. Carefully life the brushes off and slip in a business card. Measure ring to ring.

[Enforced_Leo44]

What model/year bike are you talking about? You should probably measure the resistance across the slip rings on the rotor. Carefully life the brushes off and slip in a business card. Measure ring to ring.

The bike is a ‘83 R100RS, with the brushes lifted and a probe of the multimeter on each ring, it reads 3.9ohms.

I don’t suspect the rotor to be the problem though, since when I jumper the connectors of the regulator plug, I get a proper voltage reading and it climbs as I rev

[barryh]

To get 13.3 Volts means the charging system is doing something.

Only just Just off idle you wouldn't expect to see more than 13.3 volts so when you say off idle what revs are we talking about and does the voltage increase at higher revs to 14.0 Volts or something near ?

Also is the battery fully charged and any additional electrical loads turned off. The first rule of measuring charging voltage is the battery has to be fully charged so that it isn't dragging the voltage down and giving you a lower reading than you might expect.

[Enforced_Leo44]

To get 13.3 Volts means the charging system is doing something.

Only just Just off idle you wouldn't expect to see more than 13.3 volts so when you say off idle what revs are we talking about and does the voltage increase at higher revs to 14.0 Volts or something near ?

That’s the problem, the voltage shown doesn’t climb any higher than 13.3 even if I rev higher with the regulator connected

[barryh]

To get 13.3 Volts means the charging system is doing something.

Only just Just off idle you wouldn't expect to see more than 13.3 volts so when you say off idle what revs are we talking about and does the voltage increase at higher revs to 14.0 Volts or something near ?

That’s the problem, the voltage shown doesn’t climb any higher than 13.3 even if I rev higher with the regulator connected

OK,

The 15.0 Volts you got with the VR jumpered is a bit low, you would expect it to be higher on a healthy charging system. So there is something wrong with the alternator or the diode board. I'd start with cleaning/checking all connections including the rotor brushes and if that doesn't fix it test the diode board. Rob Frankham will be along and be able to tell you how to do that if it becomes necessary.

[Enforced_Leo44]

To get 13.3 Volts means the charging system is doing something.

Only just Just off idle you wouldn't expect to see more than 13.3 volts so when you say off idle what revs are we talking about and does the voltage increase at higher revs to 14.0 Volts or something near ?

That’s the problem, the voltage shown doesn’t climb any higher than 13.3 even if I rev higher with the regulator connected

OK,

The 15.0 Volts you got with the VR jumpered is a bit low, you would expect it to be higher on a healthy charging system. So there is something wrong with the alternator or the diode board. I'd start with cleaning/checking all connections including the rotor brushes and if that doesn't fix it test the diode board. Rob Frankham will be along and be able to tell you how to do that if it becomes necessary.

Oh no wait I wasn’t saying that 15v was the max it would output, if I remember correctly going higher than 14.5-15v could damage the battery and the rest of the circuit so I stopped there, I’m sure that it would climb higher than that

[Rob Frankham]

OK...

1) 13.3 volts is too low for an airhead charging circuit with a fully charged battery but, as has already been said, if the battery is low (or in poor condition) it can pull the charging voltage below the optimum. FWIW, the optimum should be regarded as between 13.75 volts and 14.2 volts.

Advice...

a) Make sure the battery is fully charged and, if you are in any doubt about it's condition, get it checked with a modern battery tester. An auto electrioc workshop will do it... you can probably get it done a Halfords if you trust them. I wouldn't trust the 'testers' you get of Ebay or Amazon for about £20.

b) Once you are sure that the battery is good, run the bike for some time at charging revs and see if the voltage climbs to a good level. It shouldn't take more than ten minutes even with a fully discharged battery.

2) The test with the regulator out of circuit is good but I would expect the voltage to reach around 16 volts or higher. You won't damage the battery by having it at that level for a couple of minutes. Many boost chargers go way above that level (although, don't get me wrong, bost charging a battery to often isn't good for it!) and a short period won't hurt. Run the engine for a couple of minutes at above 3,000 rpm and see if the voltage stabilises at the higher voltage...

3) Check the stator windings. If one of the windings is open circuit or shorted, the alternator will still charge but at a reduced rate. Remove the three connectors on the right hand side of the stator (looked at from the front) and measure the resistance between each pair. (that is, if the terminals are labelled v, w and x, measure V-W, W-X and X-V). The specified value is 0.62 Ohms but you'll need a very good multimeter to get an accurate reading for a resistance this low. It is sufficient to note that the readings you get are in the ballpark and, crucially, all the same.

4) This brings us to the diode board. Here is a link to a testing procedure I prepared some time ago. https://robfrankham.com/diode-board-test. Be aware that there are no fewer than 11 diodes on the board and the failure of any one of them can cause the symptons you are suffering.

5) Needless to say, as you work through the system, make sure all connections are clean and that all wiring is in good condition.

Most of these tests are quite simple and within the abilities of most people with a multimeter. Be systematic and you'll find the issue.

Good Luck

Rob

[Enforced_Leo44]

OK...

1) 13.3 volts is too low for an airhead charging circuit with a fully charged battery but, as has already been said, if the battery is low (or in poor condition) it can pull the charging voltage below the optimum. FWIW, the optimum should be regarded as between 13.75 volts and 14.2 volts.

Advice...

a) Make sure the battery is fully charged and, if you are in any doubt about it's condition, get it checked with a modern battery tester. An auto electrioc workshop will do it... you can probably get it done a Halfords if you trust them. I wouldn't trust the 'testers' you get of Ebay or Amazon for about £20.

b) Once you are sure that the battery is good, run the bike for some time at charging revs and see if the voltage climbs to a good level. It shouldn't take more than ten minutes even with a fully discharged battery.

2) The test with the regulator out of circuit is good but I would expect the voltage to reach around 16 volts or higher. You won't damage the battery by having it at that level for a couple of minutes. Many boost chargers go way above that level (although, don't get me wrong, bost charging a battery to often isn't good for it!) and a short period won't hurt. Run the engine for a couple of minutes at above 3,000 rpm and see if the voltage stabilises at the higher voltage...

3) Check the stator windings. If one of the windings is open circuit or shorted, the alternator will still charge but at a reduced rate. Remove the three connectors on the right hand side of the stator (looked at from the front) and measure the resistance between each pair. (that is, if the terminals are labelled v, w and x, measure V-W, W-X and X-V). The specified value is 0.62 Ohms but you'll need a very good multimeter to get an accurate reading for a resistance this low. It is sufficient to note that the readings you get are in the ballpark and, crucially, all the same.

4) This brings us to the diode board. Here is a link to a testing procedure I prepared some time ago. https://robfrankham.com/diode-board-test. Be aware that there are no fewer than 11 diodes on the board and the failure of any one of them can cause the symptons you are suffering.

5) Needless to say, as you work through the system, make sure all connections are clean and that all wiring is in good condition.

Most of these tests are quite simple and within the abilities of most people with a multimeter. Be systematic and you'll find the issue.

Good Luck

Rob

That’s one hell of a detailed answer, many thanks Rob, I’ll make sure to follow the steps as I go and update here!

[Blapper]

I'm a bit surprised that Rob didn't tell you about his earth bypass mod. It is excellent and may be worth investigating first as it is easy. Basically, it works because these old bike rely on a low resistance between the earthing point of the diode board and the torturous route to the battery negative terminal. As the years go by, there can be more and more resistance at the engine casing joints in the earth path which reduces the effectiveness of your charging system. The way to check is to clip a voltmeter across your battery and connect a 2.5mm² wire from earth at the diode board (best crimp a ring terminal to the wire and trap it under one of the screws that hold the diode board down) and make it long enough to reach the negative terminal of the battery - effectively bypassing the engine. Start your engine and rev it to 3000rpm and touch the wire on your battery negative terminal if the battery voltage increases you will know there is a voltage drop across the joints which is limiting your charging current so make that bypass wire permanent but put a 40A fuse in series with it in case you ever forget to reconnect your original 16mm² battery negative connection at the gearbox and go to start your engine. Lack of a fuse will make that 2.5mm² cable catch fire.

Hope that helps. I incorporated it in my rewire anyway because why not?

Thanks Rob.

Andrew.