Dynamo Draining Battery.

[Nimbus]

I would be extremely grateful if someone would be able to tell me how to rectify this fault. I have a 1940's 6 volt, negative earth, motorcycle with no lights, or fancy electronics, and the fully charged new battery will go completely flat after 30 minutes with the engine running. The red charging light stays on and someone has suggested that this indicates there is a current drain to the dynamo and that the dynamo is trying to run as a motor. Using a multimeter on the battery terminals indicates that the battery is being charged.(?) So far I have replaced the battery, regulator, bushes with no effect. The dynamo is of the Bosch type and the regulator is the correct one. I've tried isolating the warning light but the symptoms remain the same. Would connecting a suitable diode somewhere in the wiring be a practical solution ?

 

[James]

You should be reading 6.8 to 7v across the battery terminals when the engine is running over 2000rpm if it is charging.

 

[Nimbus]

You should be reading 6.8 to 7v across the battery terminals when the engine is running over 2000rpm if it is charging.

Hello James, Thank you for your reply. It's reading 7-8 volts with the engine running and 6:30 volts when the engine isn't running. I can't understand how the dynamo can be both charging the battery and draining it at the same time. If that is what's happening. Regards, Tom

 

[Avo Mk8]

I'd be inclined to try measuring current rather than voltage, and see where it's going!
Preferably without blowing up the meter ?

 

[Nimbus]

I'd be inclined to try measuring current rather than voltage, and see where it's going!
Preferably without blowing up the meter ?

Okay, thank you for your advice !

 

[James]

Are you sure your meter is only measuring DC, putting AC into a battery can have some strange Effects.

 

[Nimbus]

Are you sure your meter is only measuring DC, putting AC into a battery can have some strange Effects.

Hello James, Thank you for taking an interest. Definitely measuring DC. Since starting this thread I've spoken to a retired auto electrician and he thinks that, in spite of what I've been told, the electronic regulator may be incompatible with the dynamo. Tomorrow, I'm going to try using a mechanical regulator that he will loan to me. He has also suggested that I recheck the polarity of the dynamo. Hopefully, the whole thing will be sorted soon as it's starting to drive me bonkers.

 

[brianmoooore]

IIRC, isn't it possible for a dynamo to reverse its polarity? Seem to remember that back in the day, when changing from positive earth to negative earth, you 'flashed' the dynamo. Could this have happened accidently?

 

[Nimbus]

IIRC, isn't it possible for a dynamo to reverse its polarity? Seem to remember that back in the day, when changing from positive earth to negative earth, you 'flashed' the dynamo. Could this have happened accidently?

Hi Brian, I'm beginning to think that, as you suggest, the dynamo is incorrectly polarised. A friend of mine who is an "expert" polarised it but, thinking back, maybe not using the correct procedure for this design of dynamo. That would explain most of the symptoms. I'll have another go when I get down to my workshop this afternoon and report back.

 

[Nimbus]

Thank you to those who replied to my post. I'm still in a state of shock due to my discovery that, when the previous owner of this motorcycle had the dynamo reconditioned by a company with little experience of this kind of work, they incorrectly connected the internal F & D terminals. The terminal marked F on the body of the dynamo was actually D and vice versa. Can you believe it ? Obviously this unit was not properly tested or this elementary mistake would have been found. I'm pleased that the fault has been sorted but not very happy about all the time and cash wasted.

 

[James]

Glad you got a good result in the end

 

[DPG]

And post a picture of the bike so we can have a look at it!

 

[Nimbus]

I ought to include a 'photo of me looking 30 years older due to the stress of the dynamo issue.

 

[DPG]

I ought to include a 'photo of me looking 30 years older due to the stress of the dynamo issue.

Nice. Suicide shifter looks well engineered. How much of the frame is original?

 

[Nimbus]

Nice. Suicide shifter looks well engineered. How much of the frame is original?

The frame is completely original but not the girder forks. There's an article about the bike in this month's Classic Bike magazine if you should find yourself browsing in WH Smiths. The Nimbus models in the mid 1930's would have had foot clutch/hand change.

 

[DPG]

Tank looks well between the frame. What engine is it?

 

[Nimbus]

Tank looks well between the frame. What engine is it?

It's a Danish Nimbus. It's 80% Nimbus but I've added the supercharger and built it from bits to resemble a Brooklands racer that never existed. This is a fake 'photo of it in the paddock at Brooklands in the 1930's.

 

[telectrix]

I ought to include a 'photo of me looking 30 years older due to the stress of the dynamo issue.

awesome, but where does wifey sit?

 

[Nimbus]

awesome, but where does wifey sit?

she's got her own one with a pink tank.

 

[telectrix]

funny. i'm looking for a restoration project, barn find perhaps, a 1950's /60's British M/C 500 ish cc. AJS, BSA, RoyalEnfield. Arial,Triumph, any. any one know of anything going?