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6 cyl Bristol cars Type 400 to 406 - restoration, repair, maintenance etc

401 Dynamo

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  #1 (permalink)  
Old 17-12-20, 07:02 PM
Join Date: Apr 2019
Location: Hampshire
Posts: 45
Default 401 Dynamo

I have had an email from a French 401 owner asking if anyone might have any ideas as to why the dynamo in his car is not charging when the engine is idling?

As the owner of a V8 with an alternator I cannot help!
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Old 18-12-20, 12:07 AM
Join Date: Jul 2008
Location: Sydney, Australia
Posts: 386

X Series ,
I'm not an Auto electrician but I know the regulator is usually adjusted to cut in at about 1200rpm , or the charge generated at 1200 rpm.
This is NORMAL function of a generator/regulator system.
I'm sure there is plenty of information if the French owner searches Google

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Old 18-12-20, 03:35 PM
Join Date: Jul 2008
Location: Bury St Edmunds, Suffolk, UK
Posts: 235
Default 401 Dynamo

If Geoff's idea does not satisfy the gentleman perhaps a check of the brushes may help.
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  #4 (permalink)  
Old 19-12-20, 07:20 PM
Join Date: Sep 2010
Posts: 239

Not an auto electrician either, but I have a little understanding.
The dynamo output goes through the control box which has two cut outs.
The dynamo uses an electro magnet and the first cut out is also controlled by electro magnatism.
When dynamo output is low the first cut out breaks, stopping output going to the car. At that point power flows from the battery giving a negative charge on the ammeter.
With the cut out open power from the dynamo is fed back down the field cable to the electro magnet, energising it and causing the dynamo to produce more output. When it reaches about 12 volts it is producing enough power to energise the cut out and close it, allowing power to enter the system. Once it exceeds the draw you will get a positive charge through the ammeter.
Finally when the voltage gets to about 14.5 volts the other cut out opens, stopping power to the field and reducing dynamo output to stop the dynamo over charging the battery.
The important question is, at what engine speed will the dynamo produce a positive charge with all the lights on?
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