C39 dynamo problem.

[vee12eman]

Hi,
I have a 1950s Austin Cambridge fitted with a B series engine, positive earth and using a C39 generator/dynamo. Recently I got the car running but the ignition light does not go out, indeed it gets brighter with increasing rpm. Not an E-type, hence the post in “Pub Chat” , but some of you are running similar units on E-types and other cars. I suspect the dynamo itself - the wiring loom is completely new and I think correctly connected, though I may have made an error. Lucas electrical systems are much the same at this period, so I thought I would ask if anyone knows why the light glows brighter as the rpm rises?

This is my first dynamo and first positive earth car. Any help is much appreciated.

Regards,

Simon.

[abowie]

IIRC this was a good discussion.

viewtopic.php?f=4&t=11571#p94214

Rick Astley's (yes, he never lets you down) book Classic British Car Electrical Systems is the best text out there to understand this stuff.

I'm guessing you have a RB106 regulator (big domed top held on with a wire clip). I think that what's happening is your voltage cutout isn't working or the dynamo isn't generating enough power to trip it.

This is my take on how the ignition warning light works with a dynamo and the RB106 (and by inference RB310 and 340 in the E types.)

The ignition warning light connects between the ignition switch and the D terminal on the regulator.

With the car not running and the ignition switch on, a small current flows from the battery into the regulator via the A terminal, the voltage cut out winding and thence to the D terminal. This lights the ignition warning light.

Once the generator is turning and producing more than around 13V the voltage cut out is triggered, current flows into the relatively low resistance current windings in the regulator and thence to charge the battery, and the globe is effectively shunted out of the circuit by virtue of its higher resistance.

[abowie]

As an aside, in searching for the Austin Cambridge wiring diagram I found this site.

https://www.workshopautomanuals.com/

This seems genuine and has loads of manuals available for free anonymous download.

[vee12eman]

Hi Andrew,

Much appreciate your reply, it has given me plenty to think about, not least the fact that I have Rick Astley’s excellent book as a digital version on Kindle. I know what I will be reading as I await the fireworks later on!

Happy New Year.

Thanks

[Series1 Stu]

Hi Simon

It may just be that the dynamo needs re-polarising by grounding the winding. Instructions should be in the manual somewhere.

Happy New Year!

Regards