No charge from 6 v dynamo.

[Ray White]

Having got the car to run (see 'no sparks ' thread) I now find that the ignition warning light will not go out and the ammeter shows a discharge; regardless of r.p.m.  As the car has not been out for quite some time (before the Pandemic) I am wondering if the dynamo has lost it's residual magnetism?  

I have tested the dynamo - a C35M - and found  relatively high resistance from both terminals.

I would like to know if the dynamo needs "flashing"... or is this a last resort?  For example, should I try driving about to see if it starts to charge, or investigate the possibility of a sticking brush or something else.

To be honest, I am not sure what to do first.

Any advice would be appreciated....

Ray.

[David Stepney]

Dynamos are fairly easily to test on the car. Take a wire from both the D and F terminals on the dynamo, connect them together and then test with a voltmeter when the engine is running. The voltage should rise to anything up to 15 volts ir more as the engine speed increases.

However, given your previous comments regarding the ignition switch, I would see if there is a poor contact in that first. Try wiggling the charge/lighting knob and see if that brings the charging circuit to life. The dynamo field windings earth out via the charging part of the switch, either directly (Winter) or through a resistor (Summer). If they are not earthed, you won't get any charge.

[Howard Wright]

Hi Ray

I would also check that the cut out is working. As the revs rise the cutout should close and allow current to flow to the battery.  Without use the cutout can stick open.  

Cheers

Howard

[Ray White]

So,  there may not be anything wrong with the dynamo (despite the ignition warning light staying on and the ammeter showing a discharge regardless of RPM) and I should first look elsewhere? 

Ray.

[Steve kay]

As above, I would check the cut out first.

[Ray White]

I checked out the cut out and the points are working freely. I had the CF1 professionally rewound and restored a few years ago and it still looks nice. I overhauled the dynamo at about the same time so am a bit irritated that it seems to have stopped working. I will do the check with the engine running later.

I seem to remember the ignition switch can crack causing an intermittent short.?........but how would that happen if not being used?

[John Mason]

You can check your dynamo with nothing more than a 6 volt bulb. Connect a wire from a test bulb (use a spare headlamp bulb if you have one.) to a good earth and the other wire to the dynamo. I cannot recall which of the two connections on the top of the dynamo you connect to. No doubt somebody on the forum will know but I doubt if any damage will be caused if you connect to the wrong one while you do a quick test. If the dynamo is charging the bulb will light and get brighter as the revs are increased. Do not rev to high otherwise the bulb will glow really bright and burn out. Don’t ask how know.

John Mason

[Bob Kneale]

Hi Ray
Sounds very much like you have lost some residual magnetism. Flashing the field will probably restore it (take a length of wire from a +ve connection -the horn + or starter motor cable/battery lead at one end the FIELD terminal at the other). With the engine running at a fast idle touch the field terminal with the end of the wire, and the contacts should close, the IGN light goes out and then rev a bit more/take it for a run and you should find it's charging. Ive done it on my box dynamo many times and inevitably Bob's yer Uncle!
If you're positive earth for each + read -

See you at Moreton -with luck?

  Good luck Bob

[Ray White]

Hi Ray
Sounds very much like you have lost some residual magnetism. Flashing the field will probably restore it (take a length of wire from a +ve connection -the horn + or starter motor cable/battery lead at one end the FIELD terminal at the other). With the engine running at a fast idle touch the field terminal with the end of the wire, and the contacts should close, the IGN light goes out and then rev a bit more/take it for a run and you should find it's charging. Ive done it on my box dynamo many times and inevitably Bob's yer Uncle!
If you're positive earth for each + read -

See you at Moreton -with luck?

  Good luck Bob

Hello 'Uncle'  Bob. !! 

Loss of residual magnetism was my first thought...as per my original post!

Trundles is negative earth as per original.  I will give it a go and hopefully get some charging back.

[Ray White]

As suspected, the fault was a loss of residual magnetism. A quick flash on the field terminal and the dynamo is now charging. The ignition warning light goes out first, followed by the ammeter needle swing to the right showing 'charge'.

Thank you everyone for your help and advice.