Overcharging

[Peter S]

I have an overcharging problem. Gradually rising the revs from tick over the output form the D+ terminal is fine, rising to 6v and with the warning light dimming. However as soon as the cut out closes the voltage suddenly rises to 10v and the warning light comes back on very brightly. I thought it could be a short between the D+ and field terminal. However when I disconnect the dynamo terminals and measure the voltage from the D+ terminal at 2,000 rpm it was reading 1.5V which sounds about right.

Has anyone had a similar problem and a cure?

Regards

Peter

[DF9053]

Hi Peter,

Dynamo's are a myterious area and I am sure there are others with more knowledge than me - however to start us all off some back ground would be good.

Has this dynamo been working over the last few months, or is it a new installation on a restored car? What type of dynamo - DEL or DSS? Do you have the summer/winter charge set up?

The field coils and D+ are connected (through the summer/winter charge resister) as the dynamo spins and voltage builds up the D+ is directed into the field coils which in turn energises them further creating more current/volts. The peak output is set by the third brush which taps off the armature current for the field.

Let us know more and we can put our thinking caps on.

Cheers
Jeremy

[Peter S]

Hello Jeremy,

The dynamo is a Lucas DDS and the car doesn't have the summer resistor. The dynamo was rebuilt by Wood Autos due to a worn shaft and has been working well. Therefore it is a new dynamo in a restored car. I adjusted the output by moving the third brush and I also cleaned the cutout points. I did this to get the the optimum performance. I wish I had left it alone, so its something I have done I am sure of that. The issue appears to be the field coils getting overexcited but I cant understand why the warning light comes back on very brightly when the cut out closes. I am thinking this could be caused by a problem with the cut out which is causing the D+ feed to the warning light to go straight to earth.

Appreciate your views and assistance.

Peter

[DF9053]

One thing springs to mind and that is to check that the removable cover on the dynamo is not touching the brushes, there should be a gasket under the cover but over time this gets worn and can fall out. The wires and brush holders can short to the cover.

The other thought is that moving the third brush has caused this to come close to the case and shorting as the output rises, try running the car with the cover off and in the dark you may see some sparking which would help identify the possible short.

Hope this helps

Jeremy

[Peter S]

Hi Jeremy,

A couple of good suggestions, i do have the cover loose at the moment so I can look into the dyanmo, but it could be arking. I will try over the weekend. I also thought about take the warning light bulb out to see if there is a back feed into the f terminal.

Regards

Peter

[ian judd]

It might be worth having another look at the cut-out. The last time I had a cut-out fail the warning light came on very brightly, I had smoke coming from behind the dash board and the yellow plastic in front of the warning light melted! I fitted another cut-out and the problem (whatever it was) disappeared.

Ian

[Peter S]

Thanks for all the advice. Over the weekend I isolated the issue down to the dynamo. I readjusted the 3rd dynamo brush and all is OK. Therefore it must have been shorting out. I was going to dismantle and try and trace the problem but the thought of removing the dynamo again made me think again. I will wait if it happens again! By the way during the fault tracing I found a resistor wiring on the ignition switch side of the ign warning light. The resistance wire is wrapped around the under dash interior light switch. Is this normal?

Regards

Peter

[DF9053]

Often this method was used to lower the voltage in the ignition warning lamp but I don't think this was used on the ohc Minors. My understanding wsa that the summer/winter charge resister was in the dynamo or on the back of the dash panel, but I am sure others will put the matter right if I am wrong.

Glad the problem has been resolved

Cheers
Jeremy