charging question.

[Ray White]

Please could someone give me an idea of why my dynamo/cut out is behaving differently to the way it has always done in the past...  With the switch on full charge the engine is started but has to be revved quite hard before the ammeter flips over from discharge to charge and the light goes out.  When the engine settles down to tick over the needle flips back to discharge and the light comes on as normal.

(The car is 1930 with 6 volts.  The dynamo is a later Ruby type with a resistor added for Summer charge.  The CF1 cut out has been professionally rebuilt a few years ago.)

[John Mason]

You could check that the dynamo brushes and springs are good and not sticking in there guides. Also make sure all your wiring from the dynamo to the cut out is good together with the engine earth strap is in good condition and fastened tight at both ends.

John Mason

[bob46320]

in spite of the cut out having been re-built, I would clean the contact surfaces by rubbing them with a bit of clean printer paper. Costs nothing and would rule out that problem. What you are saying is that the dynamo is charging, but the cutout is not operating correctly - i think.

[Ray White]

Thanks gents. I will follow your advise and see if I can find the fault tomorrow.

It would be ironic if there is a fault with the (expensively) restored cut out because for 20 years I was happy with just a heavier gauge "hot" resistance wire.

[Bob Culver]

The resistance wire only affects Summer charge. If failed will be none. As above, Sounds like worn, stuck or oily brushes.
The contact points are silver and should not be abraded with anything that might embed. If must be trimmed a clean fingernail file applied very gently ideal.
As previous posts the basic test of a dynamo is to isolate, link F and Output and connect an old 12v headlamp bulb between the link and earth. The bulb should glow fully at lowish rpm and get very bright as revs increased. if you have an analog meter should show 6v at revs below normal cut in revs,  and exceed 12v at gentle engine revs.
Such antics often safer with the fan belt off.

[Ray White]

Today I had a chance to take a closer look at what is going on and the issue seems to be with the ignition warning light. When I switch on, the light comes on as normal. When I start the engine the ignition light remains on until I increase the revs and it goes out when the ammeter switches over to charge as normal.

The difference is that the light stays OFF as revs are reduced and doesn't come on again even when the ammeter shows discharge. If I turn the engine off and switch the ignition back on again, the light comes on again as normal.

All I have done so far is clean the cut out contacts with a piece of paper. The cut out points were just a little dirty but nothing to speak of.

[Peter Johnson]

There can’t be a problem with the ignition light. It is just a bulb wired between the battery and the dynamo which lights up when there is a difference in potential between them.

The symptoms you describe suggest that the contacts of the cut out are not being thrown apart as the voltage at the dynamo diminishes. I figured out what was going on once, but have subsequently forgotten.

Peter

ETA. Take a look at http://www.austin7.org/Technical%20Artic...refresher/

There are two coils in the cut out. Coil from D+ causes the cut out to be pulled in. Current then flows through the points from D+ to A charging the battery. A second coil in the path between the points and A will kick open the points if the current flow is reversed from A to D+.

[Bob Culver]

The light comes on when the contact opens. some reverse current is necessary to trigger that , ie volts somewhat below battery. usually only ocurs at very slow idle . Even then the light responds only to difference in dyn and battery volts and will only just glimmer . When or if dynamo stops producing competely glows bright.
 The reverse current is not easy to read on the ammeter as it is additional to the coil current.
 When switch off, after cutout opens, the stilll spinning dynamo feeds the light which is earthed via the ign coil so the lamp responds to difference between dyn and earh and the lamp  is quite bright for a jiffy whilst the dyn is spinning.

If light reluctant to go out suggest low dyn output, but then would expect noticeable lamp at very slow idle?

[Ray White]

If the cut out points are not separating when the voltage is reduced what can I do to fix it please?

[Peter Johnson]

What is the condition of the battery? Has it been charged with a proper maintenance charger? I suppose that a low battery voltage could cause the contacts not to be opened. The cutout is a balance of the forces from two solenoid coils and a set of springs.

I suppose that a cut out that engages too early may also exhibit the same problem. An experiment to establish the cut in voltage may be beneficial.

Peter