Hello everyone,

There is pretty exhaustive coverage of things charging related already on here, but I don't think I have seen this covered.

Today I managed to get my dynamo charging again (yippe!), about 6amps at 40mph, which looks good to me. All it needed was a good clean - I'd put too much grease in the distributor gear when I put it back together last year, and this had oozed where it shouldn't have and mixed with a bit of carbon dust to get up to mischief with the brushes - all cleaned and sorted now.

So I was slightly annoyed to find my charging troubles had just moved up to the cut-out. It cuts in at fast tickover-ish and then stays cut in as the amps rise. If I then let the engine go back down to tickover, the cut-out doesn't cut out. If I manually pull the contacts apart they stay apart. If I stop the engine, the contacts do separate.

I got the voltmeter out, but don't really know what I should be doing with it. I measured the volts between the dynamo pos and earth terminals on the cutout box. With the contact open at tickover I get a nominal reading of less than a volt. If I then raise the revs the volts start to rise to about 3. Then the contacts close and the reading jumps to 6.5 (presumably I'm measuring the battery there?).

So, the questions:

Is it the rising volts or the rising amps that actually close the contacts (or is it in fact, as seems likely to me, watts?). To my limited scientific mind, the dynamo could be throwing out enough power to close the contacts without actually matching the voltage of the battery.

Can someone tell me how to test the operation of the cutout accurately?

Are there any adjustments I can carry out to the cutout once I have been able to test it accurately?

Thanks!

Geoffrey

PS Then once I've sorted out the cutout I'll get round to the half charge...and the steering...and the brakes!

Hi Geoffrey,

The cut-out has two windings. The first is a high resistance winding between the dynamo terminal and earth. As the engine speeds up the dynamo voltage rises until the pull-in value (e.g. 6½ volts) is reached and the contacts close. From here on the second winding takes over. This is a series winding connected between the contacts and the battery terminal. The magnetic field produced by this winding helps to keep the contacts closed, buts its main effect comes into play when the engine slows down and the dynamo output falls below the battery voltage. When this happens the current reverses direction, flowing from the battery back to the dynamo. This causes the series winding to produce a magnetic field which opposes the (weaker) field produced by the first winding, and this forces the contacts apart.

The symptoms you describe could result from the cut-out being connected the wrong way round, or from a negative earth cutout being fitted instead of a positive earth type (or vice versa).

Hope this helps,
David

Thanks David.

The car is wired negative-earth. The dynamo positive cable goes to a terminal marked as positive on the cut-out (and the field cable goes to a corresponding one labelled F). How would I tell if things are, in fact, connected the wrong way round? If the battery was sending a charge back to the dynamo, would it not register as a negative voltage on the volt meter (or am I now really showing my ignorance of elastic trickery?!). I'm not really sure where to start with this one!

Thanks again,

Geoffrey

Barry Riseley is good on these matters. Barry where are you? Bit concerned not to have had your input lately.
Ron

What model car and cut out do you have Geoffrey?

Hi Dave,

The car is a late 35-early 36 Ruby. The cutout is marked under the cover "model CER TYPE" with the number 332 above. There are six terminals: A, F1, Aux, E, +D, F2. Only "A" and the last three are connected to anything. +D and F2 appear to go to the terminals on the dynamo.

I hope that makes sense!

Geoffrey

Geoffrey,

I think the correct cut-out for your car should be a CFR2, the earlier Rubies were CFR, I'm not familiar with the CER type, although a CFR2 unit I have is part number 332151, (yours is 332). There's an excellent Ruby wiring diagram (CFR / CFR2) on the Dorset Austin Seven Club Site HERE. I think your horn and wiper should go to AUX, which you say is empty at the moment.

Jeff.

There is a colour version of the wiring diagram at the Essex Austin Seven website www.ea7c.com

Thanks both,

Both horn and wiper work...I guess they're not fused then, as well to know! I'll have a methodical look through the wiring diagram and see if I can work out where I'm going wrong. If I can't fathom the wiring diagram for an Austin Seven, that doesn't bode well for coping with more modern machinary!

Does anyone know what the physical difference would be between a pos-earth and neg-earth cutout?

Thanks!

Geoffrey

You have got me thinking about cutout polarity. In an electromagnet like the cutout voltage coil, current flow in one direction will magnetise the core with (say) a North pole at the top. This will attract a soft iron part like the cutout contact armature towards the core. Current flow in the opposite direction will magnetise the core the other way and give a South pole instead, but it will still attract the soft iron just as effectively. The polarity of the voltage coil of the cutout therefore doesn't matter. When we add a second (current) winding, this must have the correct sense relative to the voltage winding so that it helps it pull in when charging the battery, and helps it drop out when discharging. Since the charging current is in the opposite direction for positive earth, the correct sense is indeed maintained. The upshot of this is that the identical design of cutout works for either positive or negative earth. See my Ruby wiring diagram on the Dorset club site - no connection with the colour one on the Essex site.

I think we may be getting somewhere here. Thanks John for your very clear drawing - even I go some way to understanding it!

For a start I think I have a mis-match between dynamo and cut-out. There is no half-charge box on my dynamo but my cut-out is the earlier CFR (not CFR2) type. That might explain why the half charge switch has never worked....

When I get the time I think I'll print off the diagram and check everything is attached where it's meant to be.

Geoffrey

.

Just posted a bit under "Dynamo Problem" which may be relevant???


Mike

Re half charge . If your cutout resistance is burnt out You can restore the half charge by adding an external resistor. For details see Austin Seven Clubs Association website under 'Tricks & Tips'--'Split -charge Fix'.

A further thought about cut-outs....(I bet I'm amking myself popular!):

I have established that my Mk 1 Ruby has the correct cutout (CFR), but the incorrect dynamo (the later sort without the half charge shunt). It is wired neg-earth.

If I install a later CFR2 cut-out to match the dynamo, will the car have to be re-wired positive earth to match? Or will a CFR2 operate "backwards"?

The other option would be to get another (earlier type) dynamo to match the CFR cutout and the neg. earth polarity.

Just going through my options!

Geoffrey