Could someone please advise on how to safely test the functioning of a CAV cutout. I am rebuilding a 25 Chummy which has been unused for years. I would like to test the cut out whilst it is currently off the car, but do not want to take any risks which may result in damage. Thanks.

Location: U.K.

Paul,

Start with a visual check of the connections underneath the cut-out, and check that pressing the moving piece on top of the cut-out makes the contacts close satisfactorily. Unfortunately I don't know what the 'pull-in' voltage for these units is, but you may be able to use a fully-charged 6 volt battery and a couple of wires to test it. Connect the negative terminal of the battery to the body of the cut-out (please note that a common cause of poor charging is the cut-out being insulated from the bulkhead, usually by too much paint). Take a lead from the positive terminal of the battery and touch it onto the left-hand (I think) terminal of the cut-out: the contacts should close. If they don't, gently press the moving piece on top and the contacts should close quite easily. Removing the wire will cause them to open again.
Testing the other winding in the cut-out on the bench needs a rather more complicated set-up – it's probably easier to do this on the car.

Any large sparks probably mean trouble – for safety you could connect a 10 amp fuse in series with one of the leads.

If any problems, please call.

David

Hi Paul

The cut in voltage is somewhat above 6v. The post “Bench Testing a CFR2 cutout” may be of interest. Some have drawn attention to the Cornwall club site info on cutouts. The low resistance winding must not be fed direct from a battery as will draw many amps and destroy the contacts if closed.

This info is from me who experts claim "knows nothing about Sevens" so use carefully.

Location: Auckland, NZ

According to "Lucas Workshop Instructions for Motorcycle Control Box MCR2" the cutout should cut-in at between 6.3 and 6.7 volts and it should drop off at between 4.5 and 5.0 volts with a reverse current of between 3.0 and 5.0 Amps. I see no reason why the CFR2 cut-out should have any significantly different specification from it's motorcycle equivelant.

Ian Mc.

Location: Shropshire

Perhaps wires have been crossed here - we're not talking of Lucas CFR2s, the question was about the C.A.V cut-out type E on a 1925 model - and I seem to recall my auto electrician saying there were differences on mine. I'd presume Paul with his experience has seen something so to be asking the question? Cheers, Bill in Oz

Location: Euroa,Victoria

Bill, thanks for your comments, the difficulty I am having with the CAV type EA from my 25 Chummy is to identify which terminal goes to which winding. For testing the operation of the cutout one would obviously connect an earth to the negative on the 6v or whatever source, the question is then, which of the two terminals do I touch the positive source without risking damaging a winding. Perhaps I'm being over cautious but CAV cut outs don't grow on trees any more. David Cochrane makes a suggestion and whilst I don't doubt his judgement all printable comments would be welcome.

Location: U.K.

If, IF, your cut out is the kind (aren't they all?) with the two windings I can't see how you could damage it Paul as long as you keep the negative (or the positive) grounded. Application of the other pole on the battery side will do nothing at all (no current because open) and on the dyn side will activate (or not depending on the voltage of the battery) the relay. The parallel winding is some tenth of ohms so not much power implied.
Hope this helps.
Renaud

Location: Windy, misty, Brittany

If it is this one



Then use a multi meter to buzz out the connections. D goes to the contacts via the fuse and A goes to the other contact. Ground will be the case of the cut out.


Else, if it's this one



Then the fuse is in the switch panel. The diagram shows a winding to ground attached to CD (D) which can be measured using a multi meter. The CB (A) winding is only connected when the contacts are pushed together.


In principle, connecting only to D and Ground should pull the contacts together, connecting to A and Ground should push them apart.

I use a variable voltage power supply to check the pull in voltage according to the values (approximately) in the red book.

Peter

Thanks everyone for your input, think I'm sorted now.

Location: U.K.