Dynamo fuse?

[Graham Honnor]

On my way home from the Bromyard Speed Festival the ignition light suddenly came on very brightly and
then went out and the ammeter showed no charge even though it had been on the side light setting (therefore
winter charge).
When I got home I found the fuse had blown in the cut-out, which probably is the reason for the lack of charge, 
but I'm not sure yet why the ignition light came on.
I shall replace the fuse first and then start looking into the bright ignition light, but I cannot read what the fuse
is supposed to be.
Could someone therefore please tell me what I need to get and also what the other fuse is which is also in the cut-out.
The charging system is bog standard '37 Ruby complete with 35M dynamo and CFR cut-out.
Any pointers to where I should be looking for the unruly ignition light would also be very helpful please.
I'm guessing starting looking at the brushes would be favourite but if not where next please?
Many thanks as always
Graham

[David Stepney]

The field fuse should be 6 amps. The other fuse is the auxiliary fuse and should be 25 amps. If the ignition lamp came on very brightly, this means that the dynamo was trying to charge through it. If the field fuse goes, the cutout cuts out.

My first thought would be that something has gone wrong with the dynamo. If the field fuse blows, then the field coils are drawing too much power.

The field coil resistance should be 1.7 ohms or thereabouts. If it is a lot less, then you have a short.

Hope this helps.

[Graham Honnor]

That's perfect David, many thanks indeed.
I'll pick up a few spare fuses tomorrow and start investigating the cause.
Don't be surprised to be hearing from me again though, electrics aren't my
first language.
Thanks again
Graham

[daveg]

This great wiring diagram by John Cornforth might help:-

http://www.da7c.co.uk/technical_torque_a...ing_v3.pdf

[Graham Honnor]

That really is useful too. 
Thanks for that Dave.
Graham.

---

I'm just looking at available fuses and there are slow and quick blow fuses.
Which should I be using please?

[David Stepney]

If the dynamo needs attention, you could do worse than get in touch with Trevor Fawcett. His email,address is: trevazit@hotmail.com. He overhauls Austin Seven electrical items as a hobby. He has just overhauled my (spare) distributor for me as I was so impressed by the standard of the refurbished one I bought from him on ebay.

[Graham Honnor]

Thanks again David, I'll keep a note of that.

[David Stepney]

Trevor is currently advertising his C cab van on Car and Classic. His phone no: is on the advert.

[Bob Culver]

On the later cars the field fuse seems to protect only the field resistor. It is only in circuit on Summer charge. An intermittent connection beween cutout input, including the contacts,  and battery would cause the bulb to glow bright and likley blow a moderate rated field fuse if on Summer.  Rating is is not critical. With all ok  should still charge on winter without fuse (removal a convenient way of switching off dynamo on long day trips). All assumes your car is later wiring. Does the ign bulb still work?
The standard checks for dynamo operation covered recently and many times before.

[Graham Honnor]

Hello Bob
Many thanks for your input.
It was on the winter setting when things went wrong. 
Firstly I noticed the light came on very brightly and when I looked at the ammeter 
it wasn't showing any charge.
I switched it to the summer setting and back again but the bright light continued so 
I returned it to the summer setting and left it there until I got home.
The first thing I did was check that the ignition light came on when I turned the ignition on,
which it did, and it was it's usual dim glow.
I then looked inside the cut-out, which has only done around a 1000 miles from new, and noticed the 
small fuse was blown.
At this stage I decided to buy some new fuses and will then start checking other things.
I do know that I need to adjust the 3rd brush as it does charge at around 10 amps and has
consequently boiled some water out of the battery a couple of times in the past when I've forgotten to 
switch it back to summer.
I normally only have it on winter mode for 5 minutes or so after I have used the starter, so
that it can replenish the battery a bit. Well that's my way of thinking anyhow.
I shall fit the fuses in the next couple of days when I have time and then do the usual checks on
the dynamo.

To be continued...

Graham