Discharge/charge

[Nick Salmon]

Could an Electrickery Wizard please give me a little guidance. c35M dynamo and CFR2 cutout on my RP.

For quite some time I have been struggling to get the third brush adjustment sorted. The cutout pulls in and lets go at suitable revs. The ammeter shows a rising amperage as revs increase. However, on summer setting I can't get the amps up enough, and on winter setting I can't get the amps down enough.

I suspect it may be something to do with the following:

When the ignition is switched on (engine off)  the ammeter shows a discharge of nearly 15 amps. Switch off, rotate the dynamo slightly via the starting handle, switch on - normal 1 or 2 amp discharge. (May need to do this a few times until the 'normal' discharge shows).

Leave ignition on, rotate dynamo slightly - 15amp discharge again.

I've not seen this on any other of my Sevens so can anyone shed light please? Is it a bad segment on the commutator?

[John Cornforth]

Hi Nick

It sounds to me as though the cutout isn't opening, allowing current to feed back into the dynamo. This needs to be sorted out before any other investigations.

You could prove a point by sliding a very thin piece of insulation between the cutout contacts to make 100% sure of no conducting path. If you still see a heavy discharge when you turn on the ignition with the engine stationary, that would point to a wiring fault.

[Nick Salmon]

Thanks John. Have done as you suggested and the discharge is still showing. (I also substituted a different cutout with the same result). So now I will investigate the wiring.

[Bob Culver]

Hi Nick. 
 
I presume you have a copy of John's Ruby circuit diagram, the first step in any diagnosis and a must for all owners. (Saves drawing your own.)
You have me with that problem. If current was somehow getting back to the dynamo a good commutator would make steady contact. it is easily examined. The dyn can be isolated partly or completely,  and in latter state explored with an ohmmter coomparing with the resistances  in the diagram. (beware battery volts with ohmmeter)
I presume the field is connected to the 3rd brush and that it is not set up as 2 brush with the third ornamental.

[Nick Salmon]

By a process of elimination I have determined that the 'ignition on' discharge is only happening when the points of the distributor are closed. I have substituted another distributor and get the same discharge.

Would be very grateful for guidance on where to go next...

(p.s. I have removed lights connections to the ignition switch just in case they were causing an issue)

[John Cornforth]

Hi Nick

My Ruby diagram is here:  http://www.da7c.co.uk/technical_torque_a...rubies.htm

Apart from wipers etc the previous generation A7s are very similar.

If the discharge is CB points related, that really only leaves the circuit through the ignition coil primary.

With the points closed, the standard A7 coil draws about 4 amps.  Once the engine is running, the duty cycle is about 50% so the current then averages out at about 2  amps.

Your 15 Amps sounds mighty high.  Has someone changed the coil to a non-standard low resistance one ?

Try temporarily disconnecting one side of the coil LT to prove that the 15A is indeed flowing through it.  If you can, check the primary resistance of the coil - the standard coil is about 1.5 ohms.  To read this accurately requires a decent quality meter.

Is the ignition warning lamp behaving normally ?

Are the brake lights off (sorry, but I have to ask the obvious !)

[Nick Salmon]

Thanks Bob and John. I have pretty much proved that it is the ammeter itself that is at fault. Shall get a replacement, try it out, and report back.

[John Cornforth]

Hi Nick

I'm just wondering if you have the type of ammeter with terminals, or the type that has a turn or two of cable clamped to the back. If it's the latter type, then too many turns will produce more magnetic field for a given current, and thereby make it over-sensitive. Sorry. I can't remember how many turns it should be.

[Robert Leigh]

I recollect that it is a single complete loop.

[Nick Salmon]

John/Robert

The ammeter on there at the moment is a two terminal version.