Field Coil resistance

When you say "the resistance of the instrument" is that with the probes connected (ie should be zero ohms)? That is normally minimal and when measuring things like field coil resistance I would just ignore it. What reading do you actually get on the field coil (your connections are correct assuming nothing else is connected eg regulator / cutout)? Aree you sure you're using the right range?

That's an enormous internal resistance. I'd try another meter. My cheap (<£10) meter shows something like 0.1 ohms with the probes connected together.

Previously George Phillips wrote:

Events moving quickly. I was misreading the meter. The Ohm scale runs in reverse. Doh! So the resistance of the meter was around 0.3 Ohms. Nevertheless I carried out a motor test only to find it was spinning in the opposite direction to the arrow on the drive casing (there is no label on the dynamo). I've reversed the leads on the brushes and it now spins in the same direction (anticlockwise) as that arrow. I'll do further tests tomorrow. Thanks for your help, Ian.

It might be a good idea to re-read page 185 of your" Classic M/c Electrics Manual"as having changed the leads around you may need to repolarize, It seems to depend on whether you have + or - earth when back on the bike.

I've had that problem before but I think it's down to not spinning it fast enough hence not enough current to excite the field coil - especially if it's been idle for a while as the field will lose its residual magnetism. I find it useful to connect a 6 volt battery across the field windings then spin it and measure the voltage from the D terminal. Only for a short period of course........

If your dynamo has not been working all the time you have had the bike, try some basic dynamo maintenance.

Make sure the brushes are in good shape and slide freely in their mounts, and they are under some spring pressure holding them onto the commutator. Look at the commutator - is it in acceptable condition and clean? Find a clean lint free cloth and press it to the commutator and turn the engine over to clean it

A dynamo which has not been working for years will not have the residual magnetism needed to excite the field coil and generate current. Make sure the battery is holding a charge, and flash the dynamo. It's very easy to do. Remove the two leads from the dynamo. The lower terminal is for the field coil. Take a close look for a letter F Take a wire from the battery feed side and touch it a few times to the field terminal. Expect a spark. After a few touches, you should get a small blue flash. The job is done.

It's worth doing a dynamo output check. Make a short lead to connect the D and F terminals on the dynamo. Get a volt meter, put the earth probe on a good frame earth or the dynamo body, and the feed probe in one of the dynamo terminals - it does not matter which. Now start the engine. At idle, you should get around 2V, and at a fast idle, around 7 or 8 volts. Do not rev the engine..... you can damage the dynamo if you do.

Now reconnect the D and F wires to the dynamo. Do you have a charge from the dynamo?

Paul

I'd have thought that if it motors well the field coil is in decent condition - if it's really zero resistance it wouldn't do that. I'd prefer an original to some of those available which can be of dodgy provenance.

Is your new field coil 6 V or 12 V ?

Paul

Interestingly, my ES2 dynamo was showing only a tiny voltage when spun with an electric drill when I checked it yesterday. I looked at the brushes and both were looking glazed, so I used some fine glasspaper wrapped round a bit of dowel to remove the glazing.

I then got a healthy 8 volts when spun (D and F connected).

It's not something I've previously thought about (the resistance across the commutator ie between the brushes was about 1.5 ohms with the brushes in their original state) but a nice quick and cheap fix.