Dynamo not charging

[Stuart Giles]

Hello again. Wanted to go to Tesco today, thought I would take the 7. Battery down again, and its just a new one. Voltmeter says 5.8v. Has a battery isolator fitted. Gave her a crank, tested the voltage from the D, and its 7.8-8.1v.

At that voltage the cutout should operate; even if only briefly.

The ammeter shows a 10a discharge when I switch on, and wont rise above 0a.

As Bob says, you shouldn't be seeing anything near 10A discharge from the ignition alone.

I clicked the points on the cut out shut, the ammeter rose from 6a discharge to a 4a discharge. Cant see the cutout working. Any clues would be appreciated.

The cutout has two coils on the bobbin. The current coil (the one you can see the big fat windings on) and the voltage coil; this has much finer wire and has always been wound (and therefore hidden) on the inside of the current coil on all the ones I've looked at. The voltage coil is connected from the Dynamo input on one side and to the chassis earth on the other. The current coil is in series when the cutout is operated -i.e. it's connected via the cutout contact from the cutout input to the output.

This is what should happen re. the cutout: When the voltage from the dynamo rises with revs, the voltage coil operates the cutout connecting the output to the ammeter via the contact. Now the current coil has positive (going towards the ammeter) current going through it, so along with the voltage coil, it provides the magnetism to hold the contact shut. When the revs drop, there isn't as much voltage going through the voltage winding, but on its own, it still won't release the cutout contact -But also, as the volts drop the current flow reverses in the current winding i.e. it's now going towards the dynamo; so now the two magnetic forces are in opposition, so the cutout releases. When you rev the motor, the sequence is repeated.

A good earth for the cutout is important, otherwise the voltage coil can't work. If the voltage coil is dud, and you operate the cutout manually, it will often stick operated because the opposing magnetic force from the voltage coil will be missing. When testing a cutout off the car, there should be a resistance reading from the cutout's dynamo input terminal to earth (this is the voltage coil reading) there should be a near short circuit reading from the cutout's dynamo input terminal to the output terminal when the contact is pressed in with your finger. Any resistance reading you see here is that of the Current coil; if these readings are both OK, the cutout should work.