Dynamo not charging

[Tatenlyle]

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

[David Stepney]

The most common cause of dynamo failure is worn brushes.

[Tatenlyle]

Can the brushes be worn and still knock 8v out? Could it be reversed polarity with all the flat batteries, or could the cut out be faulty. How can I test it

[David Stepney]

The answer is that yes, a dynamo with worn brushes can still produce voltage, but not sufficient current. The quick way of testing to see if the brushes are in good condition is to slip the band off at the distributor end of the dynamo, and inspect the brushes visually.

[Bob Culver]

I was becoming concerned. We had not had this enquiry for several months!
Analogue voltmeters are often in error, but digital ones go haywire near running engine. The coil should draw about 4 amps, half that running. Presumably the 10 amp discharge is with lamps on. Even the later dynamos should not be adjusted to more than balance 10 amps.
When prodding the cutout beware not left locked down.
Has the battery ever been fully charged? For all unmodified Sevens and especially cars not in regular use battery optimum maintence is a bit of a trial. especially if sealed calcium grid battery. A modest summer charge rate backed by a permanet bench smart charger probably ideal.
Do you have any battery charger? The ideal bench charge ritual depends on the charger capacity and the type of battery.

[Tatenlyle]

Thank you both. The 10 amp discharge is when I turn the key. Could be a wiring fault. Now I put a new bush in the dynamo last year, I may not have got the 3rd brush rightly adjusted, would that matter

[David Stepney]

The dynamo should give about 8 volts if it is in good order. Test it by disconnecting the wires from the field and dynamo connections on the dynamo itself. Replace with two lengths of wire to each terminal on the dynamo, one to the D connection and the other to the F terminal. Link the free ends together and put the linked ends to one side of an analogue voltmeter (as Bob has said, a digital instrument will be affected by the electrical noise from the HT side of the ignition). Connect the other end to earth and start the engine. AS the engine is revved, the dynamo should start to show a voltage.

If it does and it rises to more than 8 volts, then the problem is going to be either in the wiring or in the cutout.

[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.