Dead dynamo?

[davidpidge]

I need to start by saying I have zero knowledge of electrical systems, car electrics and all that pixie magic.
Anyway..
I was driving along the other day when the engine cut out and I rolled to a stop. Ignition light on but couldn't start with a pull (altbough it did make a few feeble attempts at cranking over). Started easily with a jump from another car and ran fine so turned out the battery wasn't charging.
Assume its the dynamo but how do I confirm? I put a voltmeter on the 2 wires coming out the back of the dynamo and got 0.1v which went up to 0.3v when engine revved up. Battery reading 11v and no change with engine running.

Next question...
The car has positive earth (1956 car originally 948cc engine but swapped for 1098cc engine a few years ago.) Should I have changed anything after the engine swap in relation to the battery earth? Can that damage the dynamo?

Cheers.
David

[svenedin]

To test your dynamo connect one lead of your voltmeter to the D terminal at the back of the dynamo and the other lead to earth. At 1,0000 rpm engine speed there should be about 14 volts.

If your dynamo is good the other possibility is that that the voltage regulator is not closing its contacts to allow the dynamo to charge the battery.

Also worth checking the basics that the battery terminals are clean and the contacts tight

[davidpidge]

Thanks for the reply. Battery is new with new cables and connectors etc all checked and tight.
Can you visually check the regulator is working? I see a little thing that looks like a switch. Will that close when charging?

[svenedin]

I am an electrical idiot so not well placed to advise you. The workshop manual advises how to test the voltage regulator. The contacts can become dirty and sometimes it will not work properly until the contacts are cleaned. Also worth checking that the fan belt is not slipping on the dynamo pulley.

Stephen

[Peted7202]

Have a look at this

https://www.howacarworks.com/electrical ... r%20tester

[davidpidge]

Cheers that's very helpful.

[geoberni]

Let's answer your points in order.....

I need to start by saying I have zero knowledge of electrical systems, car electrics and all that pixie magic.
Anyway..
I was driving along the other day when the engine cut out and I rolled to a stop. Ignition light on but couldn't start with a pull (altbough it did make a few feeble attempts at cranking over). Started easily with a jump from another car and ran fine so turned out the battery wasn't charging.

Correct. The reason it cut out was probably because the battery was not charging and had got to such a low state it couldn't provide enough power to the Coil for a decent spark to be generated.
It must've been really low and I'm assuming you didn't go far after the Jump start.

Assume its the dynamo but how do I confirm?

That link that Peted7202 provided is a good basic guide to fault finding.

I put a voltmeter on the 2 wires coming out the back of the dynamo and got 0.1v which went up to 0.3v when engine revved up.

That's not going to help, what you're doing there is measuring across the Output and the Excitation Field Input.

Battery reading 11v and no change with engine running.

As already established

Next question...
The car has positive earth (1956 car originally 948cc engine but swapped for 1098cc engine a few years ago.) Should I have changed anything after the engine swap in relation to the battery earth? Can that damage the dynamo?

Changing the engine makes no difference to the dynamo polarity. If it's always been Positive Earth, then that what it is.

There's a number of possible reasons for not charging, from the Dynamo itself to the Voltage Regulator/Control Box.
See how you get on following that 'How a Car works' link.

[davidpidge]

Turned out it was just the brushes. They were wedged in with corrosion. Bit of WD40 on the springs and a clean up and they brushes moved freely. Dynamo giving 14.4v now and didn't have to spend a penny.

[geoberni]

Good to hear you sorted it.

[svenedin]

Turned out it was just the brushes. They were wedged in with corrosion. Bit of WD40 on the springs and a clean up and they brushes moved freely. Dynamo giving 14.4v now and didn't have to spend a penny.

Marvellous! A minor miracle