Dynamo behaviour

[melanddoug]

Hi,

I wonder whether any of you good good people can help ..

I'm new to moggies and haven't dealt with a car with a Dynamo before, so I'm not sure how they should behave ..

My girlfriend Mel's car is a 1971 moggy saloon, with dynamo, all unmolested and standard. It's lovely!
It's been standing 20 years dry stored and we're nearing MOT time (yay!).
I found the ignition light wasn't going out when we started it so I took the dynamo off and had a look at the brushes.. sure enough - one broken spring and worn brushes.
We replaced the springs and brushes with new ones, reassembled and put back on the car .. I didn't check/clean the commutator as I hadn't read about that at the time. Also wasn't sure whether to lubricate it or not, so left as-is.

Now when it starts, the ignition light stays on but only goes a bit dim (but still 'on') progressively as the engine is revved.
I'm pretty sure the fan belt isn't slipping.

How should it behave ? Should it go out after starting, like a car with an alternator ?
If so does this imply the dynamo is still not performing? A friend said it might be the control box (voltage regulator or cut-out switches) but I'd have thought that would give an "all or nothing" response on the ignition light.

Any advice most gratefully received...

Many thanks,

Doug and Mel

[bmcecosse]

Yes - it should go completely out. Have you swapped the car to negative earth by any chance ? Or it could be the control box - take the cover off and run some very fine emery paper (or wet/dry) between the contact points.

[melanddoug]

Hi,

It's still a Positive Earth car. I'll try cleaning the control box points. At least I know what to expect now.

Thanks!

Doug

[mike.perry]

Check that the brushes and springs went back correctly
Check and clean the comutator
A couple of drops of oil on the rear bush of the dynamo through the hole at the back.

[rayofleamington]

I'm not sure if the dynamo will need to be polarised again after standing 20 years - however if so, it's not difficult as you just short the terminal to the non-body side of the battery.

[beero]

If you want to check or adjust the control box see here

http://home.comcast.net/~rhodes/generator.html

[melanddoug]

I'm not sure if the dynamo will need to be polarised again after standing 20 years - however if so, it's not difficult as you just short the terminal to the non-body side of the battery.

1) Beero - briliant, that's filled in loads of gaps. Thanks.

Ray - I'm not sure what this means .. do you mean touch the main output (thicker) terminal to the not-earth (= Negative in my case, as the car is positive earth) side of the batter ?

Not sure what "Polarised" means in this context but I'll have a google-up and see what I can discover.

Thanks again for all your help.

[d_harris]

I have a feeling that the emery paper on the contacts in the control box would likely sort out the problem...

[bmcecosse]

It is an idea - that the dynamo may have lost it's polarisation over all these years. The field coils hold a little residual magnetism - enough to get the dynamo going, and then it should close the contacts and get the field current going - bringing the magnetic flux up to full strength. And yes - a wire from the non-earth side of the battery - and spark it on the SMALL terminal on the dynamo.

[melanddoug]

Well .. we were all right.
the brushes and springs were qorn
Repolarising the dynamo helped dim the ignition light bit more (so it did something!)
ceaning out the commutator helped even more
but leaning the regulator/cut out points did the bz.

one dynamo fixed - thanksloads to all!

2 x happy people

[beero]

Well done, what a good feeling when you cure it yourself, eh?

[melanddoug]

Yes - a case of "researched, found out how, fixed it."

luvvit !

[Dave36]

My kind of thread, thanks for all that!
my dynamo is giving me a light show every time I start the engine, but very little in the way of charge. I think the light show is arcing from the brushes.
Is there a painting by numbers instruction on stripping it down and replacing the brushes?

[MarkyB]

The long screws that hold the whole thing together can be VERY tight, I've resorted to a screwdriver whacked into the slot at right angles to get them started.

Once they are out the whole thing should pull apart.
If there is resistance tweak the springs up onto the brush holders to take the pressure off the brushes (useful technique).

If there is a lot of muck inside it will comprise of copper and carbon dust, both conductive, clean it off.

The surface of the commutator (the copper bit the brushes ride on) shouldn't be too horribly mangled and scratched according to the manual actually in practice it isn't that critical.

I've never seen one where the insulator is higher than the copper, and have given up mucking about trying to undercut it with a hacksaw blade like the book says.

To put it back together, install the new brushes without doing the screws up so tight that you bust a blood vessel, tweak the springs up onto the brush holders on the side you can reach when it's back together.

Put it back together, and use a screwdriver to lift the spring up till they bear on the brushes.

Don't use a screwdriver at right angles to do it up again!

That's the old blokes guide, so I may well have forgotten something, but it covers most of the bases .

[melanddoug]

The long screws holding it together are probably the most nervy part. If the screw shaft is corroded, I can see that they might break and you're left with a virtually un-rescuable dynamo.

One of the screws came out ok on our car, the other was a bit tricky so I used a wheel brace with wheel trim remover at other end. (bit like a huge cast iron scredriver about 1cm wide). That made life easier.

D
PS.
Mel's car has halogen headlights, plus the dynamo. It;s not on the road yet, but a thought has just occurred : I don't know whether Halogen bulbs draw more current .. Will a Dynamo be powerful enough to run Halogen lights ?

[MarkyB]

Be firm, very firm if necessary with those screws,they aren't invariable a pain, just be prepared in case yours are.
Do your best not to mangle the slots though, there is a certain creek you can find yourself up after you mangle any screw or bolt.

Will a Dynamo be powerful enough to run Halogen lights ?

It may be marginal at worst, it will probably come down to how much night driving you do.
Compare the wattage they need compared to standard bulbs.

[bmcecosse]

Section N5 of the Manual for servicing the dynamo (page N6). It really is easy (and cheap) to do. As above - don't get in to undercutting the comm! Halogen lights actually use slightly LESS current - so no worries there.