Testing a dynamo

[Nick]

What is the best way of testing a dynamo before putting power to it, or is there not much to go wrong with them? I have one on my bench which hasnt been used in 20 years or so.

Apparently it was working when removed.........

[henk]

You can easily check the inside by removing the rear cap. Check the rotor, rear bearing bush and coils.
The front roler bearing can be checked by feeling.
If all things look right, you can test it on the engine.
There's a topic about it.

[Pavel]

Having checked it mechanically, as Henk suggests, fit it to the engine and start it. Increase the revs to about half way and place a volt meter across the battery terminals. In good condition you should get a reading in the region of 13volts. Less than that it's either on its way out or the regulator is faulty.

Pavel

[super6954]

Hi Nick
As Henk said mechanical is easy to check. for function there is an old school bench check I do before it even gets on the tractor . Hold the dynamo in a vice. I join the 2 terminals together that the wires fit on, on the dynamo.
Then with a fully charged battery, take a jump lead cable from positive terminal to the 2 terminals on dynamo that are connected. then ground the other terminal to the battery and dynamo case, be careful not to short the live terminals to ground, you get a big loud cracking sound, like lightening and blue flash .

If the dynamo starts to spin on it's own like a motor, chances are the dynamo is good, and will charge right. Have had some here that a guy needed to turn by hand to get them to start to motor, those ones didn't work on the tractor . in those cases I found one that started to motor on it's own put that on and got the charging working.
when you put the good dynamo on you also need to polarise it for either positive or negative ground/earth system, to get it to work .
This has to be done or it will cause a non charge issue even on a new dynamo . There are posts here already about that if you don't already know
Regards Robert

[Nick]

Thanks for that robert, will try that tomorrow! I will also give it a visual inspection before I do anything.

I should of explained, there is no wiring loom on the tractor at moment, which is why I want to test it to make sure its going to work before I blow anything up, or catch anything on fire!

I have read about polarising them, will have to find that again as I want to make it negative earth, mainly because on our farm, batteries seem to go walkabout and if 'the others' re-connect it, I can guarantee it will be connected negative earth even if there is a huge sign saying 'positive earth' on the battery terminal!

[oehrick]

I know well enough that glasspaper is fine for cleaning commutators but emery / silicon carbide papers are not, as the abrasive embeds in the copper and continues to wreak havoc on the brushes.

Does anyone have positive or negative experiences using the finer grades of Scotchbrite on Copper ?

My next job is the remove and renovate the dynamo and I'm always reluctant to skim a true com if I can avoid it and stock of fine glasspaper running low........

[Nick]

Well it spins on its own, so hopefully its ok! I couldnt get the two screws undone, they felt like they would snap if i really tried, so I dont know the state of the brushes.

Also, something interesting, I put my multimeter across the F and D terminals when it was still, and it had almost complete continuity. When I span it clockwise the resistance went up to about 600 ohms, and when span anit-clockwise it went minus 600 ohms, so I guess this means it must be doing something!

[super6954]

Hi Nick
I think you're in luck then When we took over a farm in wales, the last owners grew oil seed rape ( called Canola in Canada) they had a shear knife that went vertically on the end of the combine header to clean cut the swath end. this would stop the crop pulling and wrapping round the reel. That had a dynamo connected like this and a belt to drive it.
I guess the motor starting on it's own was good with this as you wouldn't want your fingers near it, trying to make it motor .
Kinda think that's why this test works so good as an indicator . Many years ago I read guys used this idea in ride on battery powered trains for kids as well, so nothing new really.
Just putting the application to a different use in tractor repairs.
Never tried the ohm meter thing, but guessing it's to do with clockwise and anti clockwise current flow . only part i understand is they either work or they don't, getting 2 university degrees above rocket science for me after that. .
Regards Robert

[Pavel]

You might like to have a look at this site, Nick:
mg-cars.org.uk/imgytr/pdf/lucas.pdf
It tells how to check out a dynamo using a multi-meter when fitted to an engine -- but with the 2 leads disconnected so can obviously be done without a wiring loom in place.
The bonus is that, when all wires are later connected, the whole charging system, including the regulator, can be optimised.
To remove the long screws in order to remove the endplate and access the brushes and armature, either use an impulse screwdriver or lock on a pair of self grips. The screws are high tensile so are difficult to break.
Incidently, I've always used 800 wet and dry dipped in metho to clean armatures and never had a failure in the decades I've been doing it.

Pavel

[oehrick]

You might like to have a look at this site, Nick:
mg-cars.org.uk/imgytr/pdf/lucas.pdf

Pavel

Thanks for that Link Pavel, I'm about to start looking at my lack of charging too, last time was about 25 years back so a read through will be an excellent refresher , also good to hear you've never had any problems using fine SiC wet&dry paper on comms.

[henk]

[quote="Nick"] I couldnt get the two screws undone, they felt like they would snap if i really tried [quote]

Nick. The end plates are aluminum and the screws are steel, with a bit of rust and paint they can be hard to get loose. Put it in a vice. Get a good screwdriver were you can put a spanner on and give it a few good taps with a hammer on the screwdriver to shake the screws. Than press hard on the screwdriver and use the spanner to turn it. They will not brake, but feel like twisting because they are long screws

[oehrick]

Having fitted a shunt between Field and Output terminals on my '58 (Mk2) Major dynamo, I am delighted to find that it is working fine and I have a quick and nasty way of bumping a bit of charge into the battery (if I remember to disconnect it after a few minutes running) !

Conclusion is dirty regulator contacts, dead regulator or wiring betwixt. I don't recall ever having the instrument panel off and going purely by photo's I have seen, would this be the same (or similar enough) regulator as used in the S1 Landrover or Morris Traveller ? if so I should have a newish spare somewhere and it might be quickest to swap, I'm a little reluctant to move the panel about much as the temp / pressure gauge capillaries may have work hardened given the age - sods law predicts a crack - have others had a problem with this or should I be OK ?

Opinions welcome please

On the downside, still have a water leak which doesn't appear to be hose or pump based - there goes the never removed nosecone status !

Seems likely my hedgerow Nuffield M4 may have found a new owner to restore it