dynamo testing

Post by **phil63** » Wed May 16, 2007 12:42 pm

How do I test a dynamo with only a wire and bulb . I cant remember if I test between the terminals for a light or join the terminals together and test beteen them and earth
Regards Phil

Post by **steve4063** » Wed May 16, 2007 1:41 pm

i'd like to know also as my old unit the ignition light was on i had to really rev it to get it off.

i swapped it for another dynamo and now the light goes straight off oncew engine is running

Post by **Alec** » Wed May 16, 2007 4:10 pm

Hello Phil,

a simple test is to short the field and main terminals together then apply 12 volts to the shorted terminals and earth. If It's OK then it will spin (motor). Not if you still have the fan belt on though

You can test continuity with a bulb and wire but that doesn't prove that all is well.

Alec

Post by **bmcecosse** » Wed May 16, 2007 4:51 pm

Usually dynamos just need new brushes - unless the bearings have obviously had it. The 'motoring' test is good - expect quite a spark as you touch the wires and a fair kick from the dynamo.

Post by **8009STEVE** » Wed May 16, 2007 7:10 pm

An old test was to start the car and switch on the headlights. Rev the engine and see if the lights increase in brightness. If yes, then charging ok. If no then not.

Post by **bmcecosse** » Wed May 16, 2007 8:35 pm

Aye, but in this case the red light was said to be not going off - so obviously not charging. It's the brushes!

Post by **steve4063** » Wed May 16, 2007 9:35 pm

ok so i think its the brushes hopefully.

whats the nack of getting them screws out of the side then

i've actually broken the bit on my impact screwdriver and they still haven't buged

Post by **Alec** » Wed May 16, 2007 9:42 pm

Hello Steve,

if you are talking about the screws that hold the field coils then there is a special jig to do that. The frame fits in a cradle then the driver bit is fed down on a screw. There was a hand wheel to then actually turn the screw. The driver was kept under pressure so that it couldn't slip out.

But why on earth would you want to remove them?

Alec

Post by **alex_holden** » Wed May 16, 2007 9:42 pm

I don't think you want to undo the side screws. You only need to take the back end plate off to get at the brushes.

Post by **Willie** » Wed May 16, 2007 10:07 pm

If you are talking about the two screws which hold the end plate on they have large heads which actually protrude from the plate enough to get mole grips etc on them if they are proving difficult.

Post by **bmcecosse** » Wed May 16, 2007 10:25 pm

And DO NOT try to remove the field coils!!

Post by **steve4063** » Thu May 17, 2007 7:08 am

oops ok so i was trying to remove the wrong ones

i'll have a look at the end plate then

thanks

Post by **bmcecosse** » Thu May 17, 2007 8:46 am

It's a roadside repair - popping in new brushes. I have done it at the roadside!