MG Car Repair: 1972 2M100 Starter

Question
Hi Barrie,
It's been a while... my MG been running pretty good last 2 seasons until now.

My starter started to get touchy, especially when I haven't driven in a while.  It would just click once as it engaged, but wouldn't turn over. Got lucky by turning the key a few times.

I decided to take it out and apart for a cleaning... turns out one of the brushes is shorter than the others, I suspect this was the issue.

Problem now is, I get it all back together, and testing it before putting it back in, all I got was sparks.  (I tested it without the Solenoid)  I then tested the resistance on the terminal, which showed 0.  is this Expected?

But what is really puzzling to me, is I am testing the side of the Armature where it touches the brushes, again, I get 0 resistance touching any 2 contacts (commutator), I would have expected them to be insulated from each other.

Maybe this is expected... I don't think I damaged anything, just used Brake Cleaner to clean the Armature and Brushes.  Ideas? Suggestions?  Are my tests all wrong, and should just put the solenoid back on, try it again?

Thanks

Answer
Hi Joe.

The armature has many windings, and each winding terminates at 180 degrees from where it starts.

Each winding should have approximately the same resistance, and the windings should all be insulated from each other.

I cannot remember how many windings a 2M100 motor has, but if it has say 24, then there are 12 windings.  

The resistance of a starter motor winding will be VERY small, so your ohm meter is probably not sensitive enough to measure it.  We are talking less than one ohm.  To give you an idea, if you run the motor under no load conditions it will take about 70 Amps.  That would indicate a resistance of about 0.16 ohms including the field coils.   

Test the armature by measuring the resistance of each pair of commutator contacts.

Basically they should all read about zero ohms.

If any pair is open circuit, then the whole armature needs to be rewound or replaced.

If they all appear OK then you can reassemble the motor and connect a battery to it, but expect sparks as you make contact because you are basically shorting the battery.

By the way, the workshop manual says that the brushes need replacing when they are worn down below 8mm in length.