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MG MGB Technical - Starter resistance readings

MGB 1966 model standard starter.

Can someone please indicate what sort of resistance I should read as follows on my starter motor.

Commutator segment to adjacent segment(s)

Commutator segment to armature (IE Earth)

Are all segments interconnected or only those opposite via the winding.

I am sure I know the answers but my readings are quite bizarre.

My starter has decided to stop playing and when I stripped it down I got wildly different reading from segment to earth and segment to segment. Sometimes the segment to segment readings were open cct except for the opposite segment and sometimes they all seem to be connected together. Sometimes the segment to earth is 40 ohms, or 4K ohms or even zero. Any consistent reading would be fine by me whether it is zero or open, at least I'd know what the problem is, but which readings are correct.

Cheers
Tony
Tony Oliver

To all intents zero, since the starter takes a couple of hundred amps under load. Ohms law, R=V/I, 10/200=0.05 ohms). In practice it is probably lower than this as the cables are also limiting the amount of current that can flow to some extent.

Opposite segments must be joined together or no current would flow and the rotor wouldn't turn. But there is also the stator to consider i.e. it depends on whether the brushes are still connected and resting on the commutator as to exactly what happens. The brushes will span more than one commutator segment, and in practice there are two windings connected to each segment - the end of one and the start of the next, to give a smooth current and torque and no dead spots where the motor wouldn't run.

But unless the brushes are actually worn and can be replaced, if the motor isn't spinning and the solenoid *is* connecting 12v to its external terminals, then it is bust, and there is little point looking further except out of interest.
PaulH Solihull

Thanks Paul. The armature is out of the starter and I was just trying to check as much as possible before giving in.

When the starter was first removed I put it on jumper leads but it turned very slowly. I cleaned up the commutator and tried again and got a faster rotation and on installation it started the car once and then failed again.

I am sure that any segment to earth should be infinity but it was the segment to segment that had me as I have always thought that each winding was independent of the other.

If I am reading you correctly when you say "there are two windings connected to each segment - the end of one and the start of the next" then it seems that the "apparent" short from one segment to all the others is correct. I interpret that statement as meaning the armature winding is effectively one long single winding not multiple short ones. This is a great help as it did seem a bit unlikely that every winding was short to every other winding.

Back to the shed and I will report back when I find the problem.

Tony
Tony Oliver

From what I've seen yes some motors at least do effectively have one single circuit chaining through all the segments and turns of wire. And you are right their should be no continuity from any of them to the armature they are wound on. When that happens it's usually because it has been overheated and 'burnt out', which has burned the insulation away from the copper, a classic case of 'letting the smoke out'.
PaulH Solihull

Thanks again Paul. It was the knowledge that the windings were effectively one that gave me the clues that led me to the fault.

The insulation on one of the wires to a brush was dodgy and I repaired that but the real fault was a short to earth in the armature. Needing to get the car mobile as my second car is on loan a used a method I learnt long ago when I first started playing with cars.

I poured enamel paint (it should be winding varnish but needs must) down the windings from the drive end of the armature and left it overnight to soak through to the commutator end. It's about as rough as you can get as far as repairs go but it worked and the starter is working as advertised.

Don't know why it shorted and don't really care as long as it lasts for a few days so. It gives me a bit of time to source a replacement. Many thanks for the info.

Tony
Tony Oliver

Tony, your '66 model would have the "throw out" type of bendix starter motor.These get clogged up with carbon dust from the worn brushes.I've found in the past that a good clean out in this area works wonders. My wife had string of Minis in the seventies & this was the first thing I always checked when they had starting problems. Barrie E
B Egerton

"I poured enamel paint (it should be winding varnish but needs must) down the windings from the drive end of the armature and left it overnight to soak through to the commutator end."

Impressed!
PaulH Solihull

This thread was discussed between 21/01/2012 and 23/01/2012

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