Alternator fault, not seen this one before!

SydneyRoverP6B

SydneyRoverP6B

Well-Known Member
Staff member
After a 265 Mile (427km) drive on Saturday, over which my Rover ran perfectly, the alternator then promptly failed. :shock: The ignition light remained off and the ammeter showed a discharge. The ignition light would also remain off when the ignition was switched to position 2, the opposite to what it is supposed to do!

Now the ignition light is connected in series with the field circuit within the 18ACR, so if the bulb fails, then the alternator will not charge. Checking the bulb showed it to be working correctly. The voltage regulator was swapped for another that I was carrying with me, again it did not fix the problem. Checked the brushes which were reasonably new, again they were fine.

I then removed the brush box so as to allow an inspection of the slip ring. All appeared to be normal, except when measuring the field resistance. A reading of 3.2 ohms is normal, but what appeared on the meter was a reading in the Megaohms :shock: .

I separated the two halfs of the alternator so as to allow a full inspection of the rotor, and then this was discovered.



As can be seen, the winding had broken.

The wire was duly soldered back together prior to sliding a length of heat shrink tubing into place. Movement of the wire was then prevented by squeezing it into place around the 'T' piece, which can be seen in the photo behind the break in the wire.

Measuring the resistance again this time delivered the correct 3.2 ohms. Today another 265 Mile run, and I am happy to say that the alternator performed perfectly!

Ron.
 
I would hazard a guess that the wire had the very smallest of nicks in the copper from manufacture causing a stress point which eventually fatigued the metal due to rotational force and heat from ambient temp and temp from the current very small that the current will have been.

I have seen it many times in electric motors that I have had to repair.

Graeme
 
Hi Graeme,

Thanks for sharing your experience, never realised that this could be common place.

Do you think having soldered the two pieces together, the wires side by side over say 5mm or so, that this would be sufficient to keep the rotor running for some time to come?

Ron.
 
Really depends how immobile the joint is, solder does not make for a strong mechanical joint and it will fracture or go "dry jointed" over time, thats not to say that it wont last a few years or potentially more. If neither end has any movement ( before rejoining) then it will last longest.
Were it mine and I wanted not to revisit it at some time in the future I would mechanically secure the joint by 1 of 2 methods.

1: lay the two wires side by side with sufficient overlap , more is better, then wind some smaller gauge copper wire ( pre-tinned if you have it) multiple times in a close wound fashion ie no gaps between adjacent wires over the 2 side by side wires ( single layer is fine) then solder the combination with sufficient heat and time to ensure the solder flows throughout the joint and to the original joined wires.

2: if there is not sufficient wire length available for overlap a copper crimp joiner can be used, I would then likely heat and solder the crimped joint. I would not pre-tin the wire ends before crimping as this may cause a higher resistance joint over time. You would also not want to use to much heat if you choose to heat and solder the crimp as this I would guess de-tension the crimp a little.

Graeme
 
Hi Graeme,

Thanks for those suggestions. Fortunately, the location of the wire now prevents any movement, with the soldered join held firm.

I am hoping that it won't give any problems well into the future.

Ron.
 
Throwing the armature wires off commutator segments is a common enough problem that a sign of quality in an electric motor armature is epoxy potting of the wires at the commuter end. They are under quite a bit of stress.

Though in this case I'd favour a nick and fatigue as the root cause.

Yours
Vern
 
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