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MG MGB Technical - Alternator problem

My 77B recently suffered an electrical fire involving the brown wires from the starter motor to the alternator. There was evidence that this had happened before (the brown wires were actually red). I replaced or repaired all the affected wires and installed a new alternator.

With the brown wires to the alternator safely disconnected I turned on the battery disconnect switch and the ignition switch to verify the ignition light was working. Then I proceeded to install the browns to the alternator (forgetting to turn off the battery switch), and brushed a brown against the alternator case resulting in a large spark.

Now, with the battery switch off and all wires installed, I am measuring full voltage across the battery switch. At this point can I repair the “new” alternator by replacing a component or am I “fried”, needing another new one?

Paul Brown, Woodstock, GA
PAUL BROWN

Paul,
Brushing a Brown to the case shouldn't hurt the alternator as all you've done is short the battery.
Measure the resistance to ground on each of the alternator pins and if there is no direct short eg zero ohms, go for it and throw the battery switch.
But keep an eye out for any wires getting a bit hot!

Best of....
MGmike
M McAndrew

Thanks Mike,
With all connections to the alternator removed there is zero resistance to ground on the large terminal, not looking good!
Paul
PAUL BROWN

Usually, the problem you described is due to a battery being installed backwards. This frys several wires going from the alternator to the starter motor solenoid. When you momentarily touched the outside of the alternator, with a brown hot wire, it shouldn't have done any internal damage to the alternator. RAY
rjm RAY

rjm RAY, you are correct, thank you.
The battery was removed to use in another car that had a flat battery. It was reinstalled in the B incorrectly! Expensive lesson learned!
Thanks again, just glad to get the problem solved.
PAUL BROWN

Regardless of installing the battery backwards, you will get a big spark if you touch a brown to the alternator casing.

Also with the alternator connected and a battery cut-off switch open you probably will 'see' 12v between the cut-off switch contacts, especially with a digital meter. These are very sensitive and reverse leakage of even good alternator diodes will be enough for it to display full system voltage. An analogue meter will typically show less voltage, and a test-lamp probably no voltage at all. Unless any of the circuits in the car are connected, like the courtesy lights from a door being open, in which case all of them will probably indicate that 12v is present.

With the wiring disconnected from the alternator it depends on which way round you connect your ohmmeter to the output terminal and the case as to whether you will see zero (or nearly so) ohms or infinity. An ohmeter puts out a positive voltage on one probe and a negative on the other. If the probes are connected so that the voltage from them is the same as the battery would be (if connected correctly ...) you should see infinity. But if you have the probes the other way round you will see zero ohms, for exactly the same reason that if you had connected a reverse battery to the output terminal you would have blown the alternator diodes. It's complicated by analogue meters typically having the positive voltage on the negative probe and vice-versa i.e. counter-intuitive, and digital meters having the positive voltage on the positive probe and the negative on the negative. Complicated even more by digital meters probably having to be switched to their 'diode' setting before you read anything either way.
Paul Hunt

This thread was discussed between 27/10/2015 and 28/10/2015

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