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MG MGB Technical - disaster

wife drove my mg b 1978 run fine. put it in the garage. about 10 days ago dead battery. 0.0 volts. took battery out went auto zone to try chrager but dead battery. new battery now. when try connect put one post. when tryed the second big flash of sparks. took both cables off. resistance check ground starp to from frame 0.0 ohms good. check from the battry cable that goes to engine area. 0.0 ohms. looks a dead short wire some place. the car ran fine when she shut off the engine. no one touched the car. any ideas to look for it.

earl
RE HALLE

Earl,
assuming all the various switches (lights, wipers, heater etc) are in the off position, I would pull the lead off the alternator and check again.
Then pull the fuses and finally check the overdrive cables if you have one.

Best of...

MGmike
M McAndrew

Sounds like a dead short on permanent un-fused leads. First check the main battery lead to the starter. Then the brown leads from the starter motor which are always live. The brown from the starter to the alternator sometimes short out on the alternator body.(not common with plug in type) The brown also feeds the starter switch which can fall apart. All purple leads are always live, check the lighter socket these can go dead short, also the horn wire at the top of the column, it can get worn through.
Allan Reeling

Hi Earl, Bit of a problem. You will have to isolate the fault to find it. So get a wiring diagram and it appears you have about 5 brown wires plus battery feed going to the starter motor. This feeds the headlights, starter relay, fuse box and 2 wires to the alternator.
Disconnect 1 at a time and check each time if the fault has gone to isolate the faulty circuit. Start with the main battery wire at the starter motor and then the brown wires this will also prove the starter motor is not faulty.
Also test each time in both directions.
1 on the wire removed to see if the fault is there.
2 on the terminal it was removed from.
One of them should still show the fault or it has been disturbed and gone so examine that area and no need to go any further as nothing to find.

When you do find the faulty circuit you will need the wiring diagram. Try to pick a point in the diagram about half way along the circuit, Isolate that point to determine which half the fault is in then half the faulty section again to narrow it down.

Good luck Ken


K Stuckey

Sorry, having brain fade, purple wires ARE fused hence shouldn't be an issue.
Allan Reeling

Earl. Hook up a test light between the negative terminal of the battery and the disconnected cable clamp. The light will illuminate if there is current flowing. Then begin your disconnecting of the various wires brown wires at the starter. There should be three of the smaller brown wires and the large cable coming from the positive terminal of the battery. Attach the large cable to the terminal, then touch each of the other wires, one at a time, to the terminal. The test light will illuminate on the wires that are showing a load, with the bulb burning brighter with a heavy load and barely illuminating with a light load. The problem you describe would seem to indicate a heavy load, most probably an internal short in the alternator.

Les
Les Bengtson

Hence the question about battery polarity?

I assume you connected the battery the correct way round i.e. negative earth. If not then the alternator will look like a dead short, although only with your ohmmeter connected one way round, it should be very high resistance the other. Connecting a battery the wrong way round may well have blown the alternator diodes such that it shows a dead short either way round.

If you get the flash with the ignition off then disconnect the alternator as previously advised and tap the earth connector onto the post with the positive already connected if you don't have a test light to connect in place of the earth strap. If you still get a flash then the problem is in the brown wires so you can ignore the overdrive wires. Disconnect the browns and the battery cable at the starter solenoid and try again. If still a flash it is the battery cable. If not reconnecting one at a time will tell you which is the problem, and you can go from there.

It may well have been this fault that took the previous battery, in which case I doubt it is a very major short or the car would almost certainly caught fire when it occurred.
Paul Hunt

Hi just wondering been about a week now have you found the fault yet. Ken
K Stuckey

This thread was discussed between 01/01/2015 and 08/01/2015

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