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MG MGB Technical - Little help, Please

I have an electrical problem with my 74 MGB. I have checked the battery and it is fully charged but when I turn the key on the lights are dim and the Tach jumps to 5,000 revs. The starter will not engage and just makes a whirring sound. I have cleaned all of the grounding contact points and the terminals on the battery. To test the positive side I ran a jumper cable from the batery post directly to the starter and the same thing. I then ran a jumper cable from the negative side of the batter to the alternator mounting brackets and when I went to connect them I saw a small spark. So I have a short somewhere but have to believe the Tach jumping to 5,000 when I turn the key on is a clue, I am just not sure what type of clue. The car also has an after market electronic distributor. Can this be the problem? Any help would be greatly appreciated. We only have a couple of nice month here in Canada and I want to Drive. Thank-you
KCJ

It seems that you have a duff cell in the battery. Have it checked.
Iain MacKintosh

Your 74 is -ve ground so a wire from the -ve on the battery to any metal on the engine should have no effect. The spark sounds like the earth strap engine to chassis is open. Doesit spark to the chassis anywhere?
Stan Best

A wire from the battery -ve to any metal on the engine would only have no effect if the ground circuit is good. As there was a spark that implies the ground circuit is *not* good, which would account for the symptoms described.

Connect a voltmeter between the -ve battery *post* (not the clamp) and the block, and see what voltage you get when you turn the key to crank. Ideally there should only be 0.2v or so, but you may see several volts in this case. Although there is an engine ground strap that will affect cranking it won't affect the lights, so it is likely that your problem is where the clamp connects to the battery -ve terminal, the wire connects to the clamp, where the other end of the earth strap bolts to the body, or where the strap attaches to that lug.

Bad connections in the 12v circuit from battery to solenoid will have similar effects (except the sparking you mention) and I'd be checking that circuit with the voltmeter as well. And if it is a chrome bumper 74 i.e. with twin 6v batteries check the linking cable as well.

If you find a large volt-drop by testing between the battery post and the other points I mention you will be able to determine where the fault(s) lie. Bear in mind you may have more than one bad connection.
PaulH Solihull

Great suggestions Paul, Thank-you. I will try this tonight and see what I find
KCJ

This thread was discussed between 10/07/2011 and 13/07/2011

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