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MG MGB Technical - Battery connected wrong way

I installed a new battery today after the old one was no longer holding a charge. In my haste I connected it the wrong way. I realised my mistake when there was a spark (fortunately no explosion) and then smoke appeared from under the bonnet.
I reconnected it the correct way and noticed straight away the ignition light is on when no key is inserted. I insert the key and switch the ignition on and the light goes off. The car started okay, so at least the new battery has solved that problem.
No fuses have blown. The alternator is burning hot.
It seems obvious but I will ask anyway. Have I blown my alternator?
Thanks in advance.
D O'Brien

The alternator and the radio plus possibly electronic ignition! Charging light on without ignition shows 12v direct from the battery bypassing the internals of the alternator. But someone on here will give you the definitive answer!
Allan Reeling

fuel pumphave blown the diodes in the alternator and possibly the regulator as well. If it
R.A Davis

Depending on the year of the car there's capacitors in the tach that won't be happy about that and might have blown.

Max, If he didn't start it would that be an issue?
Allan Reeling

Check the wiring that goes from the alternator to the starter motor solenoid. Usually, when the battery is misconnected, the two brown wires will melt and fuse with the red wire. Repairs are fairly simple and straightforward. However, the alternator will need to have the diode trio replaced as well as the voltage regulator. RAY

Don;t forget to pick up a quart of Lucas Smoke, you definitely let some of it out. Abd as mentioned, yup Alternator is dead.

But don't feel bad, last week I dropped a wrench on a battery in the diesel locomotive I am helping get going. It fried the alternator. And a 24 volt alternator is a lot more than an MG.
Bruce Cunha

Turns out I blew the alternator and a fuse in the radio. Strangely the diode pack was fine. I was hoping that was the problem because I have a spare one and I know how to wire it as I've replaced one before.
I didn't check the regulator because I decided to just buy a new alternator.
Last one I bought I decided to save some cash and get an aftermarket. This time I got a Lucas genuine for almost the same price because it had no pulley or fan. I swapped those over from the broken unit.
So my stupid mistake ended up giving me a slight upgrade. All good now.
Just to be safe I painted a red + and a black - on the battery so it's easier to see next time.
D O'Brien

"If he didn't start it would that be an issue"

Power is applied with the ignition, so more likely if there was going to be a problem. But paper capacitors don't mind polarity unlike electrolytics, and I don't think there are any of those in the tach. It's transistors that would be more likely to fail.


Actually there are one or two electrolytic caps in certain years of the Tach. I know because I replaced mine. :-)

I was surprised as well. AFIK, the tach is in the circuit once there is power. I can't comment with 100% certainty, however. Pull the tach and open it up. If there are electrolytics and the tops aren't blown off like a fire cracker you're ok. OTOH, they're 50 years old and caps are dirt cheap.

All other things mentioned here are accurate. Diodes are like fuses. They're safety devices designed to either create certain voltages or prevent spikes from harming other components. They are one-way devices.

D. Good to hear!

One comment on the alternator. Having owned my car for all of my adult life its been a daily driver. There was a real ligitimate British Leyland electronics service man who had moved stateside. Every few years I would bring in a starter or alternator to be rebuilt.

The final year he refused, telling me he's sick of me going through this and he was putting in a French model and I had to shut up and take it!!!

Its never given me any trouble and just out of habit I have it rebuilt every couple hundred thousand miles. There are plenty of replacements these days and if you drive your car regularly its worth it. If its only for weekends and occassionaly trips the Lucas should be fine as well as original.


Maybe he wasn't doing such a good job. So far I've only had one alt failure - diode pack - since the early 70s, and that was an 'infant mortality' on a new car. I've had the V8 solenoid fail from proximity to the exhaust partly caused by an incorrect heat-shield i.e. plain tin rather than heat-insulating.

Good point. I had them rebuilt by many vendors in those days. His seemed to last longer than others. After all these miles I've come to know the car and learned that one of the best mechanics is one who knows and owns the car.

Wish the internet was available a lot sooner. Its was a desert out there previously and good information was near impossible to come by. Yes, workshop manuals. Yet not boars with people like you all here to bounce problems/solutions off.

A lot of the time the fault lies in the quality of the parts that the rebuilders are getting. I had to return the alternator, in my '75 MB, four times before the tech finally got it to last more than a week. RAY

Oh you are so correct Max. I just finished refreshing a 79 Spitfire. Had it not been for the internet, I would have either spent thousands of dollars figuring out issues or had to scrap the car for parts.

This wealth of information and technical data is got to be one of the greatest parts of the internet.

I can be working on my car, pull out my phone and have specific information or diagram of a part. Something that would have been near impossible without the internet.
Bruce Cunha

Nice to know I am not the only one to do this. My case (71 B) it was diodes and new bearings for fun. Every spring now it is a big plus sign on the positive post, and a double check on the tach for negative earth, and wonder how I could have been so stupid. Hopefully this is one of those things you only do once.
G Nicholas

This thread was discussed between 19/04/2017 and 01/05/2017

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