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MG MGB Technical - OOPS

Recently purchsed a 67 BGT. My brother was being helpful and wanted to get the car running. Knowing that the 67 was a positive ground vehicle, he was careful to hook up the battery in the correct polarity.

Unfortunatly, he did not know the vehicle had been converted to a negative ground system complete with alternator and electronic ignition.

Damage appears to be to the electronic ignition pickup. Looks like I can get a replacement for the part damaged.

Anything else I should check prior to hooking up the battery again?

Just a reminder to those who have made this conversion, help out the next person and remove the Positive Ground stickers. I am going to build a plate for the battery box that states car was converted to Neg Ground.
Bruce Cunha

There are decals available from Moss and Vicky Brit to indicate Negative Earth.

FWIW

rick
rick ingram

Someone did the same thing to my car once. The tachometer was fried.
Steve Simmons

Hi Bruce.

The alternator may have been damaged.

Don
Don

Just read this thread.
Bit scary about frying instuments etc.

My B has had it's battery out for 5 years.
It's a negative earth (was converted fom positive). Has also had the double batteries converted to a single 12V.

Can anyone tell me how do I determine which connection from the car to the battery is negative, and which positive?

My B will get a new battery soon... And after reading this thread, I'm not keen to fry instuments such as the tacho (which has just been reconditioned).

I'm no auto electrician... so any tips would be gratefully received.

Safety Fast!
Mark
Mark Duggan

Thanks all. Heading out to California on Saturday to pick up the GT. Hopefully, the dist. was the only damage. I will order the neg stickers.

I think buying these is a good idea,

Mark.

Should not be a problem. All the batteries I have seen (at least in the US) have a + next to the positive terminal. Some say POS. on the case.
Bruce Cunha

<<Can anyone tell me how do I determine which connection from the car to the battery is negative, and which positive?
>>

If you mean which wire on the car...
The negative will connect straight to the body shell. The positive will go to the starter solenoid.

Stoo
Stuart Robson

The warning *should* have been in the different sizes of the battery connectors, but it is possible to force the small one onto the big post, and they may not have been changed correctly as part of the conversion anyway.

There is no certain-sure way of telling, although if it has an alternator it is a fair bet that it is negative ground, although I have heard of people modifying the internals of an alt and using it on a positive-ground car!

Anything electronic is likely to have been damaged by incorrect connection, which includes the alternator itself which would have tried to pass a very high current, which may also have damaged its wiring to the solenoid, and the tach.

On an unknown quantity, or after a rewire, I use a high-wattage 12v bulb i.e. a headlight bulb in place of the battery ground strap. With nothing switched on this will glow at full brightness if the battery is connected incorrectly but prevent any damage to the alt. Another trick is to use a standard fuse, which should not blow on anything except cranking currents, but again will blow immediately on an alt equipped car with the battery the wrong way round. However neither will prevent damage to things like radio, electronic ignition etc. if they do not have their own protection against incorrect polarity.
Paul Hunt

The positive terminal on the battery is the LARGE one - by about 1/16" If you still have a generator & regulator, starting the car with the battery in the wrong way usually blows the points off the reg cut-out.
When converting from pos or neg earth use a jumper wire from the battery to the generator terminals to repolarise it. On a 67-71 B the tach (model RVI) has a loop wire so connecting the battery the wrong way dosen't hurt it usually.
Garth Bagnall

The early electronic tach *pick-up* is a loop, but the 12v supply is directly connected, and is is that which is liable to blow the internal electronics if reversed.
Paul Hunt

About 6 months ago I bought a mostly restored 67 MGBGT that had been sent by the previous owner to the local British Car expert for some work to make it driveable. When I picked the car up I noticed the tach did not work along with some of the lights and some other problems that should have been rectified. To cut a long story short after I had driven 150 miles home and was starting to check things over I notice the new battery had been installed negative ground. The original radio also did not work.
Now the dilemma. Was the tach changed to negative ground? Did the radio actually work? The new coil was hooked up for negative ground. The previous owner could not be contacted.

So I disconnected all of the polarity sensitive stuff and reconnected the battery positive ground. Repolarised the generator, rewired the coil and turned on the key. I touched the disconnected positive lead to the terminal on the tach and low and behold the needle pulsed. Next I replaced the radio fuse and turned it on and music started playing. Next I fired the beast up and everything worked. The only thing more relieved than me was my wife's Visa card. Maybe Lucas electrics are more durable than we think.

Dennis
D. J. Broad

Dennis! You are the man! You fixed the MG without letting any of the smoke out of the wires! Kudos!

I only wish I could so lucky as you. I neglected to disconnect my battery upon re-installing the radio on my MGC. The radio body shorted out on the support...I instantaneously had a wire frying from the radio upwards...fortunately, I "removed" the problem prior to my wife spraying the interior with Halon.

I have since decided to drive sans radio (albiet BMC original AM only)...and to be honest with you.I DO NOT MISS IT!

rick

rick
rick ingram

This thread was discussed between 14/04/2004 and 17/04/2004

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