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MG Midget and Sprite Technical - Coil white wire burn out!

As a reasonable novice midget restorer, I am totally confused with an ignition problem. When trying to start my car for the first time, I soon developed a VERY smokey situation! I eventually found that the white wire from the coil to the tacho and the ignition switch had begun to melt, as a result, I assume, of the wire going to ground somewhere.

I have replaced the wire, including replacing inside the tacho, and have checked for any grounding through the system. None occurs until I link up both sides of the coil ( white to +ve and white with black to negative).

I have checked every route for the white wire and I am totally confused as to what will have caused the burn out in the first place.

I am not experienced enough to know what to do next!

Bob
R Clayton

Bob,

Do you have a multimeter to check things out? If you are sure that the short is not in the wire, then it must be in one of the three components involved, i.e. the ignition switch, tach, or coil.

Charley
C R Huff

Charley
Thanks for getting back. I have a multimeter and have used it to check continuity. The wire is completely ok. The coil is new, There is the expected measurement between the +ve and -ve terminals. There is no grounding between +ve terminal and chassis. The terminals on the tacho are fully insulated when checked with multimeter, as is the white wire terminal on the ignition switch.

Thanks

Bob
R Clayton

Hi Bob,

Which year and engine do you have? The coil and therefore wiring varies according to year and model so for instance mine is a 78. They only made 1500's that year and mine is standard. However there is a wiring diagram specifically for the UK 78 model and the best copy of it is in the correct drivers handbook.

Sounds like I'm off the wall I know but the coil and therefore the wiring vary between models and years. So mine is a ballista coil (more wires) to get more oomph when starting and has wires to the tacho as the tacho uses the coil for its input for rpm. They are not all like this. If some previous owner (PO) wasn't aware of all this then this could be the reason you and not they are restoring it.

Cheers, Dave
Dave Squire

I would check the wire inside the distributor to make sure you have no short to ground there. That would short the LT side of the coil as well and cause your symptoms.
dominic clancy

Have you also just replaced the contact breaker?
In which case you have almost certainly assembled the insulating washers on the pivot 'post' incorrectly and it is shorting directly to earth. This will overheat the entire +ve side of the ignition wiring, much as you describe.
Guy Weller

Another possiability is the wire is worn or chaffing on the metal body somewhere, creating its own ground

Have you put any screws, self tapers,or drilled into the fire wall area


And look for spiders inside the tacko.... yeah, you dont want to know and we aint going there...hahaha


Prop
Prop and the Blackhole Midget

The 1275 midget is 1971. I installed electronic ignition, but have decided to go back to contact breakers after the burn out. Can anyone recommend precisely what I should be checking with the multimeter and the distributor?

I've checked for grounding on wires and that is ok. Insulating washer on the post is fitted ok, but I am not sure about what checks to make!

I obviously want to fully check things out before furring up again!!

Bob
R Clayton

Even if the lead from the coil to the points is grounded, it still shouldn't burn out the +ve (ignition side) side wiring.

When you switch on points ignition, often the points are closed, thereby grounding the -ve side of the coil. The coil will energise and warm up, but you can leave it like that for some time before the coil will be damaged.

I'd be looking on the ignition side of the coil for the problem.

By changing back to points, and removing the electronic ignition, you may well have solved the problem. I.e, you may have wired the electronic ignition wrongly.

What electronic ignition have you got?
Lawrence Slater

I purchased an Accuspark module to fit the existing distributor. On looking up the information and the module components , I notice the red wire going to the +ve terminal on the coil from the module had been pinched through when fitting the distributor cap. Could that have contributed to the burn out?

Thanks
Bob
R Clayton

Sounds like the culprit to me. The wire on the +ve side of the coil goes to the tacho, then through the ignition switch then (maybe through a fuse then) to the battery +ve terminal. If you trapped the red wire enough that the insulation was breached and it earthed on the dizzy body you effectively have a short from battery +ve to ground. Many amps make wire burn. The bit in brackets above probably is wrong as it would make the fuse blow before anything got hot enough to smoke and cut off the current.

Rob
Rob aka MG Moneypit

As a precaution, till you have solved this by pass the tach with a piece of wire, or the tach will act as a fuse and burn its self out

Id also disconnect the wire at its junction at the right side A piller that goes to the fuel pump as it shares the same circuit

Id also agree with the pinched wire... its along the same concept I was refering to above

It might be a good idea to pull the wiring apart under the dash to make sure none of it has melted or worse melted togather...with it all tightly wound, compacted, and taped togather, the risk of hidden damage is much higher

Prop
Prop and the Blackhole Midget

Bob,
>>I notice the red wire going to the +ve terminal on the coil from the module had been pinched through when fitting the distributor cap<< - this has been the cause of electronic igniter heads going faulty before but most owners blame the unit rather being honest about the cause

but I can't remember seeing any where it was also reported that it caused a burn out
Nigel Atkins

Nigel, how do you know, if they don't 'fess up?
Dave O'Neill 2

Thanks everyone

I've done a check behind the dash an fortunately only the white wire was damaged and not enough to affect the adjacent wires.

So if the break in the red wire was the cause, I guess I can begin to put everything together again. I have installed a new white wire from the coil, through the tacho and to the ignition switch.

With everything in place, including the new contact points, can anyone recommend a sequence of continuity testing of the ignition system prior to starting up?

Thanks

Bob
R Clayton

Dave,
by reading their posts and seeing their photos - and a few do fess up later

many of course put the CB points back in which makes everything right again and no need to admit to anything - that's if there was anything to admit to of course

some have even admitted to wiring some wrong so that the electronic unit expires

I think few of us like to admit to some of our mistakes myself included of course
Nigel Atkins

This thread was discussed between 10/07/2014 and 11/07/2014

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