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MG Midget and Sprite Technical - Now turns, no spark, is it immobilized?

Hi Everyone,

The engine now turns, the starter works and turns the engine over (definite yippee this time). Plugs back in and there is fuel poured directly in float chambers. Tops on float chambers. (not sure if pump working, so what)

No spark at end of HT leads, no spark coming out of coil. (should there be one? from what I read I am thinking there isn't and the spark is made in the distributor by the contacts opening and shutting)

Immobilizer? can't find anything, the previous button I found was the horn that works just fine with battery on.

Your thoughts / courses of action would be much appreciated. Everything is as it was before I found the engine would not turn, looking grubby and old, the lights work etc. and the starter turns all the time it is asked, battery keeps going, (thats nearly new), the alternator is not being driven so I can get to plug 1, my thoughts are to check the voltage in to the coil, (need to get a tester or in line bulb for this), however your experience, insights and thoughts are still very much appreciated.


Thanks in advance,

Dave (the not quite going yet midget)
Dave Squire

with the ignition on do you have 12 volts at the input side of the coil? The output side of the coil goes to the dizzy, can you maybe rig up a replacement wire direct from one to t'other and try it again?
David Smith

David good check but it is a 78 and will therefore have reduced voltage at the pos of the coil when ignition is on, maybe 8 volts

Dave you could try a temporary connection from the battery pos to pos on the coil then see if you get a spark? it might even start, if it does rmove this connection to stop it. This is the way we hot wired cars when stealing them many years ago . :)
Robert (Bob) Midget Turbo

Hi David,
Hoping to have a tester here tomorrow, one of my work mates recons he can get hold of one.

Dave Squire

Of course Robert, I am just too long out of playing with cars. (proper cars that is that can be fixed).
Dave Squire

Bob of course 'ballast resistor' - 9v with ignition but 12v when actually turning over?
Tester? I just use a bulb with 2 bits of wire soldered to it, dim = 9v and bright = 12v :-)
David Smith

The "normal" supply circuit to the coil from the ignition switch has a resistor that reduces the voltage when the car is running. I think this gives around 8 volts. There is a second wire which goes between the started solenoid and the coil. This is live, giving a full 12 volts, only whilst the starter solenoid is engaged in order to assist with the initial ignition.

If a car fitted with this ballast system fires with the starter, but doesn't continue to run, then it is likely to be a fault with the permanent coil supply, whilst the supply from the solenoid is working. Often causing great puzzlement!

A possible immobiliser could be a hidden switch that is earthing the coil +ve supply side. The best thing would be to disconnect both the supply wires to the coil, and make a temporary supply direct from the battery +ve. But if it is fitted with a lower voltage coil it shouldn't be left to run for too long like this as it will overheat the coil.

Guy
Guy

A kill switch on the coil from PO could cause this, but it earths the contacts side (-ve for -ve earth) not the supply side. If it earthed the supply side it would cause a short circuit.

The the normal coil supply failure which causes the "fire only when starting" symptom can be caused by being the supply taken off the wrong contact of of the switching relay.

PS: Yes, with ballast resistor there is full battery voltage on the coil while the starter is cranking. Only at that stage it is usually only 8-10V due to the high starting current using the rest of the voltage to overcome the internal resistance of the battery. That's the whole point of the ballast resistor, it means that the coil is seeing its designed primary voltage all the time, especially when starting.
Paul Walbran

Thats all great; all your replies have been informative and of great value to my understanding of how it works and how to go about sorting it out.
Many thanks guys, will be having a go over the weekend.

Dave
Dave Squire

Quite right Paul. Hidden switch is on the -ve side, not +ve. That's why if I leave the ignition on with the kill switch in the immobilize position, the coil gets VERY hot! It's OK for a few minutes bu if left too long...


actually blew one coil up doing this! I had been fixing the headlights so had the ignition switched on, but it was also immobilised. I forgot and left it whilst I had some tea and the coil went bang - it popped the end off and as I had it mounted correctly all of the oil drained out. As I didn't have a spare I just pushed it back together. It still worked, but now with no oil. I bought a replacement but kept it as a spare as I wanted to see how long it would last. Some 6 months later it was still working without oil but I then decided it was time to fit my new one.

Seems they are more robust than one realises. I wonder how many perfectly good coils have been rep[laced just because the owner thinks they may be the cause of some other ignition problem!
Guy

This thread was discussed between 06/06/2012 and 07/06/2012

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