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MG Midget and Sprite Technical - HT Bits

After all your excellent previous help and much archive reading I've done the ignition fault find and my lack of spark is definately the coil / coil to dizzy HT lead.

(the coil and dizzy low tension circuits all check out with correct voltages and coil does not get hot if the ignition is left on for a short while, no spark when coil test done independent of dizzy but HT leads old as well as is everything on the car so could be leads).

So: I looked on Moss and MGOC for new HT leads and Coil to find many options. Any help with selection etc. would be appreciated. FYI short term plan is to keep points, longer term plan is to use car as part time daily so will want to install electronic ignition once on the road.

Car described in accompanying profile link above (S reg 1500 standard set up off road for 10 years)

Dave of the still not quite going but getting closer midget.
Dave Squire

you've checked the points?

for the HT leads set I fitted some from perfomanceleads (Fast Lane) not cheap but good quality and not as high priced as many others -

coil standard one for your model/year

I'm a fan of igniters heads but you still have to oil the old probably worn dissy so if you can replace that too, the 123 electronic dissy is great

if you fit an electronic igniter take care with fitting, the leads are delicate inside dissy and out, see (two and a half minutes in) -
Nigel Atkins

If your HT leads are old and decayed it is probably just as well to replace them. But as a check of the low tension side, measuring the voltages at the coil is only a part of the story.

A quick and very simple test is to remove the dizzy cap, switch on the ignition, and flick the contact points with your thumb nail or a small screwdriver blade. You should get a good spark showing. If you don't, then there is still a fault there - probably with the contact breakers. Have they been replaced ? - in which case do make sure that the insulating washers are all in their rightful positions.
Guy Weller

Right, I have got the points out and it looks like the low tension side connecting wires may have been touching the base plate of the points (causing a short). Previous owner put LT connector to points assembly on the wrong side of the spring causing a short to the points plate. (Guys help - and the owners handbook - Nigels - was the key, shows the connection in detail so I could compare).
Now have a spark - unfortunately it happens when the ignition is off and the engine turning from a wire from the battery to the starter solenoid - does that mean that I have an ignition switch fault? Or does the solenoid power give me power to the coil as well?
Doesn't help that I work on the car when alone.
As always a step nearer and yet further away however undersealed drivers floor and drivers repaired wheal arches this week so thats coming together as well. Just a bit desperate to get engine to go so all comments valued.

Thanks in advance.
Dave Squire - Notts

well done on finding shorting

pity PO did have and look at a copy of the good book :) :) :)

that wire would be for a test situation depending on how you've done then yes it's unswitched (ignition switch) so is live from the battery

I don't know enough to go further with you so best to wait for replies from Guy, (old uncle) Bob, FRM and others
Nigel Atkins

Its an easy and common mistake to make, with or without the good book!

Anyway, making progress which is good.
But I am not clear - are you saying that you can get a spark when you use a jump wire to the solenoid, but not when you use the ignition switch? If that is the case it suggests to me either a faulty ignition switch, a break in the wiring either to the switch or between switch and coil, or possibly a hidden immobiliser switch set up to earth the coil.

If yours is a later model 1500, the supply direct to the coil from the ignition switch is at the reduced voltage (ballast resistor coil). Whereas the supply to the coil when the solenoid is activated (starter turning) is at the full 12 volts. You should still get a spark either way, but it may appear a bit weaker if just via the ignition switch.
Guy Weller

I was probably saying I was so euphoric that in retrospect I am not sure exactly what position the switch was in. On the job again today so will be checking my work and reporting back.

The spark by the way seamed good but when like me you've not had to look for a spark or seen one for - mmm - a long long time (like 30 years or so) all sparks look good!

Anyways its onward; I am pressing on with more this afternoon!

Thanks all Dave
Dave Squire - Notts

Having gone and checked:

1) I do get a spark when the ignition switch is off and the key out when remote starting with a wire from the positive to the solenoid to turn it over from under the bonnet. I will go under the dash and look at the wireing. The horn has been rewired to a seperate switch so this could be the problem. I will check ignition switch if I can work out how to.

2) I am not sure if the HT leads (which I numbered before replacing) are connected to the dizzy in the correct order as I now have no confidence in the PO's work. I know the cylinders fire 1, 3, 4, 2 and 1 is at the front (next the rad) and 4 is to the rear (next the gearbox). It seems from the number of sparks I am getting that the order they are connected to the dizzy may be wrong. I need to know which HT lead is connected where on the dizzy cap so I can rule this out as a reason for not starting. (I seem to remember seeing a top of cap diagram somewhere but can't find it).

Back to it, will check this thread frequently today if anyone has ideas.
Dave Squire - Notts

obviously it depends on you have the firing order correct by here's FRM excellent photo of HT leads cap to plugs

your cap might be at a slightly different angle and you might have an overlong HT lead set but as you'll see from photo the HT lead order is traight forward

Nigel Atkins

ETA: remember firing order goes anti-clockwise as you look at the cap
Nigel Atkins

Thanks Nigel, thats great.

My underbonnet looks complete crud compared to the one in the pic.

Mutter: work harder, work longer, .....

Its the same! very good to know.
Dave Squire - Notts

easier access for cleaning/servicing on a Spitfire
Nigel Atkins

Looking at a wiring diagram for late 1500's with ballast coils, the reduced voltage for normal running (using a resister wire) is fed from the ignition switch.

Starting circuit (coil gets 12volts) is fed from the starter solenoid - so by using the solenoid, current goes to the coil also - only for the time the starter relay is energised.

You should be able to test both ends of the wire to prove as noted above. Shown as white/light green tracer on diagram.


richard boobier

Thanks richard,
That explains the two voltages arrangement as the higher voltage is from the solenoid while starting and the lower from the ignition switch when the starter has been disconnected. I can clearly see the resistor on the diagram on the ignition switch side and now understand the two wires on the plus side of the coil.
Also allows me to clearly identify the wireing diagram to use in the Haynes manual as mine is a very early 78 registration and I have been wondering exactly which diagram was for my car.
Now marked diagram with label to ensure no messing in the future.
Dave Squire - Notts

Your description in 1) is exactly as it should be.
"I do get a spark when the ignition switch is off and the key out when remote starting with a wire from the positive to the solenoid"

Wiring direct to the solenoid will also provide a feed to the coil whilst the solenoid is energised (starter turning) But that feed will cut out the instant the car fires and the starter motor stops. That arrangement is just a temporary means of operating the starter motor in cases when the ignition switch or wiring is faulty.

You need to check that you get a spark with just the ignition switch on (engine not turning over). Turn ignition switch clockwise to the furthest click position before the starter setting (i.e the sprung setting). The red ignition light should glow, but may be not so obvious in bright sunlight. At this position you should have power to the coil and should also be able to get a spark across the contact breaker points if you remove the dizzy cap and flick the points with a small screwdriver. If you don't get a spark with this arrangement, then you have an ignition switch or wiring fault.
Guy Weller

Hi Guy,
Yes! and yes! I get spark at points when ignition turned to running (not starting) position. (that is ignition on starter solenoid not engaged)

Checked voltage of this with tester (tester checked directly on battery and reads 12V) and am reading 6V at the coil positive when switch in running position. So is this Ok do you think? Of course the battery has done a lot of work today so I will retest this voltage in the morning after a slow overnight charge. I seem to remember you saying in some thread it ought to be about 8 or 9V? Confirmation of voltage would be very helpful.

All looks good to me if the 8 or 9V reading is correct for the running setting to the coil in the morning so unless you can think of anything else I will get on with trying to get petrol to the carbs. (I will fetch some fresh and put in float chambers methinks. I have old good quality legal cans inherited with car with 10 year old petrol in and the guage for the tank reads about 1/4 full. Any advice on getting rid of old petrol?).

Then, Shell ordinary or V-Power this is the question?

Dave Squire - Notts

Ignition sounds OK to me.
You need to check the plug lead / firing order as others have said, identifying which lead is #1, connected to cyl #1 on the compression stroke and then working from there. Often the leads are in lengths such that there is at least one that will only reach to its correct plug, and then with that known, the sequence to the other 3 is set. The best is if the leads are numbered or marked in some way.

Yes, best to use fresh petrol for getting it started if only because it eliminates an unknown element. But as for disposing of old petrol - well I would use it in the car once you know that it is starting ok, topping up maybe 25% when you fill up with fresh. Better than wasting it!
Guy Weller

Thanks Guy. Thats been really usefull and helpful. Good to know all is where and how it should be. Will get some fresh petrol for use in float chambers tomorrow and go from there. And thanks for confirming my thoughts about old petrol being generally Ok.

Thanks again, I expect my fuel system will be my next thread.

Cheers, Dave (a bit nearer going 1500)
Dave Squire - Notts

No guarantees, Dave ! ;-)
Guy Weller

This thread was discussed between 25/06/2012 and 01/07/2012

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