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MG Midget and Sprite Technical - Ignition Problems

Hi can any one shed any light on the following problem,

Having recently removed the engine for my 1500 Midge, for a rebuild, I also removed the solonid and coil, I was careful to make notes of the wiring connections.

Now having put every thing back I have the following problem.

Coil permanently live once battery connected, engine starts and runs fine, but cant switch it off even disconnecting the battery she still runs, all other electrics work fine but only when the ignition switch is on, which is correct.

Any suggestions as to cause welcome

Thanks
B Lodge

When I wired my ignition switch wrong because I forgot to note connections or take photos the coil got hot

but bearing mind mine is a 1275 and I was moving the connections round on the switch to work it out and I can't remember if I started the car until the end but it cerrtainly didn't run by itself that I can remember

oh, I confusing stuff again mine was the switch yours might not be

Someone will be along soon that'll know
Nigel Atkins

For the engine to keep running the coil MUST be fed with a permanantly live wire. first check the colour of the wires on the 2 coil connections?
Robert (Bob) Midget Turbo

As Bob notes, for the engine to keep running, the coil must be receiving a full time source of power. Removing the battery from the system will only allow a running engine to run off the alternator with possible damage to the alternator. If the input wire to the coil is constantly live, there is a real possibility of an electrical fire if the engine stops when the current can travel to ground. (With a points type dizzy, if the points are closed, current is flowing to ground. With other systems, the effect is the same.)

Disconnect the wires going to the coil and, with the ignition switched off, test the wires for current. If there is current present, there is a problem up stream of those wire connectors. DO NOT PUT THE WIRES BACK ON UNTIL YOU FIND OUT WHERE THE CURRENT IS COMING FROM. INSULATE THE ENDS OF HOT WIRES AND LEAVE THEM DISCONNECTED UNTIL THE PROBLEM IS FOUND.

If you have a hot wire, with the ignition switch off, get a copy of the wiring diagram and trace back the circuits to find out why the switched wire is hot with the switch off. It will be either a bad ignition switch or an always hot (brown wire circuit) wire that has shorted over to your white wire circuit somewhere.

If you have an electric fuel pump, it is also a part of the white wire circuit and may have power flowing to it. Worth checking out as fuel and electrical shorts are a dangerous combination.

Les
Les Bengtson

I'm pretty sure I know what it is. The clue was that you had fiddled with the starter solenoid. On some older cars they put a feed from one of the starter solenoid terminals that only has power when cranking and send it to the coil. What this is for is so that when cranking the power to the coil does not go via the ballast and so you get full crank volts at the coil. Just a help for starting. This is the only reason for having the the ballast. 9V normal running. When cranking the ballast is bypassed so might have about 10-11V so coil actually gets a boost when it needs it.

I checked my manual and the 1500 USA midget has this feature the ealier Midgets don't. What it looks like you have done is the wire that goes from the solenoid to the coil is one the wrong termainal at the solenoid. You must have put it on the battery side so now it has battery power all the time. You need to get that little wire colour code WLG so think white with light green trace and put it on the solenoid terminal that only has power when cranking.

and best not to disconnect a battery with car running because without the battery load to stabilise the voltage then the regulator can sometimes struggle to kepp the voltage down can also create a voltage spike which is not good for cars with electronics. Safer to pull one of the low voltage wires off the coil.

Greg H

The white wire on the coil has been connected to a permanent live, most likely at the fuse box where the brown and purple are live irrespective of what the ignition switch is doing. Run a wire from the fuse box switched lives (White) to the coil and disconnect the existing, this should get you out of the situation, then you can trace the fault at your leisure.
Allan Reeling

What Greg says.
Dave O'Neill 2

Did 1500's use a ballasted coil? Had one once but couldn't remember.
Allan Reeling

Thanks all for your help, Allen a good temporary fix which I have now employed, and does the job a treat, at least now I should be able to get her through the MOT Tomorrow.

B Lodge

Not a good temp fix if you have a ballasted coil.

Your best bet would have been to disconnect the white/green from the solenoid, until you worked out which terminal it should be connected to.
Dave O'Neill 2


What Dave says. lol

The book says it has a ballasted coil which is why it has the bypass. Now if someone has put a 12V coil on it then the bypass serves no purpose at all because it has battery voltage all the time. Downside is that when cranking the 12v coil only gets 10 or 11 volts (voltage drop at battery when cranking) and the bypass circuit on starting just gives the same thing so no boost when cranking.

So if it has a 12v coil on it no drama but if it does still have the ballast and so a 9V coil then I wouldn't leave it too long otherwise you'll be looking for a new coil too.
Greg H

Put a meter across the terminals of the coil. 3 ohms = 12v coil, 1.5 ohms = 6/7 volt which should have a ballast resistor in series when the car is running.Always worth checking, cars of this age have had all sorts done. Yes the coil will not like 12 volts if it's intended to run ballasted, but I did say it was a get out of trouble card. The resistor wire, if it follows MGB practice, will be white/pink and hidden in the loom. If the coil is 12v then hopefully it will have been by-passed. Check it all out and sleep peacefully!!
Allan Reeling

http://www.britanniaparts.com/media/catalog/product/cache/1/image/5e06319eda06f020e43594a9c230972d/6/8/68000_81.jpg

That's a picture of the earlier solenoids, but it's enough for reference. The brown wires all go on the top spade terminals, and the big batt cable goes on the top screw post.

The red/white cranking feed wire clips onto the left middle spade.

There is no spade in the picture, but on the raised bossing on the right middle of the soilenoid is the ballast bypass feed terminal. The white/green wires go here.

The bottom screw post is for the thick lead to the starter motor.

On the coil, just to double check, put the white/green on the + and the white/black on the - terminals.

That should solve everything (provided it worked right in the 1st place!) without having to make dodgy bypass wires that will burn out your coil...
RoadWarrior

This thread was discussed between 29/03/2011 and 01/04/2011

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