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MG MGB Technical - coil

ive just purchased a coil from a local auto factors, i gave them my reg number and they came up with coil called a super gold but it tells you to use with a 1.5 ohms ballast resister. do some coils not need a ballast resister as i can't see one any where in the engine bay ,i don't no if it will be ok with the ignitor ii electronic ignition .
daz

Daz,

Which year is your car?

If it's a model with the ballast resistor as standard, it is actually a wire in the harness that goes from the fuse box area to the front of the car. The ballast wire goes to the front, across to the other side and back, so you can't see anything, other than the connection on each end. White and pink from memory, but check your wiring diagram to be sure.

Hook up a volt meter and measure voltage at the coil when cranking, and with the engine running, you'll soon see if there's a ballast or not.

David
D Balkwill

its a 79 ,what am i looking for on the volt meter ,can i use a standard coil even if there a ballast resister fitted ive just swopped it for a coil which does not need a ballast resister.will it make any difference ?
daz

forgot to say there is two spare wires now since the ignitor was fitted a green and white and a black and white , both do not show any voltage .
daz

Daz,

If your car is wired with a ballast resistor, then you will measure less than 12v on the coil when the engine is running. If there is no ballast resistor, then you'll see more than 12v.

If you have more than 12v, then your new coil won't like it for very long. If you can't find the ballast wire in the harness, then you can buy them as a separate part. They are made with a resistance wire inside a ceramic body, about 80mm long by 12mm square. Fit to the inner wing near the coil, and wire in as your wiring diagram.

If you don't have the coloured diagrams, you can download from here. Yours is probably the last page.

http://www.advanceautowire.com/mgb.pdf

David
D Balkwill

the later cars do have a ballast resister in the wireing loom , mine as been played with to fit the aldon ignitor . ive checked and there is 12v at the coil .ive relaced the coil but still have the the same problem ,it will not re start when the engine is hot .ive noticed the earthing point next to the dizzy is very rusty .ive gave it a good clean up , do you think that could be the problem ?. i will check tomorrow .
daz

Rubber bumper cars have the harness ballast and need a 6v coil which measures 1.5 ohms or so at the primary. There should be two white/light-green wires on the coil +ve on these cars. Chrome bumper cars didn't have the ballast and need a 12v coil which measures about 3v at the primary. theer should be a single white wire on the coil +ve of these cars. The problem is that most cars have been molested by past owners so you really know what you have, especially if electronic ignition has been fitted. Get the wrong combination of coil and ballast or no ballast, and you will either get an overheating coil with burnt points or a weak spark

Both ballasted and non-ballasted ignition system will show 12v at the coil +ve unless the points or trigger are closed and the coil is drawing current. Only then will ballasted systems see a voltage drop at the coil +ve, unballasted systems will still show 12v. This is very difficult to measure with some electronic systems if they are designed only to draw current through the coil when the trigger is being fired by rotation of the distributor. Testing a points system is just a matter of turning the engine until they are closed, or simply tapping a known good ground onto the coil -ve while you measure the voltage on the coil +ve. For an electronic system it is probably be best to remove all the electronics connections from the coil, just leave the ignition feed from the harness to the coil +ve, then you can connect the known good ground to the coil -ve as before. You need to know what your coil primary resistance is first, and only if it drops to 6v with a 1.5 ohm coil, or stays at 12v with a 3v coil, do you have the correct combination. The other thing to be wary of is that some aftermarket electronic systems utilise a brief high-voltage pulse to the coil from the electronics, and these coils can be very low resistance, maybe only 0.1 ohms, and these should *not* be connected direct to 12v and ground.
Paul Hunt

i think this as got the better of me ,i have 8 volts at the coil with with the ignitor ii conected and 12 volt when disconected and 12 volt with the engine on .
ive cleaned the main earth at the battery and a earth under the brake servo.the coil fitted was a aldon flame thrower which does not require a ballast resister.the green and white cable which is the supply shows 12 volts , im not sure if this as a ballast resister in the wiring some where as it is 79 model .the engine worked before when connected to the flame thrower .may be i need a coil which needs a ballast resister .can you tell me what the black and white cable connected the - side does as it does not make a difference when disconnected when the car is running . the car starts sometimes and runs ok ,then other times it splutters .its not starting at all untill i wait for the engine to cool down , the funny thing is it will kick up every time when i remove one of the leads .i hate things beating me but it may be going to a auto elestrician thanks for got to say there is a spare green cable and a single black and white cable just to the side of the coil .
daz

If you measure the voltage with the wire disconnected it will always show the battery volts regardless of whether there is a dropper resistor in place or not. If you have spare coil then measure its resistance (it should be close to 1.5 ohms for a CB car) Then measure the volts with the wire disconnected and then connected to ground via the coil post the answers here and I will tell you what resistor is there, if any. BTW Paul H had a blonde mopment above, 3v is not something you will ever see but 3 ohms is the resistance of a standard CB coil.
Stan Best

Ah yes, Stan, but a grey moment rather than a blonde :o)

8v with power connected and coil drawing current is about what you would get with a 12v sport coil connected to a ballasted ignition supply, and that will give weak sparking. To use that coil you will have to bypass the ballast, which entails running a white wire from the whites or white/browns which should be on the front of the 2nd fuse up in the fusebox to the +ve terminal of the coil.

But I really don't know why so many people with rubber bumper cars knowingly fit 12v coils and bypass the ballast, the factory system supplies a boosted voltage to the coil during cranking which makes starting easier, or less likely to fail any rate. Any gain from an uprated coil is more than outweighed by the loss of that starting boost.

As far as the white/blacks on the coil -ve go, there were originally two, one went to the points but is now replaced by the Ignitor, the other goes to the tach. Disconnect that and you will have no tach.

Originally there would have been two white/light-greens on the coil +ve, one from the solenoid which supplies the boost voltage during starting, the other from the ignition supply via the ballast resistance. If *both* the white/light-greens have been disconnected, and a direct unballasted feed supplied to the coil already, and that is showing 8v, then it has some very poor connections, and will be giving weak sparking and possibly intermittent ignition.

But I still don't see how the engine runs at all if you have the red from the Ignitor connected to the coil -ve and the black to the +ve (I seem to remember you saying that) except it isn't difficult to make them polarity insensitive, and maybe that's what they have done now. I'm amazed that there was no protection in the original ones and reverse connection would destroy them!
Paul Hunt

i have a green and white cable connected to the coil, which shows 12v when disconected from the coil , 8v when conected and back to 12v when the engine is on . i have a spare green and white cable cut short and not used .are you saying the green and white cable as a ballast resister conected to it , if so should i change the coil or get another supply .please can you explain where exactly i should get another ignition supply from thanks
daz

the spare green and white as no voltage showing , to take it back to standard would i join the two green and wires together and connect to the + on the coil .i don't mind getting another coil i just want to drive the car thanks.i still want to keep the electronic ignition .
daz

The 12v disconnected and 8v connected still indicates you have a 12v coil in series with a ballast resistance as I said before. It goes up to 12v with the engine running as the system voltage is then about 14.7v and not 12v, but is still giving weak sparking. If you have an ignitor or other aftr-market ignition system you may well also be getting intermittent ignition. You need to measure the resistance of the coil primary (between the two spades) to be certain what the situation is, as you will get the same voltage readings if a 6v coil *is* fitted, but for some reason you have two ballast resistances in circuit (I've known that happen). A 12v standard coil measures about 3 ohms, a 12v HV coil about 2.5 ohms, a 6v standard coil about 1.5 ohms, and a 6v HV coil about 1.2 ohms.

The best situation for a car with points is to leave the ballast in circuit, have a 6v coil (1.5 ohms or 1.2 ohms), connect both light-green/whites to the coil +ve. This will give about 6v on the coil +ve with the engine stopped and the points closed. Then check you have *battery* voltage on the coil +ve during cranking, which should be about 10v with a good battery.

With an Ignitor, the ignitor red must be connected to full ignition voltage, not via the ballast. If you have a 6v coil it should have two light-green/whites on the +ve, and the Ignitor black and tach white/black on the -ve. If you have a 12v coil the same full ignition voltage as the Ignitor red must go to the +ve, and the Ignitor black and tach white/black to the -ve. In this last case it doesn't matter whether the light-green/whites are connected to the coil +ve or not, if not connected they must be taped up and put of harms way so they can't short on anything.
Paul Hunt

so , with the ignitor i need a 12v supply , i have found two fuses one a 30 and the other a 17 the larger fuse is the bottom one and the next one up is the 17amp . you said i can put the other ones safe and hide them out the way .which one shell i conect to as they give a supply with the ignition on . i can't understand why it worked before connected to the ballast supply when the flame thrower was being used .i now have a pure gold standard coil but i will not run on the same green and white wire .thanks
daz

All four fuses in the fusebox should be the same rating - 17amps rated, 35 amp blow. It matters very much which terminal of the fusebox you power the ignitor from! The bottom fuse should have brown on the front and purple on the back. Connecting the ignitior to this you wouldn't be able to turn the engine off! The top two fuses are for the parking lights, so if the Ignitor was connected to this the engine would only run when the lights were on.

The 2nd fuse up should have white or white/brown wires on the front, and green on the back. You are not powering the Ignitor via this fuse, the white or white/brown is the unfused ignition supply *to* the fuse and hence the green wires. You are simply tapping your Ignitor onto this white or white/brown circuit to pick up an unfused ignition supply for the ignition.

As I said both 6v and 12v coils will *work* off the ballasted supply, but the 12v coil (which you seem to have now by the voltage readings) will give weak sparking, which rather makes all your expense a waste of time and money. But whenever swapping coils, even if you think you are buying a standard coil for your car, chrome bumper or rubber, you must always measure the primary resistance as coil labelling is often confusing if not actually incorrect.

Ballasted ignition (rubber bumper cars) needs a 6v or 1.5 ohms coil. Unballasted ignition (chrome bumper) needs a 12v or 3 ohm coil. There is nothing to be gained by fitting a 12v coil to a rubber bumper car and bypassing the ballast, all you are doing is losing the starting boost function which may make the car a non-runner in adverse conditions.
Paul Hunt

thanks for your help,ive been doing bits to the car all day, repairing the high beem on the stalk and i found some earth cables not screwed to the chassis ,the sprayers must of forgot about it .i spoke to aldon and they told me that the ignitor works with a 12 v coil so i connected a new ignition live (second terminal up )and locked it in place in the white and brown clip . ive been on a decent run with no problems as to yet .are you saying the other side the red cables are the fused side thanks daren
daz

Yes, the red wires (which should be on the back of the top two fuses) are the fused side. The single red-green for both those fuses is unfused, as are (should be) all the wires on the front of the fusebox.
Paul Hunt

thanks for your time ,i think ive cracked it , the red cables are fused and the brown and white are not .does it make a diffence as there are connectors front and rear in a banks of two .
daz

Forget the red, they are parking lights.

For the ignition you need to use the brown/white wires, i.e. the front of the 2nd fuse up. There are two spades, and you can use either. However I would expect both to already be occupied, one with a spade with two wires in and another with one wire. Rather than cutting in to wires or using Scotchlok connectors it would be better to use a 'piggy-back' connector on the end of the wire leading to the Ignitor. Remove the connector with the single white/brown from the fusebox, fit the Ignitor wire in its place, then fit the removed white/brown to the male on the back of the piggy-back connector.

See http://www.maplin.co.uk/Module.aspx?ModuleNo=42729&&source=14&doy=Search for piggy-backs.
Paul Hunt

its all worked out in the end, i threaded the wire through the hole on the spade on the fuse block then pushed the connector back on , it not going anywhere thanks for all your time .just going to have it tuned now , its been stored for a while . the engine as 110 bhp at the wheels but i think its running a little rich thanks daren
daz

BTW, you must take the same supply to the coil +ve if you have a 12v coil, which it looks like you have, or you will be getting weak sparks.
Paul Hunt

This thread was discussed between 21/09/2008 and 30/09/2008

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