MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG Midget and Sprite Technical - Ballast resistor and electronic ignition

I am pretty sure my standard Jan 78 - 1500 has a ballast resistor wire in the ignition system.

Can anyone confirm this for me? On the other hand if it is a ballast resistor unit system is it possible to verify this visually by telling me where the resistor unit would be if one was fitted? I have searched a lot, found there is not a lot of electrical stuff, and can't find a physical resistor. Would be good to know for certain from someone who knows what to look for and where to look.

Fitting AccuSpark. There is a method for detecting ballast resistance and I will use it as well but I would really like independent confirmation if poss to put my mind at rest.

For those of you who know the story so far the rear oil seal is fine :-) and the therefore improved speed now it does not throw oil onto the clutch is very nice ;-). Feels like it is going better the more revs it gets. (Like running in). I will need to address the suspension (handling) once the AccuSpark is fitted.

Thanks in advance, Dave
Dave Squire

Sometimes there isn't an identifiable ballast resistor in that it is a bit of wire, made from some metal with a higher resistance than copper, which is part of the wiring loom.

Rob
Rob aka MG Moneypit

it is a resistive wire it runs to the front end of the loom just around the radiator where it is spliced to a wire that gose to the coil.
mark 1500 on the road Preston Lancs

Thanks fellas. Its a long wire from the fuse box then.

Cheers,
Dave
Dave Squire

Dave, FYI, I ran a long wire following the regular loom back through the fire wall, behind the dash and into the spare white terminal at the fuse box.

To check for the ballast you could measure resistance between white terminal (at fuse box is probably easiest) and the +ve at the coil (brown and white). I *think* if it is ballasted you should get about 3 ohm resistance. Will check mine when I go to the garage in 5 mins or so.

Malcolm
M Le Chevalier

Dave,

You can check if the resistor wire is functioning (or still present) by turning on the ignition and checking the voltage at the coil + terminal should be about 6 volts if resistor is working.
Also check the coil fitted is a 6volt coil.

A 12volt supply is only provided to the coil by the second feed to the coil + during engine cranking via the solenoid to assist in cold starting.



R
richard boobier

RE: my post from earlier, I measured 2.8 ohms resistance between white wire at fuse box and white and green wire at positive of coil (not white and brown as I said earlier, memory failed me on wiring colours there!).

2.8 is close enough to 3 for me :-)

Malcolm
M Le Chevalier

Quote "richard boobier, North Somerset, United Kingdom"

>>You can check if the resistor wire is functioning (or still present) by turning on the ignition and checking the voltage at the coil + terminal should be about 6 volts if resistor is working.

Richard, Just to clarify I believe that test is only good if the contact breaker is 'closed'. If it is open, you will measure 12v on both sides of the coil....

Jim
J Smith

I think it only gives 12v at start up then runs on 10v the 12v is supposed to make it start better as the coils are only 10v coils , so with ignition on it should be 10v thats how i understand it i could be wrong .
mark 1500 on the road Preston Lancs

I believe that 6v is correct.

The impedence of the ballasted coil is 1.5ohms as opposed to 3ohms for the non-ballasted coil.

The 10v figure is correct for the instrument voltage stabiliser.

Jim is right about the points needing to be closed in order to read 6v at the coil.
Dave O'Neill 2

Ok Dave, from memory, if it's like mine the splice in the wrapped loom between the resistance wire and the 12volt feed is adjacent to the dissy, this wire goes to the front of the car and comes back to the dissy. the only way to confirm is to partially unwrap the loom and look for the resistance wire join. If you do this you can uncover the join for future reference e.g. a 12 volt coil can be powered from here. Hopefully someone can confirm this
HTH Rod
R W Bowers

Thanks for all your thoughts and help. I had to abandon installation last night as I could not find the spade connectors. (workshop is a tip at the moment). Will be acquiring some today and probably go with installation tonight.

I plan to do the wiring like Malcolm suggests except my fuse box is in the engine bay on top of the drivers foot well.

Cheers, Dave
Dave Squire

It's not so much as getting 12V when the starter is operating, as getting full battery voltage. With the large currents drawn off by the starter the internal resistance of the battery uses up a big chunk of the 12 volts, leaving a much lesser voltage at the battery terminals. A ballasted coil runs at this voltage when the resistor is in-line (normal operation) and then when the starter is operating and the resistor bypassed the coil still has its operating voltage and behave effectively.
As to to the coil voltage, my measurements show about 8V.

It's one of the best retro-mods you can do to a pre-ballast car, imprioves starting no end, especially in colder weather when the starter draws more current to overcome the greater drag.

If you don't have a ballasted coil and the car is reluctant to start, a good little trick is to run jump leads to the coil, not to the whole system. That way the coil is getting almost the full 12V (a little bit of voltage drop occurs as current is drained back through the small coil feed into the starter circuit, but not much) and a bigger healthier spark results. When the main battery is struggling with the starter this is much more effective than simply jumping the whole system. So long as the starter turns even slowly the right voltage at the coil will do the trick. A bit like the effect of using a crank handle for those that remember them ...
Paul Walbran

Right... note to self... think twice, type once! :-)

I have been thinking about this more this morning. If the coil in a ballasted system is 1.5 ohm, the ballast resistor should be ~1.5 ohm in order to drop the voltage to 6 v at the coil whilst running.

I measured 2.8 ohm resistance, which seems very high. My thinking that the ballast resistor should be ~ 3 ohm is me confusing it with the impedance of a non-ballasted coil. Wrong.

This makes me wonder if I have a dud/dirty connection somewhere thus pushing up the resistance slightly (I measured "terminal to terminal", rather than "just the wire") , in turn lowering the voltage at the coil and giving me a weak spark maybe?! I shall investigate further tonight and report back!

Measuring from green/white at coil to white at fuse box is still correct I believe.

Finally, Dave, when I said "I ran a long wire following the regular loom back through the fire wall, behind the dash and into the spare white terminal at the fuse box" I did mean that it has to come back out of the firewall again to attach to the fuse box :-) My fuse box is also on the top of the footwell in the engine bay (standard position).

Laters,
Malcolm
M Le Chevalier

Of course Malc. I forgot about the battery being there on most cars. My battery is retained in the boot and not by the heater. (its a quite big beasty off an escort) :-)
Dave Squire

Right, my final conclusion is my multimeter is too rubbish to take any meaningful measurements. I get the resistance of the coil and the ballast wire at anything between about 1.8 to 2.7 ohms depending on how I wiggle the connectors etc.

So, in summary, check there is *some* resistance between white and white/green and that should tell you if there is a ballast resistor in there :-D

Malcolm
M Le Chevalier

Malcolm
Don't be too hard on yourself there mate
Yes the two types of coil are 1.5 ohm -non balasted and 3.0 -internaly balasted meaning the non balasted coil will need an external resistor of some description
There are several aftermarket resistor available, and I've had them anywhere between 1.0 and 3.0 and have worried but all seem to operate ok
The prefered operating voltage is around 7to8volts under load - points closed but don't panick if it's down a bit as operating supply voltage will increase from 12v to 14plus after the engine starts
willy
William Revit

This thread was discussed between 11/11/2014 and 13/11/2014

MG Midget and Sprite Technical index

This thread is from the archives. Join the live MG Midget and Sprite Technical BBS now