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MG MGB Technical - Ignition ballast or not?

Hi All, just a 'check' question. Have just tested if I have a ballast wire in the system. My volt-meter is showing 11.05 volts. Am I right in assuming that I have no ballast wire in the system? A simple check I know but my car is not original spec. The car is from 1977 USA model, all the smog bits are removed, the engine is a reconditioned one from March 2008, Ivor Searle specification 846F, with a Weber 32/36 DGEV carb. Im also going to check if I have the right coil ie non balast.
Thanks for any response
/Moss
Moss

What colour is the wire supplying the coil? Actually two wires if it's ballasted.
Dave O'Neill2

Hi,

11 V would indicate that you don't have a ballast, though its a bit low. I would expect around 12.5V, assuming the battery is fully charged and OK. To check which coil you have, measure the primary resistance, around 3 ohms non ballasted, around 1.5 ohms ballasted coil. A ballasted coil, with a ballast resistor would give about 6V at the terminal.

Herb
Herb Adler

Moss. The factory system had a resistance wire on the North American specification cars from the mid 70's. Your 77 would, definitely, have the resistance wire--if the wiring is still factory original. There should be two wires, white with black trace, going to the + (input) side of the coil. One comes off the starter solenoid and is powered (12V) when the starter is engaged. The second is one one incorporating the resistance wire which drops the voltage down to about 8 volts when the ignition switch is in the "run" position.

But, the original Lucas 45DE4 distributor has commonly been replaced in these cars due to it being unreliable. Many of the Lucas 45D4 "Euro-spec" distributors (a points type) sold here in the US have instructions recommending that the system be rewired to a full time, 12 volt input. Such kits, normally, include a Lucas Sports Coil as part of the package and, when such a conversion is made, the Sports Coil, or other form of full time, 12V input coil must be used.

So, what are the wires going to the coil? What is the resistance across the two primary terminals of the coil (1.43 to 1.58 ohms for the "6 V" coil and 3.1 to 3.5 ohms resistance for the 12 V coil--from the factory workshop manual). A gold colored coil, having about 2.7 ohms resistance is a Lucas Sports Coil.

As I remember (but, it has been many years since I checked) the coil was powered at about 8 volts, checked with the engine running, with my 77 and 79 cars. But, I seem to remember that the voltage reading was higher when the engine was not running which may account for your readings.

Les
Les Bengtson

Only rubber bumper cars had the ballast resistance, and hence the 6v coil, North American and everywhere else.

If the points (or trigger) are open then you will see the same voltage at the coil +ve on both systems. You have to have an earth on the coil -ve, then measure the coil +ve, to tell whether there is a ballast in circuit or not. There should be about half the system voltage at the coil +ve of ballasted systems - *if* you have the correct coil (1.4 ohms primary). If you have a 12v coil (3 ohms primary) on a ballasted system you will see about 8v. If you have a 6v coil on a 12v system the only way to detect that is to measure the primary resistance, as the voltages at the coil +ve and -ve will be the same with either coil.

To measure the primary resistance of a coil you must remove the wiring from at least one spade terminal.

With the engine running you will get completely different voltages on a ballasted system, as it is switching between 6v and 12v (more likely 7 and 14), but that also depends on what distributor you have, as it also depends on the dwell. 45D4 will be about 11v with a 14v system, 25D4 will be a bit more. 45DE4 (Opus) will be less i.e. closer to 7v, and 45DM4 will be a lot closer to 7v at idle, increasing as the revs are increased. After-market electronic systems will depend on whether they are variable dwell or not, closer to points if not, closer to 45DM4 if they are. That's with the correct coil.
Paul Hunt

Thanks for response.
The reading for the primary resistance of the coil is 3.4 ohms - wiring disconnected on both sides.
There are 3 wires connected to the coil. 1 wire coming from the dis - low tension wire. And 2 wires connected to the other side. 1 wire goes to the starter solenoid- I presume as it goes into the blue plastic covered wiring harness leading downwards and to the side of the engine on the inner wing. The other wire goes into the blue plastic covered wiring harness which leads up towards the alternator.

So the volt reading with the engine not started but ignition switch on is 11.5 v and the resistance for the coil is 3.4.
Surely this must mean that there is no ballast wire in the system or at least it is bypassed?

/Moss
Moss

Potentially (ho ho) wrong coil then, as that is ballasted wiring.

You will have to earth the coil -ve terminal before you measure the voltage on the coil +ve, that is the only sure way to make sure the coil is passing current. Which is the only way you will know if the ballast has been bypassed or not, when taking voltage measurements.

No good reason for bypassing the ballast and using a 12v coil. The ballasted system gives a better spark at high rpms (not that that is an issue on the MGB as it doesn't rev high enough), can make the difference between starting and not starting under adverse conditions, and the coil runs cooler.
Paul Hunt

Ok. My description was incorrect. As well as the low tension wire from the dis There is also a white with black tracing connected to the minus side of the coil and there is only 1 white wire connected to the plus side. I've had the B since 2010 and it is the first time I'm investigating the wiring/ignition...by more than just looking at it! The reason is that I want to try one of those inexpensive electronic ignition modules that fits in the dis - which is a 45D as fitted to engine spec 846F (accordoing to Haynes).

/Moss

Moss

Hi Moss,

The white wire with black trace is the tacho sense wire.

Herb
Herb Adler

And the white implies a non-ballasted coil feed. However as a 77 would have had a ballasted feed it's obviously been modified, so you will have to satisfy yourself that it really is a non-ballasted feed and not a ballasted feed pieced out with a bit of white for some reason. The only way to do that is turn on the ignition, connect an earth to the coil negative, and measure the voltage on the coil positive.
Paul Hunt

The reading says 11.4 volts. So I've concluded that the ballast wire is no longer part of the ignition system or removed.

Have now fitted the electronic ignition module - an inexpensive one. There is a difference - the engine starts quicker and engine turn off is instant - as for performance, not really. Had to adjust the BTDC til 11 - 12 degrees instead of the specified 10 degrees. Now I just have to adjust the tappets - I think they are slightly out. A slight misfire in neutral - the timing mark on the crank pulley moves slightly to 12-13 degrees at a steady rhythm in tune with the engine turnover (or what ever it is called). Have been out on a long run and the plugs are a nice mid to light brown.

Next jobs are fitting the new carpets, renovating the second hand SU HI4 twin carbs I bought on e-bay, bleeding the brakes and new brake hoses. And replacing the drain plug on the rear axle - like a fool I used the wrong tool - I have managed to split the top of the drain plug facing down towards the ground . Off to a workshop. Silly design me thinks, but then I would say that now! No leeks though!!

/Moss
Moss

The axle drain plugs are a pain. I've welded a 3/4" bolt stub into both mine. Makes checking the level a positive pleasure ...


Paul Hunt

A useful adaption.
/Moss
Moss

Re: tappets - when cold - should they be 15 tho or 13 tho and should I turn the engine over by hand or use ignition? I've just tried by hand turning slowly which is easy enough but I notice that I am not precise enough on each turn ie on each revolution the engine seems to turn over a little bit more or less on each turn. So the gap at the tappet is not exact for when adjusting.

/Moss

Moss

15 thou cold, take the plugs out and either nudge the car forwards in 4th or turn the engine by hand. They are supposed to be adjusted by the 'rule of nine' which means when a rocker is fully down you adjust its complement, i.e. nine minus the one that is down. If you can't subtract but can count from 1 to 4 then count from the nearest end to the one that is down, then count the same number from the other end for the one to adjust :o)

However I always had trouble getting repeatable gaps i.e. it seemed to change from adjusting all of them to double-checking all of them, until I realised that my back three are still changing at the strict rule of nine point, so now I adjust at the maximum gap. That has made them a bit quieter, as well as consistent i.e. they don't need adjusting from one service to the next. What that says about my cam I don't know, but I have found the same thing on a pal's car as well.
Paul Hunt

Thanks for the comments Paul. I'll try with the maximum gap by turning engine by hand. Thankfully I can subtract AND count to more than 4!
/moss
Moss

Bought a sports coil to finish off my attention to the ignition system...and the difference is very noticeable.
The engine runs smoother and, dare I say it, there does seem to be more power over 3500 revs.

Moss

This thread was discussed between 22/05/2013 and 08/06/2013

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