MGB & GT Forum

I have a curiosity and I know the experts will know the answer-

I think I have a good understanding on the two alternate set-ups for the ignition system... With or without a ballast resistor (or resistor wire) appropriately matched to the 3.0 or 1.5 ohm coil. My two Bs have one of each set up. I also appreciate that owners convert... most often it seems in the direction of eliminating the ballast resistor and going with a 3.0 internal resistor coil.

Is one set up better or more advantageous than the other? Was there an evolution from one approach to the other?

They moved to ballasted ignitions similar to many manufacturers. Whether you need one or the other depends on your ignition. For example I have an electronic ignition, which requires a 3 ohm coil. In my setup a PO had put in the correct coil but failed to bypass the ballast resistor, which caused poor ignition and idling. The ballast resistor is within the wiring loom on later cars, rather than being a discrete resistor. If you're happy with your current ballasted ignition you may wish to upgrade to a discrete resistor per Rick Astley's MGB Electrical Systems book.



Edited 1 time(s). Last edit at 2021-09-28 01:59 PM by Bmaschine.

The simple answer is, if you have a points ignition you will get improved cold weather starting with a ballasted ignition and your points will have a longer life.

For a street driven B, unless you're having a problem I would leave it "as is".

The unballasted setup is slightly simpler but not by much.

As noted, the resistor wire can fail since it has a high nickel content making it brittle. If the ballast wire were to fail on the road, it can quickly be bypassed to feed BAT voltage to the coil; won't run forever but it should get you off-the-road.

Boy- found a really good explanation to my question on Moss-

https://mossmotors.smartsupportapp.com/articles/85-Does-my-car-have-a-ballasted-or-non-ballasted-ignition-system

Steven really captured the essence of it. The problem with non-ballasted ignition systems is that, when cranking (particularly during cold weather) the battery voltage may drop down to 8 or 9 volts. So, a 12 volt (non-ballasted) ignition coil is only able to put out about 3/4 of its normal operating voltage. This is non-optimal for starting.

Almost all American manufacturers had gone to ballasted systems much earlier than MG. Arguably, the variable climate in North America made this more important than in England.

A ballasted system does have some added complexity - and complexity means higher likelihood of something going wrong. That said, it isn't really all that much more complicated and generally works fine.

Of course, many electronic systems do not require (and often are hampered by) ballasted systems. In that case the straight system is a better fit.

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Terry Ingoldsby
terry.ingoldsby@DCExperts.com

Terry- Thanks. Between the answers here and the Moss videos I have a much better grasp. One follow on question... why would electronic systems not benefit in the same way by helping overcome the decreased voltage during engine cranking, etc.? Are you referencing modern retrofit electronic systems or the old factory systems fitted to the Bs 76 and forward? (Pretty sure those cars had original electronic ignitions and ballast resistor setups...?_



Edited 1 time(s). Last edit at 2021-09-28 03:59 PM by Andrew321.

Theoretically, electronic ignitions would also benefit from the increased voltage during cranking.

Though with OEM type electronic ignitions it's not needed since they produce more spark energy than a points type ignition.

The issue with retrofitting an electronic ignition such as the Pertronix on the MGB is that they require a constant +12V to operate properly.

To accomplish this, the red +VCC wire from the Pertronix needs to be connected to the white wire at the fuse box to get a continuous +12V and not to the ballasted positive "+" side of the coil.

So what Terry said is correct... i’d like to add just a bit... the non ballasted coil (reading 3 ohms) requires a full 12 volts to produce full power spark. The ballasted coil (reading 1.5 ohms) operates on about half that to produce that same full power spark but to function correctly needs the ballast resistor (another 1.5 ohm) to work for a total of 3 ohms. The ballasted systems ability to provide full power spark at reduced voltage is why they went to this type system.... jerry o

No the reason is what I stated in post #3 above. It's purpose is to give better cold weather starting.

I assume Jerry meant at reduced voltage while on the starter motor. So I think he was explaining the same thing in a different way. I have included yet more reasoning in this article I wrote on pertronix several years ago. It describes just how important voltage (and hence coil current) is for making a decent spark. The effect of reduced voltage is not linear. Fred

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'Anyone who likes liver, can't taste it'
'If you want to repair car electrical systems successfully, learn Ohm's Law'.
'You can't shake hands with a snake'

From personal experience and customer feedback, I can definitively say that our cars run better with the early wiring system of a 3 Ohm coil powered directly from the fuse panel rather than feeding a 1.5 Ohm coil with 10.5V from a ballasted wire. Each additional electrical connection through a relay, ballast wire, etc... WILL reduce voltage to the coil's primary windings resulting in a reduced HT power output. Its common to see 1-2V lost in wiring before the coil even with no ballast resistor. That should be addressed as its not Lucas's fault connections have never been cleaned in the white circuit. Key tip - if you've lost voltage to the coil you've also lost grounding capabilities so address the WHOLE circuit, not just the first half. Without proper grounding nothing electrical works properly, including your coil, points, condenser...

Let's get one definition straight here as well. Ballast = resistance. All coils are ballasted, some wiring harnesses are ballasted. The term is being grossly misused, which causes misunderstandings. What I believe is meant is internal (coil) versus external resistance. The goal in most systems is 3 Ohms (internal + external).

Starting with or without a ballast wire bypass is extremely similar in voltage, since the starter can bog the coil power down to 8V regardless of 12 or 10V being offered to the coil, and more often than not poor starting is a result of an improper fuel mixture. If its caused by weak spark that's usually from a worn distributor or from improper timing for the fuel being used. Do my customers suffer from poor starting with points? Simply no unless their fuel mixture is improperly adjusted.

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jeff@advanceddistributors.com

Great explanation Jeff. Thank you. Just a data point... I recently reached out to the site with problems with my 79B conking out during a long summer weather drive. With your help and others, I finally traced it to a missing ground wire in the distributer (broke off, left, never replaced). Besides curing the stall out problem, I have noted how much faster the car starts now even after sitting awhile. Does have a Petronix unit (before and after the fix) - forgive me for that... LOL

Understanding that voltages are being reduced thru poor connections as well as starter draw during start ups it makes sense to me that the ballasted coil (1.5 ohm) that requires a much reduced input voltage to produce full output, would be preferred vs that same reduced coil input feeding a coil that must have 12 volts to provide full output. If that coil input reduction falls below the 12 volts required for full output as you stated possibly 8v, wouldn’t that potentially be somewhere around a 1/3 reduction in coil output ?