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MG MGB GT V8 Factory Originals Technical - Ballast Resistor/ Coils/Wiring/Relays

Over the winter I re-routed all the wiring under the passengers front fender on my 74 ˝ MGB Rover 3.5 V8 to clean up the engine bay. All the wire was re-routed to relays, new fuse box and junctions under the glove box. All relay systems check out.
Here is what I have, from a relay
85 goes to the starter
87 goes to the coil
86 to ground
30 to the white brown wire on the Lucas Starter relay
At the coil I have the Black white wires which go to the Pertronic Igniter and to the tach. On the + side I have the Green White wire which comes from 87 on the relay and over to the red wire on the Pertronic Igniter
For clarity I have included the instruction I received to originally hook this up. I should mention that the following worked just fine until I improved things

1) buy a "generic" relay from the auto parts store, of the type used for
driving lights, etc, and mount it on the right inner fender, nrear the
two relays. This relay is a small cube, about one inch on a side, with a
small mounting tab.

2) Disconnect the white/green wire from the + post of your coil to the
starter (make SURE you disconnect the right w/g wire!), and reconnect it to
terminal #85 0f your new relay.

3) connect relay terminal #86 to a good ground.

4) connect relay terminal #30 to the white/brown wire on your existing
ignition relay.

5) connect relay terminal # 87 to the + post of your coil.

6) connect the other end of the white/green wire you removed from the +
of your coil to the brown/white wire at the solenoid Both the b/w and the w/g wire will connect to the same post on the starter solenoid.

This circuit leaves the ballast resister in the circuit when the car is
running, but bypasses it for starting, just like the original MG design.
This modification is needed because your starter doesn't have the by-pass
function built in like the MG starter did.

So what I have now is a engine which turns over but won’t start. I have power at the coil + power at the Pertronics Igniter + while turning over the engine, but no spark through the coil wire.
I was wondering if I had inadvertently removed the ballast resistor while shortening the wires? (just read the archives and see one fellow mention the White Pink wire contains the ballast resistor, I removed this wire)and if this would account for it not firing
So what can I do now to get my car running.
How can I prove it is the lack of ballast resistor? And how can I correct it if it is?
Lots of information but I am would have put it back on the road today if she had started. Would love to get her on the road and drive her around for a few days before I head up to the British Car meet “The Rally In The Valley



The standard coil ballast is a length of pink resistance wire contained within the main loom, with white extensions at the fusebox end and white/light-green at the coil. With the ignition on you should either see 12v or 6v on the coil +ve, 12v if the points are open, 6v is they are closed. Can't speak for a Petronix, for faulting purposes disconnect it from the coil and use a ground wire on and off the coil -ve to simulate the points. If you have disconnected the ballast, but have connected the coil boost correctly via your relay (simulating the extra contact on the original starter solenoid), the car would start but die as soon as you released the key. Under most circumstances the boost relay is not required but can make the difference between starting and not starting under adverse conditions. To simulate the solenoid boost contact with a relay all you need is to connect terminals 30 and 86 to the white/brown on the starter relay, 87 to the coil +ve (white/light-greens) and 85 to ground. This seems simpler than the changes you have done, and disconnecting a white/light-green from the coil should not be neccessary.
Paul Hunt

Hi Paul, thanks for responding. I am still having trouble.
Sunday I did a bunch of checking, I still have no spark from the coil wire. I even tried running a wire from the alternator + to the coil + and still was unable to get a spark through the coil wire. All I manage to achieve was to have the engine continue to turn over until I pulled the wire to the alternator.
I did a continuity test from the center post of the coil to the + and I had continuity, I also checked from the center post to the post and had continuity there as well, this was on a MSD Blaster 11, I also checked out the original coil from the 4 cylinder and it too had continuity from the center post. A friend of mine checked the 2 coils he has (which started his car) and he DIDNT have continuity between the center post and the + or the -. His coils have a internal ballast resister.
I borrowed a third coil from 74 MGB, mine is a 74 but the numbers on the bottom of the coils matched. At first when I checked it there wasnt continuity between he posts. I installed it on my car and still didnt have any spark.
I received your e-mail this morning and after work once I arrived at the shop I checked 3 coils, now all three have continuity between the post. Now I dont know what is going on, maybe I wasnt careful enough when I check the coil I borrowed, or maybe I am some how blowing out the coils????

This was my set up before reading your e-mail
85 goes to the starter and is connected with the Brown white wire at the starter
87 goes to the coil (only wire to coil +)
86 to ground
30 to the white brown wire on the Lucas Starter relay
On the original Lucas starter relay
W1 Red white to the ignition switch
C1 Brown joins other browns
C2 Brown White connected to the other relay and to the starter
W2 Black

At the coil I have the Black white wires which go to the Pertronic Igniter and to the tach. On the + side I have the Green White wire which comes from 87 on the relay and over to the red wire on the Pertronic Igniter.

After reading your e-mail I located and re-attached the in-loom ballast resister as follows
Now from a group of white wires, a white wire goes to one of those rubber connectors (I call them 4 plexs ) at that point it splits, the White pink (ballast resister wire)goes to a LGW wire which goes to the coil, the LGW wire from 87 on the relay is still attached to the coil +.
The other wire from the split is a LGW wire and goes to the distributor.

But this didnt make any difference either, still no spark. Now I dont know if it is because I have screwed up or the coils are screwed up, I am receiving 12.9v at the alt and all the coils + with the ignition on.

So I looked at your instructions To simulate the solenoid boost contact with a relay all you need is to connect terminals 30 and 86 to the white/brown on the starter relay, 87 to the coil +ve (white/light-greens) and 85 to ground."
So your are suggesting I re wire the above relay then? No problem but what happens to the LGW wire which is now running from the relay 85 to the starter?

This seems simpler than the changes you have done, and disconnecting a white/light-green from the coil should not be necessary
Before I re-installed the ballast resister wire I only had 1 wire to the coil.

It is Canadas birthday tomorrow so I am off and will keep working at it. If you think the coils are shot then I think I might know where I can borrow one from Rover.

I guess I am sort of at a point (if the coil is shot) where I could easily install an internally ballasted coil? I am not sure how that would affect the relays I now have, but one step at a time eh? To get the thing to fire would be great.

Did I mention that this is a Rover 3.5 with a
Wilson Alternator #90-01-3106 12 Volt 63Amp 10SI
Weber Mini HY Tork Starter #810164 Hitachi S114-254S S114-254S8K
MSD Coil Chrome Blaster 2 Coil #8200
Leads Taylor Vertex High Performance Pro Series 8mm 078355
pertronix Ignitor (Electronic Ignition) LU-183

Thanks Paul


Coils for the MGB (and all other applications I have sen) should have continuity between the HT terminal and both LT terminals. Measure the resistance of the LT winding, if it is 2.5 ohms or higher it is a 12v coil and does not need the loom ballast resistor. If it is of the order of 1.5 ohms then it does need loom ballast. Talk of internally ballasted coils causes confusion, whether a coil has an internal resistor or not is irrelevant, it is either a 12v coil not needing an external ballast or a 6v coil which does (on a 12v system), and whether it does or not largely depends on the resistance at the LT terminals. Running a 6v coil without external ballast on a 12v system will result in excessive current which will overheat the coil and burn points if used, can't speak for a Pertronix.

According to Summit Racing the MSD 8200 is supplied with an external ballast resistor. This should be used in place of the original loom ballast. A white wire should go from the white at the fusebox to one side of the resistor, and the other side of the resistor should go to the coil +ve.

If you have 12v at the coil(s) +ve and no spark then the problem is no switched ground at the -ve, which would normally be supplied by the points and condenser. Forget the relay for the moment, as I said before it is only needed under very adverse conditions, lets get a spark first. Disconnect the Pertronix altogether, get a ground wire and connect it to the coil -ve with the ignition on. Measure the voltage at the coil +ve. With the standard coil it should be about 6v, with the MSD and loom ballast it may be lower, which is why you should be using the MSD ballast instead, which will probably give a higher coil voltage and a bigger spark. When you remove the ground from the coil -ve it should spark, and you should also get a weak yellow spark from the HT, it may barely jump a plug gap. Now get a condenser and connect it between the coil -ve and ground. Tapping a ground wire on the coil -ve now should result in much less sparking at that terminal, and a much bigger HT spark which should jump 1/2" or more. If so, then your Pertronix is bad.
Paul Hunt

Good news. I got it started. I followed your instructions and it suggests the Pertronics is bad. So I removed them from the equation and put the points back in, that lead to a couple of other problems which was solved by replacing those points with another set of points, after which the car will now start and keep running. Now that I am convinced the wiring is correct and all is functioning as it should I will once again try the Pertronics. Worse case scenario, I have to replace the igniter with the points again. Is there any other way to tell if the igniter is faulty. It is less than 2 years old and cost almost 200.00cdn.
I would like to thank you for all you help and patience Paul.

Thanks very much.



No probs, Barrie. A Pertronix either works or it doesn't, and if the magnet collar is correctly fitted and it doesn't work it is stuffed. It is only with the relatively recent Pertronix II that there was internal protection against connecting the wires the wrong way round, with the original item you just fry the unit. Pertronix reliability, like any other electronic system, is such that wise owners always carry points and a condenser as a spare - and make sure they can be fitted, as some people seem to have to butcher the points plate to fit the Pertronix.
Paul Hunt

My car suffered problems last weekend, when it was very hot and I was stuck in traffic. It began when it wouldn't start again following a refueling stop. The engine turned over fine but wouldn't fire up, yet bump started very easily. Then it later did the same again, yet when I had left it to cool for a one hour ferry journey it fired up okay. Then it failed completely on the journey home when it just died when doing about 40 mph. The car has a Mallory twin point set up with a Mallory coil with a ballast resistor. I had a second Lucas 12 volt coil ready wired to exchange to and it fired up okay on this. I haven't driven it since, but I obviously suspect coil and resistor but how can they be tested (i.e. is it simply a matter of voltage or continuity testing?)Prior to these problems the car suffered a condensor failure and the supplier of the distributor/ resistor advised me to change from the Lucas sport coil, back to the Mallory. Any ideas?

Regards, Steve

If it bump started but wouldn't crank start the implication is there wasn't enough power to create an adequate spark with the cranking, rather than there being no spark at all. A friend had problems starting after having some work done by a well-known 'specialist' that wired a 12v coil in series with an external ballast *and* the original loom ballast. He was getting about 4v at the coil during running, about 2v when cranking.

Did you wire the 12v coil exactly the same as the Mallory? Or direct to 12v and not via the ballast? If the former that does imply the Mallory coil is bad, if it started and ran with only half the voltage available to the 12v coil.

What dwell do you have on your twin-point and how did you set it up?

The condenser shouldn't be affected by the coil, particularly a 'standard' Lucas Sport coil. A bad condenser will cause weak sparking, even with a standard coil the spark should jump at least 1/2" from the coil to ground, performance coils more than that, with no condenser at all it will barely jump a plug gap and the timing will be significantly retarded from normal. Hunt's Fifth Law: "Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals." Check that 'new' condenser.

You can do static tests on resistor and coil (the resistance of a standard 6v coil and loom ballast are about 1.5 ohms each, a 12v Sport is about 2.5 ohms, performance items will probably be less than these), but these items are more likely to fail when hot or vibrating so the only real test is to check for a spark at the coil and each plug lead when it won't start by cranking.

I've been plagued by what I consider premature failure of plugs causing non-starting and rough running when cold, usually I had to be somewhere so just put in a new set and away we went. But it then happened when I was just moving the car and had time to diagnose, and just by clipping on a timing light to the king lead and each plug lead found it was the cap that was tracking to ground even though there were no obvious marks.
Paul Hunt

Thanks Paul, I am going to check this out at the weekend when I can borrow a multimeter. As you say there is always a belief in new components, and I am obsessed with heat issues in the engine bay which runs very hot. I have bought a heat wrap kit for the manifold and having my bonnet louvred, and going to replace the ignition components; hope that does it!

Thanks, Steve

This thread was discussed between 29/06/2003 and 09/07/2003

MG MGB GT V8 Factory Originals Technical index

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