MG Car Repair: Proper coil voltage, ballast resistor, van deman

Question
QUESTION: Hello Barrie--

I've got a USA model 1978 MGB.  I just finished up doing some electrical troubleshooting (the ignition relay went bad), but during my fault diagnosis I found that the positive terminal of the coil was reading 12V with the key in the "on" position.  I had expected this to be 8V based on the wiring diagram (which notes the "resistive wire") and some other information sources that I've come across on the internet.  The car was in the shop not too long ago and our long trusted mechanic mentioned something he fixed related to the electronic ignition...I didn't pay much attention to it at the time as it was unrelated to the main carb problem it was in for, but is there a chance he could have incorrectly bypassed the ballast resistor?? Is there a sure fire way to determine if the installed coil should be running at 8 or 12V?  As is (12V) the coil (which I'm fairly certain is the original) gets far to hot to touch after only a couple of minutes of operation, giving me yet another clue that it is running in an overvoltage condition.

Thanks in advance,

Quint Van Deman

ANSWER: Hi Quint.  This really depends what make of electronic ignition is fitted to your car.  

The 78 B had a ballast wire inside the loom which reduced the voltage at the coil from 12v to about 8v, and it was fitted with an 8 volt coil.  However, most aftermarket electronic ignition systems require the full 12 volts delivered to the electronics unit.  In general, it is better to fit a 12 volt coil when fitting these units, and to bypass the ballast wire.

If you fit electronic ignition and bypass the resistive wire but keep the original 8 volt coil, it will indeed get too hot and may well burn out.  I would fit a genuine Lucas `Sports' coil in your situation.

If you have a digital ohm-meter, you can check out your coil.  Disconnect one of the small terminals from the loom.  Now connect the ohm-meter across the two small terminals marked + and -.  A 12 volt coil will read between 3.0 and 3.5 ohms, whereas a ballast coil will read between 2 and 2.5 ohms.  If the reading is less than 2.0 ohms, then you have a special coil designed for electronic ignition ONLY.  Some aftermarket systems (such as Pertronix) recommend such a coil, which they market under the trade name `flame-thrower'.

---------- FOLLOW-UP ----------

QUESTION: Hi Barrie--

Thanks for the answer! Here are a couple of more details:
- This MG is fitted with an aftermarket electronic ignition, a Crane Cams XR-700.  Based on the installation documents I've found for this unit it is designed to work with a ballasted ignition circuit.
- The coil is a Lucas model 16C6, which measured 1.6 ohms using your method above, again based on some searching it seems as though this coil is also designed to run with a ballasted ignition circuit.

So it certainly seems like a ballasted ignition is the proper configuration for this car.  With that in mind I have two quick follow up questions:
- Is there a standard color for the resistive wire within the harness?  I don't see any obvious unused leads around the coil.
- Could running the electronic ignition in an overvoltage condition cause the timing advance to malfunction? If I run up the RPMS in the current configuration the timing advances far off the scale (I would guess more than 30-40 deg BTDC) which seems to cause misfires and a hot running engine.

Thanks again for all of your help!

-Quint Van Deman

Answer
Hi Quint.  The ballasted wire is hidden inside the loom.  It seems to be a single strand of stiff grey wire covered in a pink cloth-like material.  Solder does not stick to the wire.  

Starting from the coil, it starts as a regular plastic-covered wire, coloured white with a light green stripe and disappears into the loom.  It then changes colour to pink, and the wiring supplier used a crimp terminal at that point.  At the other end of the pink wire there is another crimp connector, where it joins to 3 new wires.  Two are plain white, and the third is white with a brown stripe.  One of these terminates at the ignition switch, or possibly at the fuse box.  I have seen both.

The standard distributor has mechanical advance weights, so you cannot damage them with over-voltage.  If you have a USA spec MGB (not Californian) then the spec is 35 degrees at 4,500 rpm with the vacuum disconnected.  To my mind, this is a dangerous amount of advance.  If it were my car, I would retard it to 30 degrees (but please understand that if you make this change, you do it at your own risk).