Hi all,
An old post but the heading is relevant so:
I saw the following on a Rover P6 forum today and although I found it a little
difficult to read, it makes some possibly valid arguments:
"As someone has said above, it is easy to work out if the car is ballasted or unballasted.
This is general and overall/applies to ALL cars. Mid to late 60's, cars changed from a 12 volt coil to a 6 volt coil and a ballast.
The REASON they did that was purely for easier starting.
A car battery voltage can drop as low as 8 or 9 volts when starting. On a really cold day, 8 or 9 volts to a 12 volt coil gives a feeble spark.
What they did was they put a ballast resistor between the supply and the coil input, and changed to a 6 volt coil. So, 6 volts dropped across the ballast, 6 volts across the coil when running. But for STARTING, they bypassed the ballast, and applied the full battery voltage to the 6 volt coil while cranking. That was dropped to 8 volts = 6 volt coil can handle = HUGE spark.
They achieved that by either a separate contact on the ign switch, but on most Brit cars, they had a separate contact on the starter solenoid. Either way, they applied full battery voltage to the coil (bypassing ballast) FOR CRANKING ONLY.
The ballast is either a separate, ceramic former resistance, or it is more commonly a thin wire in the loom. That is why many are adamant "they don't have a ballast" - it's in the loom, can't see it.
A 12 volt coil has a primary resistance of 3 ohms = draws 4 amps of current. A ballasted system has a 1.5 volt resistance, and a 1.5 ohm primary on the 6 volt coil. Added together = 3 ohms = 4 amps.
The un ballasted coil is in a simple series cct. 12 volts in - through coil - to earth when points close. Draws 4 amps when points close. There will be 12 volts at the coil input WHETHER POINTS ARE OPEN OR CLOSED. That 12 volt supply is the only supply to the coil.
The ballasted system has a 12 volt supply to the ballast, and then the other end of the ballast goes to the pos of the coil. With ign on but not running or cranking, that is the std supply. The voltage at the coil pos will change if points open or closed. If ign on, engine not running and points open, there is no current flow, and NO voltage drop, so there will 12 volts at the coil pos terminal. But when points are CLOSED, cct is completed, current flows, and there will then be a voltage drop, so there will then be 6 volts at the coil pos terminal.
If the engine is cranked, there will be full battery voltage at the coil pos, as the OTHER supply bypasses the ballast.
If you run a 12 volt coil on a ballast car, you are running the coil at half voltage = feeble spark.
IF you run a 6 volt coil that is meant to have a ballast but DON'T have a ballast, it will run at EIGHT AMPS instead of 4.............. and will do it (inductors are very rugged), but will eventually fail.
One final thing. Coils are often labelled "12" and "12R". Here we have 12 and 12R, and GT40, and GT40R.
The "R" means "to be used with a ballast Resistor", and is a 6 volt coil. They did that here, because at the time, many older cars were still 6 volt. If people tried to sell them a "6 volt coil" many people argued, and COULD NOT ACCEPT that their 12 volt car needed a 6 volt coil. So, label a 6 volt coil a "12R" = fixed, ie they WOULD then accept it. But it IS a 6 volt coil.
The above is how every ballasted and non ballasted ign works. Pretty simple.
But run 6 volt coil without a ballast, and it will cook............. and misfire."
I'm fairly sure this applies to our later 'electronic' ign engines. I actually have
an anomaly on my car where releasing the ign key after a failed start, it fires. This makes
me think mine is wired incorrectly. When using my booster pack, it always starts instantly.
Nigel
