What is the best way to determine if a coil is internally ballasted or not? Can a resistance measurement be made across the coil to determine this?

 

Anyone have any experience measuring both types?

 

Never heard of such on these old Fords.

All the 12 volt ignition systems used a series resistor to reduce the current thru the points during engine run time but the external resistor is shorted during "start" to give that extra kick to bring the motor to life.

I can not visualize a coil with internal ballast because it could not be bypassed during "start".

John

 

Internal resistance can be measured across the terminals. Usually 1.5 or so. Internally resisted coils are marked as internally resisted. If you have ever had a coil overheat and catch fire you know what I mean. I always resist coils (ballast) or the Ford resistor wire. If you overresist an internally resisted coil it will still work in lower the RPM band. My Mallory uses 3.5 resisted volts in the run mode and gets 12V from the starter relay during startup. The Pertronix gets 12V all the time.

 

When I measure my coil I get about 1.8 ohms after taking the resistance of the leads out of the measurement. I also have a constant 12V so I am going to find out where the in-line ballast resister has gone. This coil gets pretty hot but I know that is an arbitrary statement. (How hot is pretty hot?) In case I can't find the ballast resistor, does anybody know the resistance and wattage of the ballast resistor? I have lots of electronic components and could estimate a value to get me to the ~6.6V range if I had to.

 

These specs are for a 1959-1974 systems, the closest I had.
=====================================
Specs for Ford conventional point type system:
All resistance measurements at 75°F
Primary resistance..... 1.40 - 1.54 Ohms
Secondary resistance..... 7600 - 8800 Ohms
Amp draw engine stopped..... 4.5 A
Amp draw engine idling..... 2.5 A
Primary resistor..... 1.30 -1.40 Ohms
=====================================

In order to get a hot spark while the starter motor is running the coil is
designed to work on about 9 volts. When the starter is running, due to the heavy
current draw, the available battery voltage drops. If you run a standard coil
with about 14 volts (charging system voltage) the coil will overheat and burn
up. The points or output stage of the ignition module is only designed to handle a fixed amount of current also. The higher voltage will force more current thru the coil and thru the module output also. To bring the current under control for normal
running, a ballast resistor is used. The value of the ballast resistor is
determined by the design of the system. Ford uses a resistor wire on some
vehicles as part of the ignition harness. This wire is made using controlled
amounts of impurities in the copper, and a specific wire size, to produce a
specified number of ohms/foot resistance. The length of the wire used then
determines the final resistance value. Ford has found this to be more reliable
than an actual ballast resistor module like Chrysler uses. The heat from the
power dissipated is spread over a larger area. The connections to a Chrysler
type ballast resistor get quite hot and are a point of failure in those systems.

If you use Chrysler style ballast resistors you need to get special crimp on terminals for the wire connections. You can get high temp crimp connectors from appliance repair shops. They use them for wires connected to heater elements. Do not use conventional crimp type wire connectors. These high temp copnnectors can also be used on the HVAC ballast resistor assy.

 

Thanks Torque1st,
These are the numbers I need to do the correct troubleshooting. I guess with no expanding and collapsing field the current draw would be higher, huh?

 

Check out the edits above. You are kind of correct about that field. The points interrupt the current flow. The inductance or resistance to a changing current cuts down on current flow also.

 

My test for a hot coil is to lay the back of my finger on the coil. If I can stand the heat it is not to hot. Do this after running the engine for 10 minutes or so. You can buy a ballast that drops 12 volts down to 3-4 volts. I like the ones from Mallory because they are small, easy to mount and work. Resistor wire is okay, but it usually run's parallel with the wiring harness, inside the cab and gets warm. I don't like warm wires anywhere inside the cab.