Ford ballast resistor
 

On the Cab 13, can anyone tell me the purpose and value of the ballast resistor shown on the wiring diagram in series with the coil. As far as I can see it only drops the primary winding current.

coil
 

Hi
I think you mean the Ford system ? The coils are rated at 4.5 Volts or something like that. The resistor is wound with ni-chrome wire , like the wire in those old electric bar heaters, the resistance value changes with temperature/current flow . It was a horrible system and the cause of many breakdowns.

Ford Flathead ignition system resistors
 

Hi Alastair,

I think it has been covered recently in the forum but I cannot seem to find the thread regarding it.

My basic understanding is that it is to allow more current to flow to the coil for starting purposes when the starter motor is drawing a lot of current ensuring a hot spark. As it heats up, with the engine running and drawing more current through the coil, it increases its resistance and thereby prevents excess current from flowing through the coil and cooking it.

As Mike said the coils are designed to run at about 4.5 volts and not the full 6 Volts of the electrical system. This resistor limits it to that voltage. So it serves two purposes: aids starting and protects the coil.

Various websites have more information on it such as Jalopy Journal, Early Ford Barn etc. Almost everyone says it must be used with original 6 Volt coils. With some aftermarket coils it is important to know the coils internal resistance to work out if a different ohm rated resistor is required.

That's a capsule description and will leave it to you for your own research.
No doubt others can add to this or point you to the original posting.

Cheers,

Resistor
 

Thanks again everyone. Yes it is a Ford. I am a retired electrical engineer and still it never occurred to me that the resistor would have varying characteristics with temperature. It seems to me that it is a brilliant idea in theory but if the engine does not fire soon after the first attempt you are in trouble with a hot resistor and a failing battery.

Except that if everything is in good order as per Ford's specs, it will start every time.
The heating of the resistor is a time thing. If the key is turned off the resistor will cool and the higher voltage will again be available to the coil immediately after the key is turned on. Under normal circumstances a hot engine will still fire up on the 4 1/2 volts.

divers bell
 

I am theorizing but I think the divers bell type dissy was adding to the erratic ignition issues. With the squat coil sitting on top of the dissy , it is in a terrible position, subject to the heat from the radiator . With the improved crab dissy setup , they located the coil away from the hot environment.

When I had the little 1941 11D truck , I was amazed at how it would start from cold on a almost dead flat battery, the motor would be barely turning over, but it would fire and start . The trouble appeared after the resistor had heated up , I used to stop on the roadside and by pass the resistor with a wire link , this bodge fix got me home more than once.

After fiddling around , I discovered that the Voltage reg. was set rather too high , lowering the Voltage setting really improved the ignition system reliability . lower Volts = less Current through that resistor and less heat.

Hi Mike,

That makes a lot of sense, Ford moving the coil from the divers bell distributor to above the block with the advent of the crab distributor. It has to be a lot cooler up there.

Your bypassing the resistor to start a hot engine is interesting too. I hadn't heard of that before but reading some of the info on hot rod and Ford Flathead V8 websites, that is not an uncommon fix/modification for improving the hot starting of these engines. Some even rig a permanent bypass switch to easily enable this function. All good I guess as long as you return the switch to the non-bypassed position after it starts!

Cheers,

Quite a number of (12volt) starter solenoids have the extra terminal to bypass the ballast resistor on 'start'

I suggest a 6volt relay could be wired into service perhaps,

Early XL XP fords had the ballast resistor as a PINK wire buried in the wiring loom from the ignition switch to the coil
Also, an inline fuse for lights neatly bound inside the insulation tape in an inaccessible place up near the firewall

.