thanks in advance for your help, i did a cab swap on my truck the cab i took off had a external resistor between ign. and coil the cab i put on did not have an external resistor to coil this cab was a 1974 just need to know what to do when hooking up the coil, can i just do away with the white resistor and hook up coil, someone told me that on the 74 the wiring had the resistor already in the wiring, is that true? i have the old coil to the 74, but the other coil looks newer and would like to use it. thanks

 

The ignition systems in 73-79 trucks, whether using points or electronic ignition, are designed to use an external ballast - meaning the coil is not rated for 12 volts. This limits the current through the points, or in the case of electronic ignition, the power transistor in the ignition module.

Whether the 390 you're running is set up for points or factory electronic ignition, it is intended to be used with a ballast. A factory wiring harness for either type of ignition system of this era will have the ballast built in to the dash harness. You must use a coil designed for an external ballast in this case. In fact, most coils designed for this application should be marked "use with external ballast."

If the wiring harness you're using now has been modified, then it's anybody's guess as to what it's set up for at this point.

 

hi, what does this built in ballast look like? thanks for your help

 

There are more knowledgeable guys than me on here, but what I understand by "external resistor" in this context is the resistor wire that runs from the ignition switch under the dash to the firewall plug. Are you certain that wire is not under the dash of your 74 cab? It is usually pink and is quite long (to provide sufficient resistance).

 

^ ^ What he said ^ ^

1973/79 F100/350 & Bronco:

COLF-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-37)

61" long / 1.30-1.40 ohms resistance / Color coded Pink / #16 gauge wire.

Or...

D1AZ-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-84A)

60" long / 1.30-1.40 ohms resistance / color coded Red w/Green stripes / #16 gauge wire.

Or...

D7AZ-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-213)

49" long / 1.30-1.40 ohms resistance / Color coded Red / #16 gauge wire.

 

hi, let me know if this is right i'm sure the wiring on cab is stock with resistor in harness, my motor which had the coil with the white external resistor hook up, i should be able to do away with the white external one and just use the wiring from my new cab ,when i pulled my new cab off of the old frame it was hook up to the coil , i should be able to use the coil that had the white resistor and just do away with it and use factory wiring from my cab right? thanks for your help