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Gentlemen,
Driveway mechanic here, I know enough to get myself in trouble.
1979 V8 400ci 4x4 shortbox
46 years, previous owners and rodents have done in my ignition harness. Installing a painless DS II harness which is partially in and the truck starts and runs. My question is the resistor wire, I have absolutely no clue where to connect it to or if its even needed? As far as i can tell or find the truck does not require a ballast resistor or is there one currently on the truck. There is a voltage regulator by the starter solenoid but nowhere to tap the resistor wire into - or I am not educated enough to know what to tap into.
Using a standard coil purchase from LMC truck. Do I even need the resistor wire, or can I simple cut & plug it?
I have seen a few "opinion" post's on here but could not find a definitive answer.
I do have a wiring diagram but it might as well be written upside down and backwards in chinese. Above my pay grade
Last edited by JMB79F150; May 30, 2025 at 07:10 AM.
Reason: Addition
The stock coil is not intended to run on a full 12 vdc, doing so continuously will cook it. It only gets a full 12 vdc to start, but when you relax the key and it returns to run, it gets a reduced voltage, as it doesn't need .... nor will it tolerate the full 12 vdc long periods.
Thank You, I think I can figure out where to tap into from your diagram. Thinking it might be easier getting a new coil with a built in resistor
Originally Posted by tbear853
The stock coil is not intended to run on a full 12 vdc, doing so continuously will cook it. It only gets a full 12 vdc to start, but when you relax the key and it returns to run, it gets a reduced voltage, as it doesn't need .... nor will it tolerate the full 12 vdc long periods.
Thank You, I think I can figure out where to tap into from your diagram. Thinking it might be easier getting a new coil with a built in resistor
You really should just stick with the original design. The reason full voltage was used while cranking the engine over was to be sure it actually started.
If you combine a weak battery and a starter motor drawing to much current ... you drag the voltage down. Possibly to the point you lose spark, like on a cold winter day with sub zero temps.
Also, if the painless harness came with a ballast resistor that isn't close to what is showing in the diagram (1.1 ohms)... Don't use it
The idea is both the coil and the ballast resistor have equal resistance (1.1 ohm). This causes an equal spit of the battery voltage, 6 volts each.
If the cranking battery voltage drops to 10 volts, then the coil is only getting 5 volts. So this is why Ford sent full battery voltage to the coil while cranking. The coil is a step UP transformer, higher input voltage gives higher output voltage
I believe on your truck, the starter solenoid "I" connection is the voltage source for the ballast resistor bypass, not the ignition switch. You just run a wire from "I" to the coil positive.
Good luck, Jim
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