I am installing a Pertronix ignition and coil and need to bypass the
voltage dropping resistor to the coil. Can someone tell me where
it is on my 74 F250, 360 motor?

 

Red/green...splices in behind the cluster

 

Most of the time it is built into the harness and you would need to open it up to replace this wire as it is not a resistor as you know it.


You are better off to use the stock power wire to trigger a relay that runs a full 12 volts to the IGN unit. This way there is no need to open the stock wiring harness.
Dave ----

 

OP has a 1974 F250. Resistor wires are not spliced, are located within the wiring harness, coil to ignition switch.

C0LF-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-37) / Obsolete ~ Available NOS & from auto parts stores.

61.49" long / Color coded pink / 1.30-1.40 ohms resistance / #20 gauge wire.

1960/70 all FoMoCo vehicles; 1971/76 and some 1977 F100/350, Bronco & Econoline.
------------------------------------------------------------------------------------------------
D1AZ-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-84-A) / Obsolete ~ Available NOS & from auto parts stores.

60" long / Color coded red with green stripes / 1.30-1.40 ohms resistance / #16 gauge wire.

1971/76 FoMoCo Passenger Cars.
--------------------------------------
D7AZ-12250-A .. Resistor Wire-Ignition Coil (Motorcraft DY-213) / Obsolete ~ Available NOS & from auto parts stores.

49" long / Color coded red with green stripes / 1.30-1.40 ohms resistance / #16 gauge wire.

Misc. 1977/89 FoMoCo Passenger Cars; Some 1977 F100/350, Bronco & Econoline; 1978/89 F100/350, Bronco & Econoline.

 

Instead of modifying/butchering the harness, use the IGN supply wire (incl resistor wire) to trigger a relay (under hood) which will supply BAT VOLT to the PERTRONIX COIL. The actual module is 12V or suppressed voltage?

 

The Pertronix module prefers the full 12v. The coil voltage will depend on which coil is used. They offer multiple choices, but best would be to use one for the Ignitor and 12v and you don't have to worry about the resistor there either.

Relay is a great way to do it.

Paul

 

What I went with was the Petronix Ignitor, Flame Thrower coil, Taylor Street Thunder
wires, and E3 plugs. Wasn't sure about the plugs, but they had a rebate. Instructions
say the 1.5 ohm coil needs 12V. I like the relay idea, however being an electronics
guy, I'm not nuts about adding another possible source of problems. I might go with
a tap off the ignition switch. Thanks for the ideas and help.

 

The way to look at it, and Pertronix isn't very clear on this, the voltage specs are really to limit the amount of current flowing through the primary. It's my understanding when Ford (and others) switched to the 12 volt system in 1956 they left the points/condenser and coil system alone, and introduced a ballast resistor in the primary ignition circuit to limit the current, so the points wouldn't burn up.

Every primary ignition circuit has a certain amount of resistance, the primary winding of the coil, plus the wiring itself. If you dig around the Pertronix website they say 8 amps is the limit - for the Ignitor module itself.

Now with this information you can play around with the numbers and see what's what. The ultra-low resistance ignition coils below 1 ohm are designed for their higher end Ignitor III and such only. A 1.5 ohm coil is designed for V8 and then bypass or eliminate the ballast. The 3.0 ohm coil is designed for the 4 and 6 cylinder engines.

Use 14 volts as a baseline instead of 12, for accuracy in calculating the amps running through the primary as this is closer to what the ignition sees in use. Also keep in mind an ignition coil is not "on" all the time, it has maybe a 60% duty cycle.

So 14 volts / 1.5 ohms x 60% = 5.59 amperes, well under the 8 amps Pertronix recommends. Actual primary resistance will be a little higher than 1.5 because of the primary circuit wiring to and through the ignition switch etc.