Davis Unified Ignition
Dave Stribling
May 8, 2015
Contributers: Dave Stribling

Resistor Wiring

I will be installing a DUI HEI-style distributor in my ’65 Mustang hardtop with an inline-six. This unit requires direct 12V power from the ignition switch. I know I would need to connect this 12V power wire to the ignition switch terminal marked “C.” This is where the red wire with green stripe is currently connected to a pink resistor wire and goes out to the engine compartment as two wires (red/green stripe to coil and the brown wire to starter solenoid “I” post). Should I just disconnect this resistor wire from the “C” terminal and insulate the end? Or do I need to remove it completely since it does not need the red/green stripe wire to the coil anymore? I would appreciate any info on this. I’ve researched the DUI wiring and most of them tell you to hook up a 12V power wire to it and bypass the existing coil wire, or connect a relay, but does not explain it in detail. Thank you.

Ben (last name withheld)
Via the Internet

The coil gets its power from two sources—the brown wire that runs from the starter solenoid “I” terminal back to the coil (when cranking you get a full 12 V to help start the car) and then from the “C” post on your ignition switch. The green/red wire is sending voltage out to the voltage regulator, and the ballast (red/green) wire that goes to the coil. You can use this 12V point at “C” on the backside of the switch, as it is switched just like you want. Cutting the ballast wire out has caused a lot of grief for people when tracking down ignition problems. Keep as much of it connected as possible, and wrap it up in the harness. If you have to chase down ignition problems in the future, you can always pull the HEI setup and install a good old-fashioned points setup. I know—I have had to do it. As for the relay-type installation, people generally use the existing coil resistor wire to trigger a standard automotive relay that gets the “load” side from a full 12V source, such as the alternator feed or battery.

Letting the Magic Smoke Out

I have a ’65 fastback with a 5.0L swap, factory A/C, T-5 five-speed, electronic ignition, and lots of Shelby goodies. Everything was fine until I used the windshield wipers (first time ever, I think). They stopped working after a minute or so. But along with the wipers stopping so did my gas gauge, signal lights, temp gauge, and electric cooling fan. Is there a relay somewhere controlling these? If so, where do I look?

Joe Amoroso
Via the Internet

The wipers get their power from the 14-amp “ACCY” fuse on your fuse block—check that first. The fuel gauge and the temp gauge get their power from the same splice in the main harness, which goes to the constant voltage relay. Make sure you have your cardboard wiper transmission arm shield in place up under the dash. This is installed to prevent the wiper linkage touching and rubbing, pulling, or shorting out any wires. Next, check your grounds behind and to the left of the instrument cluster—several ground wires come to a single point here and could be loose and not making good contact. If you are trying to run the control circuit for your electric cooling fan off of the 14-amp “ACCY” fuse, you may want to move it, as 14 amps is not enough for a typical electric cooling fan. We recommend at least a 30-amp circuit with a relay and properly fused protection.

Let us hear from you. Send your ’65-’73 Mustang questions to: Beyond Basics, c/o Mustang Monthly, 831 S. Douglas St. El Segundo, CA 90245. Send email to mustang.monthly@enthusiastnetwork.com.