Adding a Running Lights Relay to a 1991 Ford Truck

by Jon (JPR)

Having burnt the running lights part of the headlight switch in my 1991 F-250 diesel twice, I decided it was time to correct the problem and take the electrical load off the switch. This is a separate modification from the "brighter headlight mod" that uses relays to power the head lights, and accomplishes the same thing for the headlight part of the headlight switch. If you have not performed that modification, it would be highly recommended.

Before beginning, disconnect both negative battery cables. The wiring colors and fuse number may differ between model years, so please be sure to verify the correct color power wire for your truck.

To gain access to the headlight switch and wiring, remove the wiper and head light switch knobs. To remove the knobs, look for a small notch in the back of the knob. Place a small flat screw driver in this notch, press the clip toward yourself while gently pulling on the knob. The 1991 trim panel is removed next by gently lifting along the edges. Other model years may have screws fastening it. Be careful on pulling too hard, as the fuel tank selector switch is mounted to this panel and the switch mounts can be broken. Remove the switch from the panel and set aside. The steel switch panel is removed by pulling the four mounting screws. You can leave the switches attached to the panel.

Looking at the head light switch, the tan /white stripe wire is the power from fuse # 4. Tap into this wire for power to to terminal #30 on the relay. I used a premade relay plug, however you can use standard female spade connectors and 12" long 14 gage wire.

The dark brown wire next to the tan /white goes to the running lights, cut this wire. The switch side will go to #86 on the relay, to activate the relay when the head

light switch is turned on. The brown wire going to the lights will connect to #87 on the relay.

Connect a ground wire to #85 on the relay.

To protect the wiring, wrap the wires with two layers of electrical tape. Make the first wrap with the adhesive side outward, followed by the second layer adhesive

side inward. Wrapping the harness this way will keep the wires clean if you need to remove the tape later and will hold just as well.

I slid the relay down next to the fuse box. This picture shows a #8 bolt that is holding the relay next to the fuse box. Once painted the screw should not be

noticeable.

I purchased the Bosh relay with the plug from Parts Express. $2 for the relay and $1.50 for the plug.

If you use this plug, insulate the end or remove the red lead to #87a. This wire will have power when the light switch is turned off.

This modification, in combination with the traditional headlight relay modification, will make your headlight switch last far longer, then it would with factory electrical circuits. This mod will benefit you especially if you have added additional running lights, or if you pull a trailer.

Parts List:

Bosh 30 Amp 12 volt Relay (or equivelent) available at http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=330-070

Relay Plug (optional, if substituting spade connectors) available at http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=330-075

Electrical Tape