Wiring Diagram For 1971 F250 Ford Truck
The following information is reproduced directly from the 1970 Ford Body Builder's Layout book (Vol. 70 HTM2-L2). The information is applicable to all '67-'72 trucks.
1) The headlamp switches on all 1970 Ford trucks (except "W" models) employ two integral circuit breakers, one 12-amp for the headlight circuit and one 15-amp for auxiliary circuits. Connections to any point in the circuits controlled by the headlamp switch will be on the auxiliary circuit breaker, except connections to the #12 circuit (headlamp hi-beam, green wire/black stripe), the #13 circuit (headlamp low-beam, red wire/black stripe) and the #15 circuit (feed wire to the the dimmer switch, red wire/yellow stripe. Connections to the 12-13 or 15 circuits (headlamp bulb circuits) should be avoided.
If the total load on either headlamp circuit breaker exceeds the breaker rating, the headlamps or taillamps will cycle on and off indicating the overload. If this occurs, a portion of the added lights must be wired through a relay, feeding the relay coil from the headlamp switch.
The feed from added lights to be controlled by the headlamp switch should be terminated in a male bullet connector and be connected to the female bullet take-out (brown wire -- 285 circuit) on the left-hand side of the instrument panel harness (near the emergency brake). If the vehicle has roof marker lights, this bullet will be occupied by the feed from the roof wires. In this case fabricate a "Y" jumper to permit both connections to the single connector.
Rear lights to be controlled by the headlight switch can be spliced into the #285 circuit (brown wire) at any point in the taillamp harness.
NOTE: The special Camper option on light trucks provides a plug connector on the left-hand frame rail to which taillamp connections can be made directly.
Lights controlled by Stop Lamp Switch and Turn Signal Indicator
F100 thru F350 trucks are equipped with a mechanical plunger stop lamp switch which is mounted on the brake pedal arm. This switch is designed for maximum loads usually less than the fuses or circuit breaker in the circuit, but ample for normal stop lamp loads. The maximum load is 12.5 amps and under no circumstances are loads in excess of this value permissible.
If only stop lamp function is desired for the added lights, splice into the 810 circuit (red wire/black stripe) between the stop lamp switch and the turn indicator switch.
If only turn signal function is desired for the added lights, connect right-hand lights to circuit #2 (white wire/blue stripe) and left-hand lights to circuit #3 (green wire/white stripe). This connection can be made by splicing into the wires near the parking lights or near the steering column. (See note below.)
If both turn signal and stop lamp function are desired for the added lights, splice into the taillamp loom, using circuit 282 (green wire) for right-hand lights and circuit 283 (yellow wire/black stripe) for left-hand lights. (See note below.)
NOTE
1) The turn signal switch used on light trucks has a maximum rated current of 6.5 amps for right and left turning functions and 8.0 amps for stop lamp function. Do not exceed these values on the turn signals.
2) The turn signal and emergency flasher system on light trucks utilizes two flashers, one for turn signal and one for emergency flasher function. These flashers are designed to accommodate a two-light (4.2 amps) load for the turn signal flasher and a six-light load (12.6 amps) for the emergency flasher. If one additional 2.1 amp light is added to each side (total 6 lamps) the C8AB-13350-A turn signal flasher must be replaced with a C6AB-13350-B flasher. The addition of two 2.1 amp lamps to each side (total 8 lamps) will require replacing the existing two flasher with a single C8TB-13350-A transistorized flasher and, because of the complexity, is not recommended. The addition of lights without a flasher revision will result in a very fast, unacceptable flashing rate.
Added Lights or Accessories Controlled by Added Switches If the added accessory is desired to be operated with the key off, the switch feed can be connected to the cigar lighter (using a "Y" jumper) if the 15-amp fuse for that circuit is sufficient. If heavier loads than the cigar lighter circuit will carry are required, the switch feed should connect to the starter relay "battery" terminal.
For added electrical accessories that operate only when the ignition is on -- terminate the feed wire from the hang-on switch in a bullet connector and plug into the three-way accessory plug on the instrument panel harness (single black wire/green stripe). This circuit is limited by a 14-amp fuse. Fuse "blow" requires the addition of a relay, with the coil fed by a connection to the accessory terminal.
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Source: http://www.fordification.com/tech/schematics_h.htm