On my 91 Bronco, why do the headlights only operate on high beam?

Ive changed the relay and the headlight switch on the dash .. still wont turn off headlights ? Just changing parts won't fix it , need to test . Hooking up a professional scan tool , one that can read codes in the BCM - body control module is the way to start to find the problem . Then familiarize yourself with how they work.

Headlamps
Major components of this system are the body control module (BCM), headlamp relay, beam select relay, ambient light sensor, headlamp and panel dimmer switch, turn signal/multifunction switch, park brake switch, and the low and high beam headlamps.
The headlamps may be turned ON in 2 different ways. First, when the driver places the headlamp switch in the ON position, normal operation occurs. Second, when the headlamp switch is placed in the AUTO position, automatic lamp control (ALC) operation occurs. During ALC control, the headlamps will be OFF in daylight conditions or in low/high beam operation in low light conditions.
The headlamp relay receives battery positive voltage directly from the battery. The BCM supplies a ground signal to the headlamp relay for normal headlight operation. The BCM also supplies voltage to the beam select relay coil for high beam headlight operation. When the driver places the headlamp switch in the ON position and the dimmer switch is in the low beam position, the headlamp relay supplies current flow through the left and right low beam headlamp fuses to both low and high beam headlamps. Both low beam headlights are grounded through the beam select relay. When activated, the dimmer switch sends a ground signal to the BCM in the high beam position and a momentary ground signal in the flash-to-pass (FTP) position from G106. The BCM then energizes the beam select relay which switches the headlamp grounds from the low beam headlight bulbs to the high beam headlight bulbs illuminating the high beam headlights. With the headlights in the low beam position, the high beams may be momentarily turned ON or flashed by activating the FTP portion of the switch.
The instrument panel cluster (IPC) illuminates the high beam indicator when the BCM detects that the high beams are requested. The IPC receives a class 2 message from the BCM requesting illumination.
The Headlamps On at Exit and Perimeter Lighting modes can be adjusted using the steering wheel controls to select the function as displayed in the driver information center (DIC). The Headlamps On at Exit selection will allow the driver to customize the period of time that the headlamps remain illuminated upon exiting the vehicle. This period can be adjusted from 0-180 seconds. The Perimeter Lighting feature allows the driver to select if they want the headlamps and backup lamps to illuminate when the door UNLOCK button on the key fob is activated. This feature can be turned ON or OFF by using the DIC steering wheel control buttons.

DTC B2575 Headlamp Control Circuit

Circuit Description
When the body control module (BCM) senses a request for headlamp or daytime running lamp (DRL) illumination, the BCM sends a ground signal to the headlamp relay through the low beam headlamp relay control circuit. The headlamp relay will illuminate the low beam headlamps.

Install a scan tool.
Turn ON the ignition, with the engine OFF.
With a scan tool, command the headlamps ON and OFF.
Does the headlamp relay turn ON and OFF with each command?

Do you know what bi-directional control means ? Need a professional scan tool to do these type of test .

DTC B2602: Daytime Running Lamp Control Circuit Low or Open
DTC B2603: Daytime Running Lamp Control Circuit High

Without this type of info. from service repair manual an using a scan tool your missing the big picture .

Sounds like it has possibly got a defective multifunction switch,which is part of the turnsignal lever/high beam lever switch. Aside from physically diagnosing the circuit I would say replacing the multifunction switch may take care of your problem.


Hope this helps

The little wire ends and the plastig plug are available at auto elecy shop just cut and shut resolder new terminal and fit to plug. To stop this problem re occouring i would suggest fitting a relay to the head lights both high and low beam and a relay for any spots seperatly but connected to high beam only. excessive amps has been running thru the switch causing meltdown. Relays will reduce the load to the wires

Bad wiring contacts. Resecure all wiring and clean all contacts that you can get to in hope that you get it.

It sounds like the multi-function switch in your steering column has worn out. It will have to be replaced,you can't repair it. Hope this helps.

Possible hi/low relay, or headlight hi/low switch. May also be the wiring.

check the bulbs first and also the fuses theres a fuse for each side and for low and high beams, start there, good luck.

check the headlight wires for power when you have the highbeams switched on, more than likley its gonna be your dimmer switch, its really common in older vehicles

What you have sounds like a Headlight SWITCH Problem, You can check the Fuse for them (DOUBTFULL) but its a Large one @ Power distribution Box Under Hood.

The only thing that all of your problems have in common is the light switch and the dash wiring harness. The stereo is linked to the light switch because of it's illumination feature' You need to test the light switch with a test light by probing the wires at it's plug.