Headlights go out when high beam switch is activated

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 .

The usual cause is a bad ground.
Check the ground with a test lamp.

Both the headlight switch and the BCM provide a ground, or control circuit, to the headlight relay coil circuit for manual or automatic headlamp operation. When you place the headlamp switch in the OFF position, and the ambient light sensor indicates low light conditions, the BCM energizes the headlamp relay for low beam operation. You may switch the headlights to high beam by pulling the dimmer switch handle toward you until the dimmer switch is activated. When the dimmer switch is placed in the HIGH position, the indicator lamp input to the BCM is pulled low. The BCM then sends a class 2 message to the instrument cluster in order to illuminate the high beam indicator lamp.
Low and High Beams Headlamps Inoperative ?
Replace the headlamp switch. Refer to Headlamp Switch Replacement .
Did you check the headlamp relay ? Location ,left instrument panel
Did

Check that the bulbs actually work, with an external power supply.

Unless both low beam filaments of bulbs have blown at same time I would say you've got a bad switch or a wiring issue. Anything is possible I would check the bulbs first, Look for gray tint in the bulb or broken wire in the bulb. You can always put a new bulb in one side and see if it starts working, if not I would move to the switch.

Test the headlight switch as follows.

Move your car to a dark area, such as waiting until night or moving to a dark garage, to test your headlight switch.

2Turn on your car, so you can see your dashboard lights. Note whether some of your dashboard lights fail to illuminate.

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  3

Turn on your car's headlights. Note whether your car's headlights illuminate.

4Press the high beam lever to activate your high beams. Note whether your high beams activate.

5Put your car in "Park." Move to the back of the car. Note whether your parking lights are illuminated.

6View any remaining driving lights, such as lights above the license plate. Note whether these lights come on.
If more than one set of lights does not activate when you turn on the lights, you may need to replace your headlight switch. If only one set of lights does not turn on, try replacing the lightbulbs in that set and then retesting your car. If the lights still do not illuminate, you may need to replace your headlight switch.

I'm not sure about your model, but my 1991 EB Ford has a high-beam relay in the engine bay, near the battery. When you find it, replace it, and I'll bet your lights work then.

Replace the HI/LO Beam Relay.
Regards.DT

You need to start by checking the fuse for your headlights (low beams). The DRL could be be bad, but I have not seen them go out very often.

check and see if the dimmer switch is working, it's most likely stuck. it might also be the headlight relay.