High beam headlights came on by them selves and stay on untill you pull the main fuse for the high beams. Can not locate the relay that controls the power to the high beams. The low beam system works normally. Looking for a wiring diagram or help on locating the relay. I am working on a 05 Kenworth t800

I would check the fuses first. Headlights are typically on a separate fuse then the parking lights. If the fuse is fine check and see if the Bulb is burnt out but it is rare for both lights to go out at the same time.

Do you know anything about automotive electronics ? Electronic control modules .
Headlamp ON/OFF control is determined by the body control module (BCM) by a signal on the headlamp switch on input circuit when the headlamp switch is in the HEAD position. When the headlamp switch is in the AUTO position, the BCM determines headlamps ON/OFF by the voltage from the ambient light sensor.
If the headlight switch is left in the ON position, the inadvertent power control feature will turn OFF the headlights approximately 10 minutes after the ignition switch is turned to the OFF position. If the driver places the headlight switch in the ON position after the ignition switch has been turned OFF, or if the ignition switch is in the ACCY position, the headlights will remain ON until turned OFF or the battery runs dead.
The high beam indicator is illuminated when the instrument cluster receives a GMLAN serial data message from the BCM that the high beams are illuminated.
High Beams
The high beam headlamps receive voltage from the headlamp high relay located in the underhood fuse block. The BCM controls the relay on the headlamp high beam relay control circuit. High beam control is determined by a signal on the headlamp dimmer switch signal circuit from the multifunction switch. When the headlamp dimmer switch signal circuit is grounded, the BCM grounds the headlamp high beam relay control circuit, illuminating the high beam headlamps. The right and left high beam circuits are independent and are fused independently in the underhood fuse block.
Low Beams - Standard
The low beam headlamps receive voltage from the headlamp low beam relay located in the underhood fuse block. The BCM controls the relay on the headlamp low beam relay control circuit. Low beam control is determined by a signal on the headlamp dimmer switch signal circuit from the multifunction switch. When the headlamp dimmer switch signal circuit is open, the BCM grounds the headlamp low beam relay control circuit, illuminating the low beam headlamps. The right and left low beam circuits are independent are fused independently in the underhood fuse block.

Hooking up a factory scan tool an checking DTC'S - diagnostic trouble codes would be the very first step in trouble shooting this problem .
DTC B257A 00: Headlamp Switch Input Signal Mismatch
DTC 2575 00: Headlamp Control Circuit

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The problem with the headlights is a common one and it is in the low beam relay.; In the under hood fuse panel, locate the HDM relay, swap this relay with the FAN relay next to it. If the lights stay on, replace the relay. This is a solid state relay, so make sure to get the same part number.

You can't turn them on manually with the switch . You have the switch set on auto ?
Headlamps
You can use the headlamps in two different ways.
• Place the headlamp switch in the ON position for normal operation.
• Place the headlamp switch in the OFF position for automatic lamp control (ALC). During ALC the high beam headlamps are On at reduced intensity for daytime running lamp (DRL) operation in daylight conditions, or low beam headlamps On in low light conditions.
The DRL/EXT LTS fuse and the EXT lamps fuse in the underhood junction block supply battery positive voltage at all times to both of low beam lamps and to the right high beam lamp. The low beam circuit continues to the high/low beam switch, then, depending on the position of the dimmer switch, connects to the normally-open headlamp relay switch contacts. Placing the headlamp switch in the HEAD position provides a ground path for the headlamp switch input of the Body Control Module (BCM). After receiving this input, the BCM energizes the headlamp relay. When the headlamp relay is energized, the low beam circuit is connected to ground G201. The high beam circuit is divided into a left and a right side circuit.
The DRL/EXT LTS fuse in the underhood ignition block supplies battery positive voltage to both the coil and to the normally-closed contact of the DRL relay switch circuit. During high beam operation, the left side current flow is through the closed contacts of the DRL relay, and to the left high beam lamp.
The EXT Lamps fuse in the top of the engine fuse block supplies battery positive voltage to the right high beam lamp. Both high beam circuits are then connected together, and continue to the dimmer switch. When the driver closes the high beam contacts of the dimmer switch, the circuit is grounded at G201.
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.
If the headlamp switch is left in the head position, the inadvertent power control feature will turn off the headlights 10 minutes after you turn the ignition switch to the OFF position. If you place the headlamp switch in the head position after the ignition switch has been turned OFF, or if the ignition switch is in the ACCY position, the headlights will remain ON until you turn them OFF or until the battery runs dead.

Check out your fuse associate with the low beam headlight, secondly there is a relay fuse under fuse rely box under the hoos. Locate this box, and locate the relay fuse associated to the low beam. You can rotate the relay with another relay of the same spec to see if the low beam lights are coming on. I would believe that it is a fuse that is blown, check your owners manaual on the location of the fuse box. Usually it's located in the driver side kick panel.

If high beams work and low beams do not, then it is probably the high/low relay. It is part number 15016745 and has four prongs. To test if your relay has gone bad locate the relay in your fuse box. There will be two of these relays located next to each other. One controls the low beam headlights and the other the cooling fan. Swap the relays and check your low beam headlights. If the low beams work you have a bad relay.

Hello, on the van you have a high beam fuse and relay. On the low beam you also have a fuse and relay. Check the fuse/relay panel for a blown fuse controlling the low beam lights. If you do not find a blown fuse have some one to turn the key on and turn on the high beam lights. During this time locate the low beam headlight relay and place your finger on top of relay, have the other person to switch from high beam to low beam lights. You should feel a click in the relay when they make the switch from high to low beam. If not replace the relay and the problem should go away. Hope this will help you.

Thanks

On the van you have a high beam fuse and relay. On the low beam you also have a fuse and relay. Check the fuse/relay panel for a blown fuse controlling the low beam lights. If you do not find a blown fuse have some one to turn the key on and turn on the high beam lights> During this time locate the low beam headlight relay and place your finger on top of relay, have the other person to switch from high beam to low beam lights. You should feel a click in the relay when they make the switch from high to low beam. If not replace the relay and the problem should go away. Hope this will help you.

it sounds like a bad light switch

Fuse 22 and fuse 23 are left and right low beam headlamp fuses located in the PDC. 23 and 24 are high beam fuses. There is also a low beam and a high beam relay in PDC as well. If no headlights work I would check for main switch or multifuction switch (wiper/high beam) since each light has its own fuse and relay.