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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3375
Experience:  ASE Med/HD Truck Technician, NATEF Diesel Engines and Electrical/Electronics Technician, EPA 608/609 HVAC Certified, Steering/Suspension and ABS/Brakes Technician
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I have a 1999 Kenworth t600. When I turn my low beam

Customer Question

I have a 1999 Kenworth t600. When I turn my low beam headlights on , my low beam bbulbs are bright and my high beams come on dim. When I turn high beams on high beams come on and low beams get dim. Any ideas thanks brad
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 1 year ago.

My name is ***** ***** I will assist you.

You are describing a faulty multifunction switch. The switch is bad inside the turn signal stalk where the high beam switch is located.

Give it a swap and the issue should resolve.

How it should be is low beams stay the same and when you click the switch, high beams come on as well. Your switch is broken internally and crossing the light wires instead of changing them.

You want a 5 star service and I want to provide a 5 star service. Please consider that I don't know your skill level as well as I can't see,
touch, smell, hear the vehicle/machine in question. Please provide as much detail as possible and I will provide as much back.

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they are not the site feedback.

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Customer: replied 1 year ago.
Already replaced it, still doesn't work. Also replaced wire harnesses from headlights to the plug behind the bumper
Expert:  Jeremy replied 1 year ago.

Well, the one thing I would have expected it for sure isn't it.


There is a dimmer relay on the truck. That would be the only other thing that could cause that besides grounding/power issues in the harness in the cab.

If you add a ground to the circuit, there is no difference?

Click HERE for the diagram.

Customer: replied 1 year ago.
Replaced the relay, also tried running another ground
Expert:  Jeremy replied 1 year ago.

Thee can only remain an issue in the harness in the cab then. Something is crossed and working right or the wrong lamps are used. There isn't anything else I can think of. I'm stuck without actually putting my hands on it...

Customer: replied 1 year ago.
I tied clicking on your diagram but it did not work
Expert:  Jeremy replied 1 year ago.

I just checked it and it is working good. I had another check it and he had no issues either.