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Jeremy, Certified Truck/Bus Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 3212
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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No trailer tail light power from truck. 2004 freightliner

Customer Question

No trailer tail light power from truck. 2004 freightliner century, MBE, last 6: M27855
Trlr tail relay in dash only shows power to pin 87 when light switch is on.
Submitted: 1 year ago.
Category: Medium and Heavy Trucks
Expert:  Jeremy replied 1 year ago.

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If you jump the relay, do you get trailer lights?

Are you getting grounds on the other 3 pins or are you losing any of them?

Freightliner doesn't have a clean diagram for the trailer circuit on the Century. I looked.


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Customer: replied 1 year ago.
I tried jumling pin 87 to pin 30, when jumped power to 87 cuts out, like there's a breaker popping. Power returns to 87 ~1 second after disconnecting jumper. I haven't checked ground on other 4 pins (5 pin relay #3602). I have no power at pins 85 or 86.
Customer: replied 1 year ago.
This is with trailer disconnected BTW.
Expert:  Jeremy replied 1 year ago.

If you remove the pigtail, the breaker stops popping, right?

I'm presuming you have a breaker in the fuse panel for the trailer lighting versus a fuse? If so, drop a higher ap one for just testing and see if it pops. There could be a bad breaker or a short to ground, which is either in the bottom of the fuse box or at the pigtail connector on the back of the cab.

One spot these tend to short out alot is if you go to the passenger side of the truck under the hood, behind the air filter you will see a huge plug of wires that comes out of the firewall. Your trailer light wires are in that big bunch of wires. I find that to short out a lot! If you wiggle the wires around with a test light resting in the relay holder, you can see the light flickering through the windshield. That is actually a double male plug that goes through that firewall. You would remove the plug on both sides and just push it through. They are notorious for shoring out right there.

Customer: replied 1 year ago.
If you remove the pigtail, the breaker stops popping, right?"
No, this is with trailer disconnected.There is 1 30amp fuse and 1 #3602 tyco relay in fuse panel for trailer tail & same setup for trailer marker.If there is a breaker for the trailer, I don't know where it is.I should have power to pins 85 or 86 to have continuity from 87 to 30. I do not.
Expert:  Jeremy replied 1 year ago.

Pin 87 should be your constant on. Pin 86 should be powered when you get the light switch turned on.

The light switch is a smart switch where it has a dedicated wire going to the trailer circuit. Can you verify that when in marker position, you have 2 hot wires from the switch? I would check that first. If you have power there, the loss is in between. If no power, switch is bad.

I would check that first. The middle is the interupt switch. It has it's own circuit for trailer as well. The it feeds the relay.

Customer: replied 1 year ago.
On the trailer marker relay (which works) pin 86 is constant & pin 30 is switched. (2 hot wires with switch on, 1hot with switch off).On trailer tail relay (which is in the bad circuit) ONLY pin 87 comes on/is hot with switch. (1 hot wire with switch on, no constant hot with switch off).Does switch have separate trailer marker & trailer tail wires out? Is this the dash light switch you're refering to?
Customer: replied 1 year ago.
Also, does constant hot go through dash headlight switch or just switched hot?

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