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ABS trailer light on dash never worked

I have a 2001 Freightliner Columbia. The truck ABS light works with key on self test. The trailer ABS light has never worked since I have had the truck. Yes the lamp is good.

The ABS light on the trailer works with key on self test and with brakes applied key off.

My experience as an electronics tech tells me that there should be a connection between truck and trailer for a signal to turn dash light on but I cannot find one in the pigtail.

How does the truck "know" there is an ABS problem?

pigtail wires

white = ground

black= power marker lights

brown = power tail lights

yellow = left turn

green = right turn

red = stop

blue = power key on

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ABS communications occurs on the BLUE wire. This is from Wabash website trailer ABS installation maual.

Power Line Carrier (PLC)

In order to meet the (March 2001) requirement to provide a tractor-mounted trailer ABS malfunction lamp, the industry has adopted power line carrier (PLC) technology. The use of a power-line-carrier permits transmission of data between tractors and trailers without adding wires or connectors. Data is transmitted as a series of high frequency chirps that appear on the ABS full- time power line (the blue wire). The chirps vary in frequency between 100K Hz and 400K Hz. The PLC signals are transmitted between trailer ABS control units and tractor ABS control units. The tractor ABS unit has an additional output signal that controls the cab-mounted trailer ABS

malfunction lamp.

The cab-mounted trailer ABS malfunction lamp:

• Lights to indicate a bulb check at start-up

• Remains lit at start-up if a fault code is set

• Will turn OFF after two seconds if the trailer is disconnected

• Lights during vehicle operation to indicate a fault has occurred.

In order for PLC to function, tractors and trailers must both be equipped with PLC versions of ABS. All Wabash National MBS-2 systems manufactured after March 1, 2001 are PLC capable.

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