Excessive Load on Trailer Running Lamps

The trailer running lamps may illuminate momentarily and then turn off or may flash when connected to 2019 Silverado 1500 and Sierra 1500 models. When operating a trailer running lamp circuit with more than 7-8 amps of current draw, the trailer lamps will illuminate for a second and then go out or, on trailers with LED lighting, the lamps will flash about once every 10 seconds.

These conditions may be caused by a trailer with incandescent running lamps that may have in-rush current that exceeds the Trailer Lamp Control Module (Fig. 14) limits for current flow. On trailers with LED lighting, the monitoring voltage may illuminate the LED lights even when the trailer theft deterrent feature is not enabled.


Fig. 14


2019 model year trucks equipped with the Trailer Information Indicator (RPO U1D) use the K68 Trailer Lighting Control Module to control all trailer lighting through pulse width modulated (PWM) voltage. The Trailer Lamp Control Module has a current handling capability on the running lamp circuit of about 9 amps. On a trailer with a large number of incandescent lights, in-rush current can exceed 20 amps before it settles back to around 8-9 amps. During this in-rush or “marriage” current period, the Trailer Lamp Control Module identifies the current as an overload and goes into protection mode.

For 2020 model year trucks, a change was made to allow 16 amps of continuous current flow by routing the current through an internal relay in the Trailer Lamp Control Module. 2019 models can be modified to operate in the same way with the use of the 2020 design software and a re-pin of the running lamps to pin 25 using the internal relay to operate the running lamps. (Fig. 15)


Fig. 15


The update requires a terminated lead to be inserted into cavity 25 on the chassis harness X1 connector. (Fig. 16) Wiring connected to pin 5 on the X1 will need to be cut and reconnected to pin 25. The terminal in cavity 5 will remain, but must be blunt cut and sealed with a closed-end Duraseal splice to prevent water intrusion through pin 5.


Fig. 16


The updated 2020 software is managed by the Techline Customer Support Center (TCSC). A record must exist of which trucks have been reprogrammed and that the procedure covered in Bulletin #20-NA-198 has been completed. Software will not work with the production configuration of wiring. The update also changes the trailer connection detection flashing of LED lighting from every 10 seconds to every 42 minutes. If trailer theft deterrent is enabled in the trailering profile, the flashes will still occur about every 10 seconds, which is by design and cannot be changed.

TIP: Concerns of excessive flashing of LED lights when parked on 2020 models also can be addressed with the same software update. The update does not affect lamp operation of the trailer theft deterrent system. It only changes the trailer connection detection from once about every 10 seconds to once every 42 minutes when the trailer theft deterrent option is not selected. Previously, both selections would produce lamp flashes every 10 seconds with the trailer parked.

To review the service procedure to move circuit 2109 on the Trailer Lamp Control Module from X1 cavity 5 to X1 cavity 25, and program new software from SPS into the Trailer Lamp Control Module, refer to Bulletin #20-NA-198.


– Thanks to David MacGillis


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