Bulletin No.: PIT5747
Published date: 06/5/2020

Preliminary Information
PIT5747 Diagnostic Tip - Trailer Lighting Complaints With
Trailering App RPO U1D
Models
Brand:

Model:

Model Years:

VIN:
from

to

Engine:

Transmissions:

Chevrolet

Silverado 1500 (New Model)

2019

All

All

All

All

Chevrolet

Silverado

2020

All

All

All

All

GMC

Sierra 1500 (New Model)

2019

All

All

All

All

GMC

Sierra

2020

All

All

All

All

Involved Region or Country North America
Additional Options (RPO)

With Trailering App RPO U1D
Some customers may comment that their trailer lighting does not operate in the
manner expected. Diagnosing these concerns can be di cult because the issue
could either be related to the truck or the trailer. This PI is to provide some
additional diagnostic information for the symptoms below:
-Trailer lighting is inoperative
-Trailer detection is not possible
-Trailer lights ash intermittently while the ignition/vehicle is OFF

General Information
-Any current body style vehicle without RPO U1D, along with all previous model
years, are not equipped with a K68 Trailer Lighting Control Module. These vehicles
utilize relays to power the trailer connector and do not have features like trailer
detection, trailer theft, circuit diagnostics, or bulb outage detection.

Condition

-Vehicles with RPO U1D, are equipped with a K68 Trailer Lighting Control Module,
which powers the trailer lighting circuits to the trailer connector. This provides
additional features like trailer detection, trailer theft and trailer lighting circuit
diagnostics, such as, trailer bulb outage detection and over current.
2019 Model Year Vehicles - Equipped with U1D use the K68 Trailer Lighting Control
Module to control all trailer lighting through pulse width modulated (PWM) voltage.
The trailer park, stop/turn signal lamp control circuits must draw at least 55mA of
total current to be detected as a trailer or the Trailer Lighting Control Module will
not control the lighting circuits to the trailer.
2020 Model Year Vehicles- Equipped with U1D use the K68 Trailer Lighting Control
Module to control trailer lighting through pulse width modulated (PWM) voltage to
the stop/turn signal and backup lamps. The park lamps utilize an internal printed
circuit board (PCB) relay to control the park lamp circuit. This enables additional
output current for the park lights. The trailer stop/turn signal lamp control circuits
must draw at least 55mA of total current to be detected as a trailer or the Trailer
Lighting Control Module will not control the lighting circuits to the trailer.

Cause

Root cause can very, follow the information in the Correction section below.

Correction:
When diagnosing a trailer lighting complaint it is recommended to connect the trailer simulator test tool (shown
below), GM P/N EL-52641, to the truck's 7 way connector. The K68 Trailer Lighting Control Module will detect the
test tool and enable the trailer lighting outputs. Turn on the ignition and operate all the lights: park, LT/RT turn,
stop and reverse. If the lights on the test tool illuminate in correlation with the lights on the vehicle and no DTC's
or warning messages are displayed, then the truck is operating as designed and no repairs should be performed
to the truck.
Note: When the ignition is turned o , while the trailer simulator test tool is connected, it may be noticed that the
trailer lighting LED's on the tester icker/ ash randomly. This is normal and is part of the K68 Trailer Lighting
Control Module's trailer detection/trailer theft features.
Some trailers have wiring that is in very poor condition. A fault on the customer's trailer may result in DTCs set
against the trailer lighting circuits along with DIC warning messages. In some cases, this may even open the K68
Trailer Lighting Control Module Fuse F74UA or F82UA on the vehicle. Verify the customer's trailer wiring is in
good working condition.
If, after using the trailer simulator tester, the truck functions properly, you may follow the diagnostic tips listed
below to help diagnose a trailer related issue. Trailer issues are NOT covered under warranty, but this
information listed below can be used to help the customer/trailer supplier understand and correct any trailer
related issues if they so choose.

1. Some trailers utilize a trailer mounted control module to operate some or all of the trailer lights. These trailers
may use the B+ circuit from the trailer connector to power the trailer lighting circuits. These trailers may not
always be detected by the Trailer Lighting Control Module and may set faults. Possible ways to remedy this:
A. Load resistors could be added in parallel to the detectable lighting circuits of the trailer, which will increase
current draw. One known source is the Curt brand Trailer Wiring Adaptor P/N 57003, which can be plugged inline
with the 7 way trailer connector. This adaptor can be used to simulate additional lighting load, allowing the K68
Trailer Lighting Control Module to detect the trailer, however some circuit diagnostics and bulb outage detection
will not be possible. The addition of a load resistor/adaptor will be at the customer's/trailer manufacture's
discretion.
B. The customer's trailer will need to be rewired to eliminate the trailer mounted control module and have the
circuits wired directly to the trailer lamps. This option would allow for the K68 Trailer Lighting Control Module to
properly detect the trailer and provide full system features such as, trailer detection, trailer theft, trailer lighting
circuit diagnostics, and trailer bulb outage detection. This repair will be at the customer's/trailer manufacture's

discretion.
2. Some non–DOT approved LED lamps may not draw enough current for the K68 Trailer Lighting Control Module
to detect a trailer when connected, and the K68 Trailer Lighting Control Module will not enable the lighting circuits
to the trailer. Possible ways to remedy this:
A. Change one or more trailer lamps on the detectable circuits to incandescent bulbs/ DOT-approved LED lamps
to increase the current draw which will maintain full functionality of the K68 Trailer Lighting Control Module. The
addition of incandescent bulbs/ DOT-approved LED lamps will be at the customer's/trailer manufacture's
discretion.
B. A load resistor could be added in parallel to the detectable lighting circuits of the trailer, which will increase
current draw. One known source is the Curt brand Trailer Wiring Adaptor P/N 57003, which can be plugged inline
with the 7 way trailer connector. This adaptor can be used to simulate additional lighting load allowing the
K68 Trailer Lighting Control Module to detect the trailer, however some circuit diagnostics and bulb outage
detection will not be possible. The addition of a load resistor/adaptor will be at the customer's/trailer
manufacture's discretion.
3. Some aftermarket accessories, such as trailer mounted cameras, are incompatible with PWM voltage and may
not function correctly when connected to the trailer lighting circuits of the vehicle. One possible way to remedy
this is the lighting control circuits can be wired to control a relay, in which the relay will provide stable voltage to
the customer's aftermarket accessory. This repair will be at the customer's/trailer manufacture's discretion.
4. With the ignition OFF, the K68 Trailer Lighting Control Module will periodically pulse the lighting circuits of the
trailer to verify it is still connected. Depending on the con guration of the trailer lights, the trailer lights may
periodically ash as part of the trailer detection and/or trailer theft features. These ashes correspond to when
the K68 Trailer Lighting Control Module pulses the lighting circuits to ensure the trailer is still connected and is
considered normal.

Version History
Version
Modi ed

1
06/03/2020 - Created on.

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