Diagnostics and testing of trailer lights (Discovery 3)

Review

For a detailed description of the system design and operation, refer to Section 417-01 "Steering System" in the workshop manual.

Before troubleshooting the trailer lighting system, check the operation of the towing vehicle lighting system by disconnecting the trailer lighting system connector(s) from the vehicle power outlet(s).

Please note that intermittent fault messages may be caused by the cycling nature of the field effect transistors (FETs).

Inspection and verification

1. Ensure that the customer's complaint is justified.

2. Visually inspect for obvious electrical problems.

Mechanical factors

Electrical factors

Condition and installation of trailer light(s) Lamps and installation Lamp holders and installation Power socket(s), trailer connector(s) and their installation

Fuses Battery electrical junction box (BJB) - fuse 18E BJB - fuse 11E Central electrical junction box (CJB) - fuse 22P (CJB) - fuse 11E 8P fuse (CJB) - 6P fuse (CJB) - 32P fuse (CJB) - 14P fuse Trailer fuse box - Fuse to trailer electrical system relay Trailer fuse box - Fuse to trailer electrical system battery power supply Trailer fuse box - Fuse to trailer electrical system battery power source Trailer fuse box - Fuse to trailer brake lights Trailer fuse box -Fuse to trailer fog lights Trailer fuse box -Fuse to trailer fog lights Trailer fuse box -Fuse to trailer right rear light Trailer fuse box -Fuse to trailer right rear light Fuse for number plate light and left rear trailer light Relay Reverse Light Relay Side Light Relay Trailer Power Socket Relay Brake light switch Wiring harness Loosening or corrosion of connector(s) Trailer power socket(s) grounding circuit(s) Battery Junction Box (BJB) Central Junction Box (CJB) Trailer fuse box Trailer relay box Instrument cluster (IPC) CAN electrical circuits (Control Area Network)

3. If an obvious cause for the problem you identified or the customer described is found, resolve it before moving on to further action. (if possible).



4. Use the approved diagnostic system or scan tool to retrieve all diagnostic trouble codes (DTCs) before referring to the symptom chart or DTC index.

Since DTCs are stored in more than one module, a full vehicle data read is recommended.

Clear all DTCs after troubleshooting.

Table of signs of malfunction

Sign of malfunction	Possible reasons	Actions
Trailer brake lights not working	Lamp failure The fuse(s) have blown Electrical circuit failure Brake light switch malfunction	Check the condition of the lamp and fuse (see visual inspection table). Check the trailer brake light circuit. Check the brake light switch operation. Refer to the electrical manual.
Trailer brake light(s) dim	Incorrect lamp power Electrical circuit failure Brake light switch malfunction	Check the condition and power of the lamp. Check the electrical circuit of the trailer brake lights. Check the operation of the brake light switch. Refer to the electrical equipment manual.
Trailer brake light(s) are constantly on	Electrical circuit failure Brake light switch malfunction	Check the trailer brake light circuits. Check the brake light switch operation. Refer to the electrical manual.
Trailer fog light(s) not working	Lamp failure The fuse(s) have blown Electrical circuit failure	Check the condition of the lamp and fuse (see visual inspection table). Check the trailer fog light circuit. Refer to the electrical manual.
Trailer fog light(s) dim	Incorrect lamp power Electrical circuit failure	Check the condition and power of the lamp. Check the electrical circuit of the trailer fog lights. Refer to the electrical equipment manual.
Trailer fog light(s) stay on all the time	Electrical circuit failure	Check the trailer fog light circuit. Refer to the electrical manual.
Trailer rear light(s) and number plate light(s) do not work	Lamp failure The fuse(s) have blown Electrical circuit failure Trailer side light relay faulty	Check the condition of the lamp and fuse (see visual inspection table). Check the electrical circuit of the rear lights and trailer license plate lights. Check the operation of the rear light and trailer license plate light relay. Refer to the electrical manual.
Dim light from rear light(s) and trailer number plate light(s)	Incorrect lamp power Electrical circuit failure Trailer side light relay faulty	Check the condition and power of the lamp. Check the electrical circuit of the rear lights and trailer license plate lights. Check the operation of the trailer side light relay. Refer to the electrical equipment manual.
The rear light(s) and the trailer number plate light(s) are constantly on	Electrical circuit failure The relay for the rear lights and trailer license plate lights is faulty	Check the electrical circuit of the rear and trailer license plate lamps. Check the operation of the rear and trailer license plate lamp relay. Refer to the electrical manual.
Trailer direction indicator(s) not working	Lamp failure The fuse(s) have blown Electrical circuit failure	Check the condition of the lamp and fuse (see visual inspection table). Check the trailer turn signal circuit. Refer to the electrical manual.
Trailer direction indicator(s) dim	Incorrect lamp power Electrical circuit failure	Check the condition and power of the lamp. Check the electrical circuits of the turn signals.
Trailer direction indicator(s) are constantly on	Electrical circuit failure	Check the condition of the lamp and fuse (see visual inspection table). Check the trailer turn signal circuit. Refer to the electrical manual.
Trailer reversing light(s) not working	Lamp failure The fuse(s) have blown Electrical circuit failure Trailer reversing light relay faulty	Check the condition of the lamp and fuse (see visual inspection table). Check trailer reversing light circuit. Check trailer reversing light relay operation. Refer to electrical manual.
Trailer reversing light(s) dim	Electrical circuit failure Trailer reversing light relay faulty	Check the condition and power of the lamp. Check the electrical circuit of the trailer reversing lights. Check the operation of the trailer reversing light relay. Refer to the electrical equipment manual.
Trailer reversing light(s) stays on	Electrical circuit failure Trailer reversing light relay faulty	Check trailer reversing light circuit. Check trailer reversing light relay operation. Refer to electrical manual.
The indicator lamp(s) does not work	The fuse(s) have blown Electrical circuit failure Instrument cluster malfunction	Check the fuse(s) (see visual inspection). Check the electrical circuit of the indicator lamps. Refer to the electrical equipment manual. Check for DTCs indicating a malfunction of the instrument cluster or CAN system.
No battery power to trailer electrical system	The fuse(s) have blown Electrical circuit failure	Check the condition of the fuse (see visual inspection table). Check the trailer battery electrical circuit. Refer to the electrical manual.
No power when ignition is on trailer electrical system	The fuse(s) have blown Electrical circuit failure Trailer power socket relay faulty	Check the condition of the fuse (see visual inspection table). Check the trailer power supply circuit with the ignition on. Check the operation of the trailer power supply relay with the ignition on. Refer to the electrical equipment manual.

DTC Index

NOTE: Generic scan tools cannot read the codes listed or can only read 5-digit codes. Match the 5 digits from the scan tool with the first 5 digits of the listed 7-digit code to identify the fault. (the last 2 digits provide additional information readable by a manufacturer approved diagnostic system).

DTC codes	Description	Possible reasons	Actions
B1D0811	Trailer Left Turn Signal Electrical Circuit	Trailer left turn signal: short circuit to ground	Refer to the guided diagnostic program for this code on the approved diagnostic system.
B1D0815	Trailer Left Turn Signal Electrical Circuit	Trailer left turn signal: short circuit to battery + Trailer Left Turn Signal: High Resistance	Refer to the guided diagnostic program for this code on the approved diagnostic system.
B1D0911	Trailer Right Turn Signal Electrical Circuit	Trailer right turn signal: short circuit to ground	Refer to the guided diagnostic program for this code on the approved diagnostic system.
B1D0915	Trailer Right Turn Signal Electrical Circuit	Trailer right turn signal: short circuit to battery + Trailer Right Turn Signal: High Resistance	Refer to the guided diagnostic program for this code on the approved diagnostic system.
U1A1449	CAN initialization failure	Internal electronic malfunction	Check the electrical power and ground circuits of the unit. Refer to the electrical manual. Configure the module(s) using the approved diagnostic system. If a module is suspect, refer to the warranty policy and procedures manual.