Pinpoint Test E: The Steering Wheel Controls Are Inoperative

WARNING: This page is about a different car, the 2010 Lincoln Town Car. However, it is still accessible from the selected car via links, so may be relevant.

Refer to appropriate SYSTEM WIRING DIAGRAMS article, Audio System for schematic and connector information.

Normal Operation

The audio unit sends a reference voltage to the steering wheel controls through circuit 595 (LB/RD). Each steering wheel control switch function corresponds with a specific resistance value within the switch. When a switch is pressed, the audio unit monitors the change in reference voltage to determine the requested function. Ground for the steering wheel controls is provided through circuit 57 (BK).

DTC C1976 (Audio Steering Wheel Switch Circuit - Open Circuit) - sets when the reference voltage on circuit 595 (LB/RD) is too high, suggesting an open circuit.
DTC C1977 (Audio Steering Wheel Switch Circuit Short to GND) - sets when the reference voltage on circuit 595 (LB/RD) is too low, suggesting a short to ground on circuit 595 (LB/RD) before the steering wheel controls, or internally shorted steering wheel controls.
DTC B2404 (Audio Steering Wheel Switch Circuit Fault) - sets when any fault is detected in the steering wheel controls circuit.

This pinpoint test is intended to diagnose the following:

Wiring, terminals or connectors
Audio unit
Clockspring
Steering wheel controls

PINPOINT TEST E: THE STEERING WHEEL CONTROLS ARE INOPERATIVE

NOTE: Use the correct probe adapter(s) when making measurements. Failure to use the correct probe adapter(s) may damage the connector.

E1 CHECK THE STEERING WHEEL CONTROLS RESISTANCE VALUES

Key in OFF position.
Disconnect: Audio Unit C290a

Fig 1: Checking Steering Wheel Controls Resistance Values

Courtesy of FORD MOTOR CO.

Measure the resistance between the audio unit C290a-18, circuit 595 (LB/RD), harness side and ground while pressing the steering wheel controls as follows:

Switch	Resistance Range (Ohms)
VOL down	30-132
VOL up	132-312
MEM	312-642
BAND	642-1,322
No switches pressed	2,894-10,800 ohms

Are the resistances within specification?

YES : INSTALL a new audio unit. REFER to Audio Unit . TEST the system for normal operation.
NO : Go to E2.

E2 CHECK CIRCUIT 595 (LB/RD) FOR AN OPEN OR SHORT TO GROUND
- Disconnect: Clockspring C218a
- Measure the resistance between the audio unit C290a-18, circuit 595 (LB/RD), harness side and the clockspring C218a-5, circuit 595 (LB/RD), harness side; and between the audio unit C290a-18, circuit 595 (LB/RD), harness side and ground.
- Is the resistance less than 5 ohms between the audio unit and the clockspring, and greater than 10,000 ohms between the audio unit and ground?
1. YES : Go to E3.
2. NO : REPAIR the circuit. CLEAR the DTCs. TEST the system for normal operation.
E3 CHECK CIRCUIT 57 (BK) FOR AN OPEN
- Measure the resistance between the clockspring C218a-15, circuit 57 (BK), harness side and ground.
- Is the resistance less than 5 ohms?
1. YES : Go to E4.
2. NO : REPAIR the circuit. CLEAR the DTCs. TEST the system for normal operation.
E4 CHECK CIRCUIT 595 (LB/RD) FOR AN OPEN
- Remove the driver air bag module. Refer to REMOVAL AND INSTALLATION article.
- Disconnect: Clockspring C218b
- Disconnect: Steering Wheel Controls C2208
- Measure the resistance between the steering wheel controls C2208-5, circuit 595 (LB/RD), harness side and the clockspring C218b-9, circuit 595 (LB/RD), harness side.
- Is the resistance less than 5 ohms?
1. YES : Go to E5.
2. NO : REPAIR the circuit. CLEAR the DTCs. TEST the system for normal operation.
E5 CHECK CIRCUIT 57 (BK) IN THE STEERING WHEEL HARNESS FOR AN OPEN
- Measure the resistance between the steering wheel controls C2208-3, circuit 57 (BK), harness side and the clockspring C218b-7, circuit 57 (BK), harness side.
- Is the resistance less than 5 ohms?
1. YES : Go to E6.
2. NO : REPAIR the circuit. CLEAR the DTCs. TEST the system for normal operation.
E6 CHECK THE CLOCKSPRING FOR AN OPEN
- Measure the resistance between the clockspring C218a pin 5, component side and the clockspring C218b pin 9, component side; and between the clockspring C218a pin 15, component side and the clockspring C218b pin 7, component side.
- Are the resistances less than 5 ohms?
1. YES : INSTALL new steering wheel controls. REFER to Steering Wheel Controls . INSTALL the driver air bag module. REFER to REMOVAL AND INSTALLATION article. CLEAR the DTCs. TEST the system for normal operation.
2. NO : INSTALL a new clockspring. REFER to REMOVAL AND INSTALLATION article. INSTALL the driver air bag module. REFER to REMOVAL AND INSTALLATION article. TEST the system for normal operation.