Pinpoint Test "A" Cannot Change 4WD Mode DTC P1812 & P1815

Check 4X4 Switch

Remove and disconnect 4X4 switch. Inspect wiring and connectors. Measure resistance across switch terminals (White/Light Blue and Gray/Red wires). Exercise switch to all positions. If switch resistance is as specified, go to next step. If switch resistance is not within specification, replace switch. See 4X4 SWITCH RESISTANCE TABLE .

4X4 SWITCH RESISTANCE

Switch Position	Ohms
2WD	3400-4300
4WD Or Auto 4WD	100-1200
4WD Low	320-400

Check White/Light Blue Wire

Gain access to Generic Electronic Module (GEM) located behind center of instrument panel. Disconnect GEM harness connector C3. See Fig 1 . Measure resistance of White/Light Blue wire between 4X4 switch connector and GEM harness connector C3 Terminal No. 8. If resistance is less than 5 ohms, go to next step. If resistance is greater than 5 ohms, repair open White/Light Blue wire.

Fig 1: Identifying GEM Harness Connector C3 Terminals
Courtesy of FORD MOTOR CO.

Check White/Light Blue Wire For Short To Battery

Reconnect GEM harness connector C3. Turn ignition on. Measure voltage on White/Light Blue wire and Gray/Red wire. If battery voltage does not exist, go to next step. If battery voltage exists, repair shorted wire. Clear DTCs and recheck for DTCs.

Check White/Light Blue Wire For Short To Ground

Disconnect GEM harness connector C3. Check for continuity to ground on GEM harness connector C3 terminal No. 8. If continuity exists, repair short to ground in White/Light Blue wire. If continuity does not exist, go to next step.

Check Gray/Red Wire For Open

Disconnect GEM harness connector C1. See Fig 2 . Check continuity between Gray/Red wire between 4X4 switch and GEM connector C1 terminal No. 21. If continuity exists, circuit is okay, recheck for DTCs. If continuity does not exist, repair open Gray/Red wire and recheck for DTCs.

Fig 2: Identifying GEM Harness Connector C1 Terminals
Courtesy of FORD MOTOR CO.

Check BOO PID

Connect scan tool and read BOO PID data from GEM. Press brake pedal. If BOO PID data changes when brake pedal is depressed, switch is okay. If BOO PID data does not change when brake pedal is depressed, go to next step.

Check Light Green Wire For Short

Disconnect GEM harness connector C3. Check for continuity to ground on GEM harness connector C3 terminal No. 13. If continuity exists, repair short to ground in Light Green wire. If continuity does not exist, go to next step.

Check Light Green Wire For Open

Check continuity of Light Green wire between brake pedal switch and GEM connector C3 terminal No. 13. If continuity exists, go to next step. If continuity does not exist, repair open Light Green wire and recheck for DTCs.

Check Light Green Wire For Short To Battery

Reconnect brake pedal switch. Measure voltage at GEM harness connector C3 terminal No. 13 (Light Green wire). If battery voltage does not exist, replace electronic clutch relay located behind right side of instrument panel. If battery voltage exists, repair Light Green wire for short to battery voltage. Clear DTCs and recheck for DTCs.