Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. This test is used to diagnose the following:

• Reverse Switch
• Wiring Harness Circuits (RS & SIG RTN)
• Powertrain Control Module (PCM)

Fig 1: Identifying Reverse Switch Circuits & Harness Connector Terminals

Courtesy of FORD MOTOR CO.

1. 1) Continuous Memory DTC P0812: Check Reverse Switch Function
   
   This DTC is set when PCM senses voltage on RS circuit is high when it should be low. Possible causes are:
   • Gearshift Lever Not In Neutral While Performing KOEO ON-DEMAND SELF-TEST
   • RS Circuit Shorted To B+ Or VPWR
   • Faulty RS Switch
   • RS Circuit Open Or Shorted To SIG RTN
   • Faulty Powertrain Control Module (PCM)

   Ensure gearshift lever is in "N" position during KOEO ON-DEMAND SELF-TEST. Turn ignition switch to OFF position. Connect scan tool to Data Link Connector (DLC). Turn ignition switch to ON position. Using scan tool, select RS PID from PID/DATA MONITOR & RECORD menu. Move gear shift lever in and out of "R" position. If RS PID value does not change from ON to OFF with gearshift lever movement, go to next step. If RS PID value changes from ON to OFF with gearshift lever movement, problem is intermittent. Disconnect PCM connector(s). Inspect connector for loose, damaged or corroded terminals. Repair as necessary. If connector and terminals are okay, replace PCM.

2. 2) Check RS Switch Resistance
   
   Disconnect RS switch connector. RS switch is located near shift linkage. Inspect switch and bracket for damage. Repair as necessary. Using a DVOM, measure resistance between RS switch terminals with gearshift lever in "R" position. If resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, replace RS switch.
3. 3) Check RS Circuit For Open In Harness
   
   Turn ignition switch to OFF position. Disconnect scan tool from DLC. Disconnect PCM connector(s). Using a DVOM, measure resistance of RS circuit (White/Violet wire) between PCM harness connector terminal No. B10 and RS switch harness connector terminal No. 2. Also, measure resistance of SIG RTN circuit between PCM harness connector terminal No. B17 (Brown wire) and RS switch harness connector terminal No. 1 (Brown/White wire). See Fig 1. If both resistance measurements are less than 5 ohms, go to next step. If either resistance measurement is 5 ohms or more, repair open in RS or SIG RTN circuit.
4. 4) Check RS Circuit For Short To SIG RTN Or Chassis Ground
   
   Using a DVOM, measure resistance between PCM harness connector terminals No. B10 (RS) and B17 (SIG RTN). Also, measure resistance between chassis ground and PCM harness connector terminal No. B10 (RS). If both resistance measurements are more than 10 k/ohms, replace PCM. If either resistance measurement is 10 k/ohms or less, repair short to chassis ground or SIG RTN.