Diagnostic Aids

Perform this test only when directed by QUICK TEST. This test is intended to diagnose the following:

• Wiring harness circuits (VSC SIG and VSC GND).
• Powertrain Control Module (PCM).

1. 1) DTC P0500, P0501 Or P1502: Check Vehicle Speed PID
   
   DTC indicates that PCM has detected an error in vehicle speed information received from Anti-Lock Brake System (ABS) module, Generic Electronic Module (GEM) or Central Timer Module (CTM). See MODULE IDENTIFICATION  table. Possible causes for this fault are:
   • Open or short in wiring harness.
   • Faulty module(s) connected to vehicle speed circuit.
   • Faulty wheel speed sensors.
   MODULE IDENTIFICATION ⁽¹⁾
   
   

   Application		Module
   Explorer, Mountaineer & Ranger		ABS
   Pickup
   4.2L, 5.4L & 6.8L		(2) ABS Or GEM
   Ranger
   2.5L & 3.0L		(3) ABS, GEM Or CTM
   4.0L		ABS
   Taurus SHO		ABS
   (1) Module that is responsible for sending vehicle speed circuit signal to Powertrain Control Module (PCM). (2) ABS module on vehicles with 4-wheel ABS or GEM module on vehicles with rear ABS only. (3) ABS module on vehicles with 4-wheel ABS. GEM or CTM on vehicles with rear ABS only.

   Turn ignition on. Using scan tool, select VSS PID from PID/DATA monitor menu. Road test vehicle and gradually accelerate vehicle to 50 MPH while observing VSS PID. If VSS PID value matches speedometer fault is intermittent. Go to step  5). If VSS PID value does not match speedometer, go to next step.

2. 2) Check For Short To Power
   
   Turn ignition off. Disconnect PCM 104-pin connector. Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Disconnect appropriate control module. See MODULE IDENTIFICATION  table. Turn ignition on. Measure voltage between PCM connector pin No. 58 and negative battery terminal. On Taurus SHO and Pickup, also measure voltage between PCM connector pin No. 33 and negative battery terminal. On all models, if voltage(s) is less than one volt, go to next step. If voltage(s) is one volt or more, go to step  6).
3. 3) Check For Open Circuit
   
   Turn ignition off. Measure resistance between VSC SIG terminal at appropriate control module wiring harness connector and PCM connector pin No. 58. On Taurus SHO and Pickup, also measure resistance between VSC GND terminal at appropriate control module wiring harness connector and PCM connector pin No. 33. On all models, if resistance is less than 5 ohms, go to next step. If resistance is 5 ohms or more, repair open circuit(s). After repair is complete, go to step  7).
4. 4) Check Circuit For Short To Ground
   
   Disconnect scan tool Data Link Connector (DLC). Measure resistance between ground and PCM connector pin No. 58. On Taurus SHO and Pickup, also measure resistance between PCM connector pins No. 58 and 33. On all models, if any resistance reading is 10,000 ohms or less, go to step  6). If all resistance readings are more than 10,000 ohms, check for fault in ABS. If no faults are present, replace PCM. After repair is complete, go to step  7).
5. 5) Visual Inspection
   
   Turn ignition off. Visually inspect vehicle speed circuit wiring harness for any of the following conditions:
   • Loose or corroded connector at appropriate control module.
   • Loose or corroded PCM connector.
   • Incorrect wiring harness routing of vehicle speed circuits.

   If any faults are present, repair as necessary and go to step After repair is complete, go to step  7). If no problems are found, fault is intermittent. Check for intermittent fault in ABS.

6. 6) Verify If Short Is In Wiring Harness Or Another Module
   
   Determine if any other modules are connected to vehicle speed circuit. See appropriate wiring diagram in WIRING DIAGRAMS article. If no other modules are connected to vehicle speed circuit, repair short in vehicle speed circuit and go to next step. If other modules are connected to vehicle speed circuit, disconnect module(s) (one at a time) that are associated with vehicle speed circuit. Repeat step that sent you here to determine if short still exists in vehicle speed circuit. Repeat until each associated module has been disconnected or short circuit has been eliminated. If short circuit no longer exists with associated module(s) disconnected, diagnose appropriate module and/or related system. See appropriate article in ACCESSORIES & EQUIPMENT in appropriate lemon-25® maual. If short circuit still exists with all associated modules disconnected, repair short in vehicle speed circuit and go to next step.
7. 7) Test Drive Vehicle
   
   Ensure engine is warmed to normal operating temperature. Perform the following drive cycle 3 times:

   A/T Equipped Vehicles

   • Place gear selector in Drive.
   • Accelerate heavily to 35 MPH.
   • Coast down to idle speed and stop vehicle.

   M/T Equipped Vehicles

   • Accelerate and shift from first to second gear.
   • Continue to accelerate heavily to 40 MPH, not shifting higher than second gear.
   • Coast down to idle speed and stop vehicle.

   After third drive cycle, perform QUICK TEST . If any DTCs are present, go to appropriate system test. If DTCs are not present, testing is complete.