Diagnostic Aids

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

• Wiring Harness Circuits (VSC SIG & 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		ABS
   Excursion		ABS
   Pickup
   5.4L & 6.8L		ABS
   Ranger
   2.5L & 3.0L		(2) ABS, GEM Or CTM
   4.0L		ABS
   (1) Module that is responsible for sending vehicle speed circuit signal to powertrain control module. (2) ABS module on vehicles with 4-wheel ABS. GEM or CTM on vehicles with rear ABS only.

   Turn ignition switch to ON position. Using scan tool, select VSS PID from PID/DATA MONITOR & RECORD 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 VSC For Short To Power
   
   Turn ignition switch to OFF position. Disconnect PCM connector(s). Inspect connector for loose, damaged or corroded terminals. Repair as necessary. Disconnect appropriate control module. See MODULE IDENTIFICATION  table. Turn ignition switch to ON position. Measure voltage between PCM harness connector VSC SIG terminal No. 58 and negative battery terminal. On Excursion and Pickup over 8500 lbs. GVW, measure voltage between PCM harness connector VSC GND terminal 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 VSC Circuit For Open Between PCM & GEM Or ABS Control Module
   
   Turn ignition switch to OFF position. Measure resistance between VSC SIG terminal at appropriate control module harness connector and PCM harness connector VSC SIG terminal No. 58. On Excursion and Pickup over 8500 lbs. GVW, measure resistance between chassis ground and PCM harness connector VSC GND terminal 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 in affected circuit(s). After repair is complete, go to step  7).
4. 4) Check Circuit For Short To Ground
   
   Disconnect scan tool from Data Link Connector (DLC). Measure resistance between chassis ground and PCM harness connector VSC SIG terminal No. 58. On Excursion and Pickup over 8500 lbs. GVW, measure resistance between PCM harness connector VSC GND terminal No. 33. On all models, if any resistance measurement is 10 k/ohms or less, go to step  6). If all resistance measurements are more than 10 k/ohms, check for fault in ABS control module, speed sensors and speed sensor circuits. If no faults are present, replace PCM. See appropriate REMOVAL, OVERHAUL & INSTALLATION article. Program PCM. See FLASH VEHICLE IDENTIFICATION BLOCK PROCEDURE . After repair is complete, go to step  7).
5. 5) Visual Inspection
   
   Turn ignition switch to OFF position. 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 After repair is complete, go to step  7). If no problems are found, fault is intermittent. Check for intermittent fault in ABS. See appropriate ANTI-LOCK article in BRAKES.

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 one module at a time that are associated with vehicle speed circuit. Repeat step  2) or  4) (depending on which step directed you here) to determine if short still exists in vehicle speed circuit. Repeat appropriate step until each associated module has been disconnected or short circuit has been eliminated. When short circuit disappears, diagnose fault with module or related system causing short. See appropriate MODULE COMMUNICATIONS NETWORK article to determine appropriate article, to diagnose appropriate module or system. 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
   
   Connect all disconnected components. 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. See DIAGNOSTIC TROUBLE CODE DEFINITIONS . If Continuous Memory DTCs are not present, testing is complete.