Code 68
ROAD SPEED SENSOR CIRCUIT - DTC 68
The ECM looks for a low road speed signal when the engine load and speed are high.
The engine management ECM receives a road speed signal (5 volt square wave) from the instrument pack. The ECM uses the road speed signal to determine idle speed control functions. The idle speed is controlled when the road speed is less than 3 mph.
The ECM will flag DTC 68 if the road speed is 1.5 mph or less when the engine speed is greater than 2800 rpm and the mass air flow sensor signal voltage is greater than 3 volts.
Condition Required For DTC 68 Flagging
Road speed - 1.5 mph or less.
Engine speed - 2800 rpm or greater.
Mass air flow signal voltage - Greater than 3 volts.
Response time - 8 seconds.
Limp Home Default
None
Possible Faults
- Open circuit or short road speed output:
1990 - 92 MY XJ6 Sedan Range - Instrument pack output to EMS ECM, Cruise Control Module.
1993 - 94 MY XJ6 Sedan Range - Instrument pack output to EMS ECM, Cruise Control Module and Central Microprocessor.
1993 - 94 MY XJS Range - Speed interface unit output to speedometer, EMS ECM and Cruise Control.
- Defective instrument pack.
- 1990 - 92 MY XJ6 Sedan Range & XJS vehicles - Defective road speed sensor, or short circuit or open circuit between road speed sensor and instrument pack.
- 1993 MY ON vehicles - Defective ABS control module, or short circuit or open circuit between ABS control module sensor and instrument pack. Defective wheel speed sensor, or short circuit or open circuit between wheel speed sensor and ABS control module.
- Defective Engine Control Module.
Diagnostic Notes:
1. The road speed input can be monitored as a logic input using the PDU. Datalogger (0 = no signal; 1 = signal).
2. Test for a DTC reoccurrence by road testing the vehicle. Drive the vehicle under lead with the engine speed above 2800 rpm for more than 8 seconds.
3. The instrument pack provides a road speed signal for the engine management system and the cruise control system. If DTC 68 is flagged and the speedometer operates correctly, suspect a wiring or connection fault between the instrument pack and the ECM. If DTC 68 is flagged and the speedometer does not operate correctly, suspect a signal fault to the instrument pack.
4. To test the road speed sensor (1990 - 92 MY vehicles), back probe the road speed connector in the trunk. Measure the AC voltage between the red and the blue wires. At 30 mph, the voltage is approximately 1.4V AC.
NOTE: The voltage level is not critical as the speedometer is a frequency counter.
Flow Chart - DTC 68
�� Flow Chart - DTC 68:
