Misfire Vehicle Speed Sensor Disco 1

[Joey]

Not sure where I got this:

An intermittent VSS fault is identified by a freeze frame speed of above 110 mph (an input to GEMS caused by a noisy signal from the VSS), complaints of intermittent power loss, or engine stall.

NOTE: On TestBook disc DRG 0001 and DRG0002 the road speed displayed as "MPH" is actually a kilometers per hour value, NOT miles per hour. A TestBook reading of "177 MPH" is actually 177 kph. This reading, when converted to the desired mile per hour reading, is 110 mph.

One of the functions of the VSS signal to the GEMS unit is to provide information on vehicle speed. If the GEMS unit sees an excessively high speed (above 110 mph) it will initiate fuel cut off to the odd cylinders momentarily to reduce engine power. This feature is intended to prevent the driver from operating the vehicle at excessive road speeds.

Customer complaints of intermittent engine surging at highway speeds, inoperative or fluctuating speedometer, or a MIL illumination could be caused by an intermittently failing VSS.

If the MIL is illuminated with misfire codes P0300, P1316 and any combination of P0301, P0303, P0305 and P0307 stored or if the customer complaint is surging, hesitation, or loss of power at highway speeds, pay close attention to the freeze frame data, particularly engine rpm and road speed.

NOTE: As with all electrical diagnosis, the circuit between the sensor and the GEMS unit should be checked. The technician should be aware of the fact that the sensor test will not detect an intermittently failing sensor.

If the freeze frame data shows a road speed that does not conform to the engine speed (for example 110 mph at 2000 rpm or a road speed of 0 mph), an intermittent fault with the VSS could be the problem.

If inconsistencies between engine speed and vehicle speed are observed, replace the sensor. See attached DTCR samples