speed sensor fault

[noyzv8]

hey, i just checked my fault codes and it came up with Code 24 - Vehicle Speed Sensor (VSS) im assuming it has something to do with my engine light coming on when i back off at 100kmh, where is this sensor and how do i change it...?

thanks in advance.

[noyzv8]

also, do i clear the fault codes by disconecting the battery?

[Cheap6]

The sensor is on the rear/side of the transmission. It's a black cylinder about 40mm diameter with a pigtail electrical connector attached to it. Replacement requires simply unscrewing it off and and crewing a new one on.

Often the wiring in the pigtail will break inside the insulation or the sensor itself fails and replacement will fix the fault but be aware that the speed signal generated by the sensor goes to the instrument cluster before being sent to the ECM. If your speedometer works, without jumping around - indicative of an intermittent signal from the sensor - it's probably not the sensor that is faulty but rather in the instrument cluster or the wiring between it and the ECM.

You can clear fault codes by removing the fusible link closest to the passenger side of the car in the fuse box, in front of the relay box, under the bonnet for ~10 seconds. Disconnecting the battery will also work but requires tools.

[noyzv8]

the speedo works fine, its just when i back off the accelerator at 100kmh that the engine light comes on...if that is what the sensor is, i have a spare, ill change them and see if it fixes the problem.

cheers

[noyzv8]

i have changed the sensor and it still comes on, so like you said it must be something between the cluster and the ECM...any ideas?

[Cheap6]

You can check a few things.

The speed signal for the ECM is on A10 - the terminal assignations are cast into the ECM terminal connectors. The wire from A10 goes to the speedometer via the brown wire with yellow trace, the fourth terminal from the left, top row, rightmost connector behind the instrument cluster. The speedometer switches (pulses) A10 to earth via the black wire, fourth terminal from the left, leftmost connector behind the cluster.

With the ignition on there should be battery voltage between A10 and an earth.

With a digital multimeter that has a tachometer function you should see an rpm (frequency) reading that varies with road speed between A10 and an earth. If you do, it's probably an internal fault in the ECM 'cos all the rest of the wiring must be OK.

If no rpm reading, the problem has to be in the speedometer, the flexible printed circuit board or the connections behind the cluster. eg. cracked PCB track or corrosion.

[noyzv8]

just an update on the problem, ive done the calais dash upgrade and still does it with the new cluster so would this mean its the computer yes???

[Cheap6]

Possibly or the wiring between A10 and the speedo. Easiest way to check is to look for the voltage between A10 and an earth as per ^^.

Use a dressmaker's pin soldered to a length of wire and remove the plastic anti-back out comb from the connector block to access the back of the terminal (connector stays plugged into the ECM).

If you don't have a digital tachometer or frequency meter (DMM with that function) you can try using a digital voltmeter (>10 Mohm internal impedance). You will only get a vague idea from that because the signal will be a random snapshot at the update rate of the meter - you should see a constant 12V signal with the car motionless and a varying 12V-0V signal with the car moving.

You might try using a 1k resistor and an LED between the A10 and an earth (polarity sensitive). That should work. The LED will flicker at the signal rate so may only appear to dim at higher road speed.

Also try checking the continuity (resistance) of the wire between A10 and the connection behind the cluster but that doesn't check the connection at the cluster.