Question about 1999 Volvo S70
SOURCE: where is the output speed
The problem is either vehicle speed sensor or wiring harness going to sensor. In some case the ECM is defective.
Ensure that you have already replaced the VSS with good ones.
If you have not replaced them yet, then the sensor can be tested reading the waveform. This is done connecting both VSS input and output wires to a machine. The one below is a good VSS wave at 37 mph:
If the waveform is incorrect, the sensors must be replaced, as the ECM returns P0722 when VSS output is incorrect, or when there is no output.
A second test can be done reading the sensor impedance and voltage. Connect a multi-meter to the sensor. Set first on Ohms reading (impedance). A good sensor will read 560-680 Ohms at 20C (68F).
Set multimeter on Volts. Touch rapidly the sensor with a magnet and pull magnet away. The sensor will emit a weak voltage pulse. If that does not happen, then the sensor is broken. Ensure that multimeter is on high volt scale.Voltage will be quite low:
The VSS are located on the wheel shaft. Here a picture:
The VSS is connected to the ECM.
If you have already replaced the sensors you need to check the wiring harness to the ECM.
The wiring harness is tested checking that each wire going to the ECM has continuity. This is done reading impedance between each connector in the sensor , and the respective connector in the ECM. Impedance must be zero or a very low value. If it reads infinity, then the wire is broken.
A second test is done reading impedance between the two different wires at ECM connector. Impedance must be the same as at VSS contacts on sensor. If impedance is zero, then there is a short. Ensure that you choose a proper Ohms scale while doing the test.
If you have already replaced the VSS, repair or replace the wiring harness. The ECM failure returning this code is very uncommon. At most you may have a defective ECM connector.
Posted on Apr 19, 2011
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