VSS or Vehicle Speed Sensor/Auto Repair Technician Observation

This article also applies to the WSS or the Wheel Speed Sensor found
today on every automobile.

The vehicle Speed Sensor (VSS) has the job of providing the different
modules, with vehicle speed and deceleration factor. This sensor is
similar in operation to the CAM/CRK sensor and a couple of different
variants are used. The VSS signal is used by the TCM (Trans. Control
Module) to control shifting and TCC (Torque Converter) application; as
well as by the instrument cluster module for speedometer operation. The
ECM also uses the VSS signal to control fuel.

The VSS may also be an input to the ABS control module. The ABS module
uses the VSS signal to know the vehicle speed at all times as well as
rate of deceleration.

There are a few types of VSS. These are MAGNETIC, Magneto-Resistive,
REED type and Photo-Electronic. The MAGNETIC type for the most part is
the most common one and it works in more or less the same fashion as the
CRK sensor. It is always important to determine the type of sensor used.
This will also determine the type of output signal that is to be
expected. The magnetic sensor always puts out a sine wave. This type of
VSS gets affected by anything that will decrease its signal amplitude,
as in excessive air gap. The other VSS types put out a square wave. This
makes the signal much more resistant to EMF, which is the reason why
they are used. The reed VSS, for example, has only 2 wires coming out of
it. This does not mean that it is a magnetic sensor, however. In this
case a reference voltage is provided by one lead and a ground on the
other. The reed VSS will simply ground this reference signal creating a
square wave.

The wheel speed sensor (WSS), on the other hand, is almost always of the
MAGNETIC type. This type of sensor, as stated before, needs to have the
right air gap to perform properly. However, newer late model systems
(2004 & up) are starting to use hall-effect (square wave output) WSS.
The reason for it higher resistance to EMF and less of a chance that the
sensor may output a false reading.

CONDITIONS THAT AFFECT OPERATION

There are two main conditions that will greatly affect the performance
of a magnetic WSS or VSS. One is the air gap between the sensor and the
reluctor wheel (also called the tone ring) and the other is a shorted
sensor coil. By far an improperly adjusted air gap represents a much
more frequent problem. Dirt and oxidation will generally stick to the
sensing part of the VSS/WSS and interfere with the air gap. An
obstructed air gap translates to a faulty signal. On the other hand, a
larger than normal air gap translates to a smaller amplitude waveform.
This presents a problem since most MODULES have a specific threshold
recognition voltage, which is usually 1.00 volt P-P. The respective
module never recognizes a signal voltage that falls below the threshold
recognition voltage level.

COMPONENT TESTING

Testing the WSS is a fairly simple matter. With the right knowledge, a
quick and accurate diagnostic is possible, even on hard to get places.
These steps should be followed in the order presented here.

On the other hand, the VSS needs a slightly different approach to
testing. This is because of the way the VSS signal reaches its
applicable module. The VSS signal path should be traced to determine its
operation. This may also involve the trouble shooting of the "Data Bus
Systems. Therefore knowledge in data bus systems and how they work is
also needed.

WSS TESTING.

Scan the appropriate module and record any DTCs. Using a scan tool,
verify that the faulty WSS is not putting out a speed signal. A faulty
sensor reading should be at 0.00 or 3.00 MPH/KPH without a signal
output. If a scan tool is not available, then perform the tests
manually. Once the faulty WSS has been verified, proceed to perform a
visual test. Follow the wires for the WSS and determine the location of
a common connector. This will help in running further tests. Once a
common connector location has been found, proceed to verify for any BIAS
VOLTAGE. Note: The WSS is almost always a magnetic type. The applicable
module sometimes puts out a bias voltage for diagnostics purposes. This
voltage completes the circuit through the WSS coil and is used to detect
open or short circuits. NOT all systems have a bias voltage, however.
Compare the bias voltage of the faulty sensor to that of a good known
sensor. If the faulty sensor harness voltage has 0.00 volts then there
is an open or short to ground problem. A faulty sensor is the most
probable cause. If the faulty sensor harness voltage has from 0.5 to 2.5
volts bias voltage, then the wiring is fine, go to STEP 7.

If bias voltage is 0.00 volts at the faulty sensor's wiring connector
and 0.5-2.5 volts at the module's wiring harness, then the open circuit
is closer to the ABS module. Short the WSS harness connector using a
jumper wire. Disconnect the main ABS connector and take an OHM reading.
If close to 0.00 Ohms is seen then the wiring is fine. The problem is at
the ABS connector.

Checking the WSS output signal.

Connect an oscilloscope to the two WSS output wires. While taking a
scope reading spin the tire (at least once per second) and look for a
uniform sine wave. The signal must be at least 1.00 volt P-P
(Peak-to-Peak) to be considered good. A waveform with a small amplitude
is an indication of an excessive WSS air gap or semi-shorted sensor coil
windings. Note: Most ABS modules in order to recognize a WSS signal need
at least 1 volt P-P.

VSS testing.

Although the WSS or the VSS sensor is usually of the magnetic type, it
does differ greatly in the way the signal gets to the ECM or applicable
module. The VSS is used extensively for transmission shifting in TCM
applications as well as instrument cluster speedometer actuation. It is
vital to determine its signal path before any diagnostics decision is to
be made. This practice will speed up the diagnostic process. In this
article various examples from different manufacturers will be shown in
order to make the operation easier to understand.

As previously explained, trace the signal path using an electrical
schematic diagram first. This will allow you to focus the final testing
phase on the right component. Determine all modules directly connected
to the VSS. Once this is done, determine the condition of the sensor's
wiring. Connect a scope to the sensor's signal lead at the specific
module. While spinning the tires in the air, check for a VSS signal. If
no signal is found proceed to a wiring check.

3. Disconnect the VSS and short the signal wire to ground. Using a VOM
take a continuity check at the applicable module. If 0.00 Ohms is seen
then the wiring is good, if NOT then an open circuit exists. In such
cases proceed to check the VSS wiring for a breakage. Because of time
constraints it might be desirable to simply run new wiring altogether

Example 1.

For example, Chrysler vehicles use a VSS-TCM-Data bus signal path on a
number of their models. In this case, an inoperative speedometer could
be due to the signal not reaching the data bus or that the data bus is
down, since the Instrument Cluster module gets the VSS information from
the data bus. For this reason, the Chrysler scan tool (DRB III) offers a
menu choice PID for RPM reading at the data bus. If the RPM PID reading
is seen at the data bus, then it is being transmitted. The fault is
probably at the instrument cluster itself.

Example 2.

Most GM vehicles have a VSS-ECM-Class 2 data bus path. In this
arrangement, the Instrument Cluster, Radio Control head, Chime
controller and Cruise Control modules get the VSS signal over the data
bus. The ECM is the only module that is hard-wired to the VSS and it is
the one responsible for the data bus VSS signal transmission. A no-VSS
code on the instrument panel module, for example, right away points to a
possible data bus or an instrument panel problem. This can be verified
by simply scanning the ECM for a VSS signal while spinning the tires on
a raised vehicle. If it does have an active VSS PID on the network and
the other modules also see this signal over the data bus, then the
instrument cluster or related circuit is at fault. By tracing the signal
path we can determine if it is a data bus, wiring or a module problem.

Example 3.

Most mid-nineties FORD vehicles have the VSS signal hard-wired into all
the applicable modules. In this case the VSS signal is not transmitted
over the data bus. To correct any problems with this signal, normal
electrical troubleshooting techniques are applicable. If a certain
module is putting out a faulty VSS signal code then an OEM scan tool
test is the preferred diagnostic choice. By simply locating the module
with the missing VSS signal and proving the condition of its wiring, a
quick determination can be made.

This type of wiring arrangement is fast disappearing, however. Such a
system with all the sensors hard-wired makes use of excessive copper
wiring. This adds a lot of weight to the vehicle with a definite cost in
fuel efficiency, which is the reason why most manufacturers are going
the data bus way. By transmitting as much data as possible over the 1 or
2 wire data bus, a massive amount of wiring can be saved. This
translates into a fuel-efficient and simpler to repair vehicle system.
The use of different data bus schemes like Class 2, UART, SCP and CAN
will be addressed elsewhere is this book.