Theory of Operation
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.
NOTE: Some manufacturers use the VSS, also called an OUTPUT speed
sensor, to detect transmission slippage. In such cases an INPUT
speed sensor signal is compared to the VSS (OUTPUT speed sensor)
signal and a slippage factor determination is made by the TCM. These
two signals are always being compared to an internal TCM memory
table. If the signals are considered out of range, a trouble code is
set and the TCM goes into limp-in mode.
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.
NOTE:
Some manufacturers use a VSS with
a built-in A/D converter to convert the magnetic sine wave signal
into a digital square wave. An A/D (Analog to Digital Converter) is
an electronic circuit that converts a sine wave into a square wave.
In such cases where the A/D is built inside the VSS itself, the VSS
also has to have a power and ground circuit. This is needed to drive
the A/D circuitry. Some Toyota systems use this type of VSS.
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.
1. Scan the appropriate module and record any DTCs.
2. 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.
3. 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.
4. 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.
5. 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.
NOTE: Some manufacturers DO NOT use a bias voltage. That’s
why it is always a good idea to check for bias voltage at a good known
sensor first. Doing so will determine if the manufacturer is using a bias
voltage for diagnostics.
6. 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.
7. 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.
1. 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.
2. 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.
NOTE: In automotive electrical work, the use of a tone
generator signal injector is very useful. Such inexpensive equipment is
widely used by the phone companies to repair broken telephone wires.
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 Blog.
