My VSS is not working. I don't deserve a ticket...

The ECT sensor is one of the most important components on a modern car. It tells how hot or cold the engine coolant. But, there are specifics to this sensor. Making the wrong assumption will cause trouble for you, the DIY-Tech or auto-man-woman. Remember, just because the computer says so, it doesn't mean the ECT sensor is bad.

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 ďData 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. Thatís 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 sensorís wiring connector and 0.5-2.5 volts at the moduleís 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 sensorís wiring. Connect a scope to the sensorís 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.

 

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Actuators               Fuel-Injectors            Ignition-Coil           Leak-Detection Pump  GM-Series Alternator   Idle-Control Valve      EGR-System            EVAP-System      

Repair Strategies Current-Ramping      Lack of Power           Fuel-Flow Volume     Ignition Waveform       Ignition Testing          A/F Ratio Diagnosis    Minimum Air Rate      No Fuel-Pressure       No Injector Pulse      No-Start/No-Spark     General No-Start      

ECM/PCM Modes Cranking Enrichment   Warm-up Cycle         Open-Loop            Closed-Loop            Accel. Enrichment Deceleration Leaning   Idle Control             Low-Voltage Correction Clear Flood Mode     Selective Inj. Cut-Off   Limp-In Mode           Exhaust Variable Valve

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Generic DTCs            GM Cars DTCs           GM Truck DTCs        Ford DTCs                  Ford Trucks DTCs        Dodge DTCs                BMW DTCs                 Honda/Acura DTCs       Hyundai DTCs              Isuzu DTCs                  Jaguar DTCs                Kia DTCs                     Land Rover DTCs          Mazda DTCs                Mercedes DTCs           Mitsubishi DTCs           Nissan/Infinity DTCs     Saab DTCs                  Subaru DTCs               Toyota DTCs                Volvo DTCs                  VW/Audi DTCs        

 Code Setting Criteria

Dodge CSC                 Ford CSC                    GM CSC                     Honda/Acura CSC        Hyundai CSC               Isuzu CSC                   Kia CSC                      Mazda CSC                 Mitsubishi CSC            Nissan/Infinity CSC      Subaru CSC                Toyota/Lexus CSC

SRS Airbag DTC

GM SRS Airbag Code   GM Truck SRS Code    Ford SRS Airbag Code  Ford Truck SRS Code   Dodge SRS Code        Dodge Truck SRS Code Acura/Honda SRS Code Isuzu SRS Codes         Mazda SRS Codes        Subaru SRS Codes Infinity/Nissan SRS       Kia SRS Codes     Hyundai SRS Codes     Mitsubishi SRS Codes Lexus/Toyota SRS

How to Get SRS Codes

Retrieving Dodge SRS   Retrieving Ford SRS     Retrieving GM SRS      Retrieving Honda SRS   Hyundai/Kia/Mitsu SRS Isuzu/Mazda/Subaru     Retrieving Toyota SRS  Nissan/Infinity SRS

SRS-Airbag Repair Guide

Deleting SRS Codes   Dodge SRS Location       Dodge SRS Operation     Ford SRS Location          Ford SRS Operation        GM SRS Location            GM SRS Operation           Honda SRS Location       Honda SRS Operation     Isuzu/Mazda/Suba Loc.   Isuzu/Mazda/Suba Ope Kia/Hyun/Mitsu Location Kia/Hyun/Mitsu Oper.       Nissan/Infinity Location   Nissan/Infinity Operation Toyota/Lexus Location    Toyota/Lexus Operation 

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