Test Methods and Sensors

Much of the testing done on the modern electro mechanical vehicle systems will be done via a scan tool connected to the vehicle diagnostic port. Information from the vehicle system will be broadcast by the control ECU onto the diagnostic comms bus, and this can be read by the scan tool. The amount of data that is available will depend upon the vehicle system and the scan tool. However, at some point it is likely that the sensor, the sensor environment and the sensor circuit will need to be examined.

There are many different sensors and control valves and solenoids used on automotive systems. When testing it is important to know what type of sensor system is being worked on to ensure that the correct test method is used. This will then deliver meaningful results and also ensure that damage is not done to either the sensor in question or the control module that the sensor is connected to. Below is a list of the main sensor and comms circuits with a brief description of how to identify and test.

  • 5 Volt ECU supplied feed  circuits
  • 12 Volt ECU supplied feed circuits
  • 12 Volt Ignition supplied feed circuits
  • Rising / falling voltage circuits
  • Pulse Width Modulation (PWM)
  • CAN Bus
  • LIN bus
  • Twisted Pair circuits

5 Volt ECU supplied feed  circuits
These can be 2 wire and 3 wire circuit. In both cases all wires connect to a control module. General these circuits are used for low current sensing circuits such as temperature sensing, pressure sensing, throttle position etc. These circuits are low voltage and low current so multi-meters not test lamps should be used when fault finding. Most vehicles have DTC monitoring for open circuit, short circuit and operating range. This means if the sensor is disconnected while the ignition is on the control module is likely to log a DTC.
5 volt 2 wire circuits
These are commonly used on temperature sensing circuits where a variable resistance sensor is used. If the supply voltage is measured with the sensor disconnected then 5 volts can be seen, if the sensor is connected the voltage will drop somewhere between the operating range of approx 0.5 – 4.5 volts. The 2 wire 5 volts circuits are basically a series resistor circuit where the sensor is a variable resistor and the fixed value resistor is internal to the control module.
5 volt sensors

5 volt 3 wire circuits
These circuits are used when a more accurate measurement is required, such as throttle butterfly position sensor,  turbo pressure sensor.
These circuits act similar to a potentiometer, and indeed in many cases potentiometer are used, but also ‘solid state’ sensors are used, the circuit testing is similar for both.
The sensor has a 5 volt feed which will measure 5 volts either open circuit or with the sensor connected. The sensor ‘sense’ or ‘feedback’ wire will vary voltage between approx 0.5 – 4.5 volt dependant on the position / sensor applied to the sensor.
3 wire 5 volt circuit

12 Volt ECU supplied circuits
These circuits are similar to the 5 Volts circuits; all wires go from the ECU to the sensor, they are relatively low current, they are DTC monitored.
Typical examples of components using these type of circuits are Airflow meters and Electric Throttle Bodies, however the use of 12 volt ECU supplied circuits is less predictable that 5 Volt circuit in where they are used ; there are 5 Volt airflow meters and 12 volt airflow meters etc.
These circuits should be tested using multi-meter and in some cases oscilloscopes.
The sensor or feedback circuit can take the form of a variable voltage output from the sensor or as a PWM signal.

12 Volt Ignition supplied feed circuits
These circuit tend to be used for power of solenoids and actuators. The solenoid is powered by an ignition feed, and the control module will completed the circuit by ‘earthing’ when the solenoid needs to be activated. The circuits can be DTC monitored as the circuit passes through the control module.
In some cases the control module will electronically connected the solenoid or actuator earth wire to earth in a straight forward off or on matter. In other cases the control module may electronically pulse the earth wire similar to a PWM signal. The frequency of the ‘on’ to ‘off’ ratio will determine whether the solenoid or actuator is fully off, partial on or fully on.

12 volt switched sensors

Rising / falling voltage circuits.
These circuits are used for highly sensitive and safety critical circuits, such as electronic throttles.
The circuit is effectively 2 separate sensors packaged in a common housing, the sensors work opposite to each other i.e as the sensor arm is moved mechanically the voltage from sensor 1 falls, and the voltage from sensor 2 voltage will rise. This arrangement has 2 safety benefits, firstly as 2 sensors are used there is a ‘fail safe’  backup in the event of one sensor failing. Secondly it is almost impossible for this style of circuit to electrically fail in the full throttle condition, i.e whatever electrical failure may happen it is very unlucky to give a situation where electrically the throttle is reporting full throttle but mechanically the throttle is in the closed position.
Often these safety critical circuit used gold plated terminal, this maybe a handy tip to identify this type of circuit.
Throttle pedal sensor

Pulse Width Modulation (PWM)
Pulse Width Modulation circuits are pulsing signal used primarily to control of the power supplied to electrical devices. The signal is controlled by a control module which turns the switch between control module and the device on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the ‘duty cycle’ supplied to the load.
Pulse Width Modulation
These circuits can be measured with a multi-meter for open and short circuits and it is possible to measure voltage on PWM circuits however the only real way to analyse PWM circuits is with a oscilloscope.
Generally PWM signals are used in 2 ways on modern vehicles:

  • As a control signal to control high current units such as cooling fans
  • As low current drivers to control the ‘duty cycle’ of devices with smaller current consumption.

High current PWM

In a high current PWM circuit (above), the control module determines the duty cycle for the hi current motor, but it transmits this requirement as a low current PWM signal. The Motor control module acts on the low current input signal and controls the power transmitted to the Hi current motor, normally by another PWM signal.
The Control module does not control to Hi current motor directly as hi current PWM signals are not compatible with sensitive electronic circuits. Hi current PWM circuits are very electrically noisy, i.e they transmit a large amount of electrical interference which can effect other electrical signals. So the Hi current motor and the motor control unit are housed as close together as possible, this keeps the PWM wires as short as possible to reduce this radiated interference. Also, the electrical components required to drive Hi power PWM circuits can be large, and they generate heat as well as electrical noise, this means often the PWM control module need to be in air flow to cool the unit.
Typical Hi current PWM circuits are used on electrical cooling fans and high pressure electrical pumps; fuel pumps, gearbox oil pumps etc.
Normally the high power wiring between the PWM controller and the motor are as short as possible and less than 1 meter in length.
On very hi power circuits such as cooling fans the module will be very close to the motor; mounted on the fan housing itself. If the module is up to 1 meter away often a ‘twisted pair’ circuit is used (see twisted pair explanation).

Low current PWM circuit

Low current PWM (above) do not transmit the high levels of electrical interference or generate significant heat within the control module so these circuits can be driven directly by the main control module (engine control ecu, or gearbox control module).
Typical Low current PWM circuits are used on turbo waste gate control solenoids, accessory cooling pumps.

CAN Networks
Controller Area Network (CAN) is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other. It is the method difference module communicate with each other.
The CAN network topology (circuit) is basically a twisted pair using 0.5mm or 0.35mm cables. The layout can be fairly simple or complex depending on how many modules are on the CAN bus. There are numerous design rules regarding CAN networks but these are more important when designing or modified the networks than working with vehicles and diagnosing faults.
Depending on the complexity of the vehicle in question, it may have a single CAN bus or it may have a multi CAN architecture. When there are multiple CAN networks on a vehicle the CAN buses can are normally divided into a number of different function categories such as, Powertrain CAN, Chassis CAN, Body CAN, Comfort CAN, Hybrid CAN. As their name suggests each CAN is exclusive to the modules working in that system group eg Powertrain CAN may contain the  Engine controller, Gearbox controller, Gear shelter module. Chassis CAN may contain ABS module, Suspension module, Electronic Park Brake module, etc.
It is possible for a module to be connected to more than one CAN bus; the ABS module when used on a multi CAN architecture is normally connected to both Powertrain CAN (PT CAN) and Chassis CAN (CH CAN).
On multi CAN architectures it is fairly normal for the difference buses to operate at different speeds, High speed CAN, Medium speed CAN etc. The speed reference to the amount of data passed per second (bytes per second).

CAN Network

Terminating resistors
An important aspect of the CAN network is the terminator resistors. Each CAN network, no matter now small or large will have a  2 terminating resistor which are house inside modules call terminating nodes. From a diagnostic point of view, it is not important to understand the electrical purpose of these resistor do but it is important to know that the resistor are there as they will be detected when measuring the CAN bus.
Each terminating resistor is 120 ohms.
Terminator resistors

Testing CAN networks
CAN networks can be tested in a number of ways:

  • Physical wiring checks using a multi-meter
  • Visual inspection of CAN traffic using a oscilloscope
  • Reading of CAN message and CAN message payload using CAN analysing tools.

Physical wiring measurements using multimeter – Resistance test
Resistance testing the CAN network can at first glance be a little confusing as it is a parallel resistance circuit; the 2 terminator resistors are in parallel.
With the vehicle ignition off, if a multi-meter set to resistance (Ω Ohms) is connected between the CAN hi and the CAN low the meter should read the approx 60Ω.
Correct CAN network circuit with both terminator resistors = 60Ω (Two 120Ω resistors in parallel = 60Ω)
Terminating resistors

If the multi-meter resistance reading is not approx 60Ω there is a fault with the CAN network.
A reading of 120Ω would indicate one of the terminator resistors is missing from the CAN network so it is likely that the CAN has an open circuit.
A reading of 0Ω would indicate that the CAN hi & CAN low wires have become short circuited together.
A reading of Open Circuit would indicate that one or more of the CAN wires have been cut or broken.
CAN network resistant checks
Neither the CAN hi or CAN low should be directly connected to the vehicle ground or vehicle battery.

CAN network testing

 

When tested with a volt meter approx 2.6 Volts will be seen on the CAN bus; volt meter connected between CAN Hi and CAN low.

CAN traffic using a oscilloscope – visual inspection
Although it is possible to test the physical CAN with a multimeter, it is not possible to see CAN traffic with a multimeter. To actually see the CAN traffic an oscilloscope or CAN analysing tool is required.  If you have access to an oscilloscope simply connect it to the CAN hi and clip the ‘scope reference lead to vehicle GND.
CAN oscilloscope trace 200ms

Although it is not possible to understand the messages it is fairly easy to see whether the CAN bus is supporting CAN traffic. Testing with ign off is best, but it should be noted many CAN buses stay live for a few minutes after ignition off.
In diagnostic terms, we do not really care what messages are being send, just where the messages are being broadcast and can be read by modules on the CAN bus.
If the ‘scope time base is changed and capture speed is increased from 200 ms to 200 us the individual messages become clearer but again capturing individual messages is for experts only. For diagnostic purposes, module will either be able to send message or they will not; simply module / CAN working correctly or not working at all.
CAN measured at 200 us

CAN analysing tools.
CAN analysing tools are common place in vehicle engineer but aren’t really used much in vehicle diagnostics, for a few good reasons i) they are very expensive ii) they are very complex iii) these tools are used to interrogate the CAN traffic, for diagnostic purposes we are much more interested in ‘is the bus not working or bus working’
With CAN analysing tools it is possible to find individual message or groups of messages and understand the broadcast timing and the payload value ( the data contained within the message (a Hexidemical value)).
CAN analysing tools

LIN Bus
The LIN (Local Interconnect Network) network is a low-end multiplexed communication system. The LIN network uses a single wire with a single master and one or more slaves. All messages are initiated by the master with only one slave responding to each message.
LIN networks are used were it is expectable for the message speed can be slower than that of CAN networks such as window lift, mirrors, wiper and rain sensors.
When testing LIN networks there needs to be continuity between the LIN master and the LIN slave. The LIN bus should not be electrical connected to vehicle ground (short to ground) or the 12v (short to Vbatt, Ign). Multi-meters are the easiest tool for test LIN continuity. It is also possible to see LIN messages on the LIN bus using an oscilloscope in the same way as the CAN bus.

LIN bus

Twisted Pair
A twist pair is not a sensor but a characteristic of the physical wiring which are used on sensitive circuits.
Twisted pair circuits always use 2 wires and the wires are physically twisted together. The purposes of twisted pairs is to cancel out electromagnetic interference (EMI) which maybe inducted into the circuit from external means. This reduction in inducted EMI means the circuits are inherently more stable and suffer less from interference which could distort the integrity of the signals passing through the circuit.
Twisted pair circuits are used on sensitive circuits such as engine, gearbox sensors, ABS wheel speed sensor etc. They are also used on audio speaker circuits.
Twisted pair circuit