General Testing Methods
General Testing MethodsTest equipment
Suitable instruments for carrying out fault diagnosis on the car's electrical system are the ISAT scan tool and a multimeter (Fluke 88/97 or the equivalent).
Use a voltmeter (multimeter) to check the voltage in a circuit.
Take resistance readings when checking for short circuits and open circuits in wiring harnesses, connectors, switches and contacts.
Important:
Exercise the utmost caution when taking resistance readings if the car battery is connected and one of the test points is ground or battery negative. Ordinary electrical equipment that is not disconnected could spoil and distort the test readings and easily lead to incorrect information.
Disconnection of the battery is not recommended, as many subsystems store information in memories which will be erased if their power supply is turned off.
Checking for continuity/shorting
Open circuit (resistance measurement)
1 Make sure that the component or cable to be checked is not live.
2 With the instrument set to measure resistance, connect the test leads to each end of the component or cable to be checked. In a wiring harness, the resistance should normally be less than 1 ohm. In regard to components, the specified resistance is applicable.
Open circuit (voltage measurement)
1 Switch on the load (item of electrical equipment), if any.
2 With the instrument set to measure voltage; connect the black test lead to a good ground and the red test lead to the wiring side.
3 At the output of a control module or switch, take readings in the direction away from them and proceed successively towards the load. When the voltage reading drops to zero, you have just passed the break in the circuit.
4 At the input of a control module or load, take readings in the direction away from the power source and proceed successively towards the control module or switch. When the voltage reading drops to zero you have just passed the break in the circuit.
Shorting to ground (resistance measurement)
1 Make sure that the cable to be checked is not live and that the load, if any, is disconnected.
2 With the instrument set to measure resistance, connect one of the test leads to the load side of the wiring harness and the other test lead to a good grounding point in the car.
3 Carefully move the wiring harness about while checking the instrument to see whether it shows infinite resistance (OL) the whole time.
PWM signals
PWM measurement
PWM signals, i.e. communication between control modules, are always triggered by a positive pulse. That is to say, the pulse is a battery + (positive) pulse.
Other PWM outputs (inputs) are often triggered by a negative pulse. For instance, the injectors are supplied with constant positive voltage and pulsed by ground (negative) signals from the control module.
Measurement with a voltmeter
If we measure PWM with a voltmeter, the meter will show the mean voltage applied to the lead so that a higher pulse ratio will give rise to a higher mean voltage.
By means of the voltmeter we can roughly estimate the pulse ratio. At a pulse ratio of 9%, a reading of about 1.2 V (0.09 x 13 V = 1.17 V) will most often be obtained.
In the case of PWM triggered by a positive pulse, connect the red test lead to the cable and the black test lead to a good ground.
In the case of PWM triggered by a negative pulse, connect the black test lead to the cable and the red test lead to battery + (positive).
Select "Smooth" on the voltmeter if it has this feature.
Measurement with an ISAT scan tool or multimeter with a pulse measuring function
Connect the red test lead to the cable and the black test lead to a good ground. Then select positive or negative triggering. Frequency, pulse duration and pulse ratio can then be measured.
Measurement with Logic Probe
Connect the battery + (positive) power supply and a good ground. Then apply the tip of the probe to the cable. The Logic Probe shows whether pulses are present or not and gives a rough estimate of the pulse ratio.
Measurement with an oscilloscope
Pulse frequency and duration are shown at the same time as we see a display of the pulses. This shows us how pulse width changes when the input data is changed.
Points to remember when carrying out fault diagnosis
1 The SDA MkII should always be connected to SAT scan tool.
2 The data link connectors (one green and one black, black for the engine management system) are located under the right-hand front seat.
3 Read and make a note of stored diagnostic trouble codes in all systems before disconnecting the battery and any control modules.
4 If communication cannot be established with the SAT scan tool and control module, first check that the fuses are intact and live.
Then check the wiring between the control module and data link connector.
5 Also check that the power supply and correct ground are connected to the data link connector and that the connector pins are undamaged and securely located.
6 Diagnostic trouble code lists will be found in the relevant sections.
Important Diagnostic trouble codes should be used only for diagnostic purposes. It is by no means certain that they indicate a component fault.
7 To avoid damage to control modules and components, make sure that the ignition switch is in the OFF position when connecting the Breakout Box (BOB) or unplugging connectors.
8 Check that the control module ground connections and supply voltages are correct.
9 It may sometimes be necessary to unplug connectors and check that sockets and pins are undamaged and securely located.
Plug in connectors again and clear all diagnostic trouble codes. If possible, start the engine and drive the car to check whether the fault or faults still persist.
If poor contact is suspected anywhere, use KONTAKT 61 contact cleaner (part No.30 04 520) on the female pins (sockets) in the connectors.
Important Contact cleaner or grease must not be used on the oxygen sensor connectors.
10 Where specified, 12 V is to be regarded only as a guide level, not an absolute value. The test reading may be directly proportional to battery voltage but 0.5-1.5 V may be lost due to an internal voltage drop in the driver stage of the control module.
11 Where specified, 0 V is to be regarded only as a guide level, not an absolute value. Due to an internal voltage drop in the driver stage of the control module, a voltmeter connected across the control module pins and ground could give a reading of about 0.5-1.5 V, but this is specified in Test readings, control module connections to 0 V.
12 Never switch from one unit of measurement to another (e.g. from V, via A to ohms) on the test instrument without first disconnecting the instrument's test leads.
13 Resistance measurement should always be carried out with the ignition switch in the OFF position.
Important When you have finished checking, always clear the fault memory with the command "CLEAR FAULT CODES". To clear all systems, enter the command "CLEAR".