Systems:
The actual values displayed are compared with the reference values stored in the diagnosis unit and can provide clues about faulty values.
There is a classic example available here too: The values forwarded by the mass air flow meter to the control unit do not correspond to the engine load state but are still plausible for the control unit. The engine no longer works at full power. This fault can be diagnosed very quickly by reading out the respective measured value block and comparing these values with the reference values under various load states.
When must a control unit fault be assumed?
This question is extremely difficult to answer, as practical garage work often shows. Basically, it can be said that: If despite the fact that all the voltage and ground connections to the control unit and all the input sig-nals have been tested one (or more) actuators cannot be triggered at all or triggered properly, a fault within the control unit must be assumed. It is important that not only the actuators are triggered by the control unit but relays, too (e.g. ground supply from the fuel pump relay). Vehicle-specific circuit diagrams and reference values should always be taken into account during all work. They provide an accurate summary of all components and cables that are connected to the control unit. Problems occur if the dia-gnosis unit does not build up a connection to the control unit. If the con-nection between the diagnosis unit and the vehicle is OK and the correct vehicle has been selected, this fault source can be excluded. It should be checked whether all voltage and ground connections on the control unit are OK and whether the voltage values correspond to the reference values. If no faults can be found, it must be assumed that damage has occurred within the control unit and destroyed the unit.
As well as serial diagnosis (testing using the diagnosis connection) some test unit manufacturers offer the possibility of parallel diagnosis. In this case the diagnosis unit is connected to the control unit by means of a vehicle-specific adapter cable. With parallel diagnosis, all values and sig-nals of the individual pins on the control unit can be tested and compared.
This diagnosis possibility provides a solution for vehicles which do not have a serial diagnosis connection yet.
A further diagnosis possibility is to use a test box (brake out box). With this test method, the test box is connected parallel to the control unit using the respective adapter cables. The individual sensors, cables, ground and voltage supplies can be tested at the test box sockets using the multimeter or oscilloscope. With this test it is very important that the pin assignments and reference values prescribed by the manufacturer are available.
Systems:
Screen presentation, parallel diagnosis
Connection, parallel diagnosis
The engine control unit
Systems:
If neither diagnosis unit nor test box are available, troubleshooting is extre-mely difficult. Measurements can be carried out with the multimeter or oscilloscope using the necessary vehicle-specific circuit diagrams and reference values. It is important that the connectors and cables are not damaged when the test prods of the testing unit are connected. Quite fre-quently, the connector contacts are bent by the test prods and no longer contact properly. These "self-inflicted faults" are very difficult to discover later.
Which precautionary measures have to be considered?
Be extremely careful when carrying out measurements on the control unit.
Inverse polarity or voltage peaks can destroy sensitive electronic compo-nents in the control unit. For this reason, do not use a conventional test lamp. Use a multimeter, oscilloscope or diode test lamp. To delete the fault store follow the manufacturer's instructions only. With new systems, stored data can be lost when the battery is disconnected. It can then be necessary to re-adapt or code some components or systems to enable them to function perfectly and be detected by the control unit. This is also necessary when the control unit or certain component are replaced.
Adaptation or coding is only possible using a diagnosis unit.
If the control unit is replaced it must be noted that in some unit types the plug-in program memories (EPROM) have to be taken over in the new unit. New control units that have to be adapted and coded in the vehicle can only be used in this one vehicle. Installation in a different vehicle for trial purposes is not possible.
If you are not certain in your diagnosis, it is possible to have the control unit checked for a reasonable price. If the control unit is faulty it may be possible to repair it. If the fault is irreparable, it is possible to exchange the unit 1:1. If no fault is found, the control unit can be reinstalled without any problems. You can find more information on this subject on the Internet under:
www.hella.com.
Testing without diagnosis unit or test box
Testing using the test box
The engine control unit
In this issue we would like to explain the ABS braking system and possi-ble faults and diagnosis possibilities in the electronic system in detail. The main focus is not on design and function but rather on diagnosis and troubleshooting.
At the end of the 1970s developments were so far advanced that the first ABS braking system was ready for series production. The ABS braking system made it possible to increase safety during critical braking situa-tions. Different road conditions (wet, icy) or sudden obstacles led to the wheels locking on vehicles without ABS in emergency braking situations.
This resulted in drivers no longer being able to steer their vehicle. When vehicles are equipped with ABS, wheel locking is prevented and the vehi-cles can be steered at all times, even in hard stop or emergency braking situations.
The ABS system comprises the following components:
■ Control unit
■ Hydraulic power unit
■ Speed sensors
■ Wheel brakes