The Controller Area Network (CAN-Bus) is a series bus system. The voltage levels of the individual signals can be shown using an oscilloscope. The changes in voltage within a signal are too fast to be shown using a voltmeter (voltage measurement with a multimeter). The CAN bus generally consists of two leads, the “CAN low” and the “CAN High” lead (exception, single wire CAN bus, mainly in GM vehicles).
Because not everyone has an oscilloscope, you can broadly check the presence of the voltage level with a multimeter.
If you measure this directly at the control unit plug, you can establish:
If the CAN bus is completely missing in the control unit
If the CAN bus is in single wire mode (a CAN lead has no connection to the control unit)
If the CAN bus is attached to both CAN bus connections on the control unit.
In the first two cases, there is a fault. The CAN bus connection of the wiring harness should be checked.
There are three CAN bus versions of Erich Jaeger CAN bus control units:
To check the voltage level with a voltmeter (multimeter in voltage mode) measure against earth (earth = vehicle body earth). The CAN bus should be active, so turn on at least the vehicle ignition and leave it on.
Finding faulty earth:
Set a multimeter as an ohmmeter (Ω, pronounced “ohm”, the Greek letter omega).
Use the ohmmeter to measure the earth pin on the module in question against the vehicle body earth (please look for suitable earth points on the vehicle body).
The resistance measured is ideally very low, around 0Ω to 2Ω.
For CAN bus control units (JaegerCANtrolLED: 381977, 383077, 381978, 383078, 381979, 383079) an earth connection where the resistance is too high between the module and the car body, means that the CAN bus level is displaced, as the reference mass of the CAN bus transceiver does not match the reference mass of the other control units built into the car. In the worst case (e.g. several Ω) the module communication may fail.
Where there is no earth at all (multimeter measures nothing, very high Ω value) the control units will not work (as with missing voltage supply).
Example of earth pins for some control units:
381977, 383077, 381978, 383078, 381979, 383079, 321114: Control unit pin 1
321391, 321392, 321179, 321162, 321163: Control unit pin 21
321130, 321113: Control unit pin 5