Central Electronic Module (CEM), information about incorrect replacement

When analyzing replaced central electronic modules (CEM), it has become evident that many replacements have been carried out incorrectly. In order to identify the main cause for certain electrical fault symptoms more easily, this document describes the role of the central electronic module (CEM) in the vehicle and the CAN network.

This document is also intended to clear up misunderstandings and, in the final analysis, to reduce the number of incorrect replacements of the central electronic module (CEM). The document should act as a supplement to VIDA Design and Function for the central electronic module (CEM), CAN fault-tracing and software downloading.

Common misunderstandings 

• CAN-related faults

  "Do not shoot the messenger". Although the central electronic module (CEM) monitors CAN communication and verifies that all control modules are in the system, the control module is not responsible for all CAN activity. When CAN-related diagnostic trouble codes (DTCs) are present in the central electronic module (CEM), the control module is often wrongly thought to be defective.

  The central electronic module (CEM) can identify faults in other control modules as well as in wiring. When the CAN bus is active, the control module checks specific signals from other control modules. The diagnostic trouble codes (DTCs) CEM-1A5X-1A6X indicate that the central electronic module (CEM) was unable to receive the anticipated signals from other control modules. For more information:Information about the CAN network, structure and function

  Diagnostic trouble codes (DTCs) CEM-DF03-DF16 indicate short-circuits between the CAN cables (green or white) to battery voltage or ground. This does not mean that the central electronic module (CEM) is damaged, but that the control module has detected a fault somewhere in the CAN network

  1. The first central electronic module (CEM) was defective
  2. The second central electronic module (CEM) is also defective, but with a different fault.
• Discharged battery

  When fault-tracing a vehicle with battery problems, control modules with a constant power supply must be checked carefully. For example, is any lamp lit when it should be off? Is the power circuit for the blower fan still drawing current after the ignition key has been removed? Are the heater pads for the door mirrors constantly switched on? Is any relay constantly engaged? These are only a few examples.

  1. The first central electronic module (CEM) was defective
  2. The second central electronic module (CEM) is also defective, but with a different fault.
• Downloading software

  Since version 2000C, VIDA has a slightly different procedure for downloading software. This procedure must be followed when downloading software in, or replacing, the central electronic module (CEM). Incorrect downloading of software does not damage the control module, but can be difficult to rectify

  1. The first central electronic module (CEM) was defective
  2. The second central electronic module (CEM) is also defective, but with a different fault.
• Problems with the Immobilizer

  Immobilizer problems are rarely caused by faulty hardware in the central electronic module (CEM)

  1. The first central electronic module (CEM) was defective
  2. The second central electronic module (CEM) is also defective, but with a different fault.
• Incorrect relay activation

  Some central electronic modules (CEM) have been replaced mistakenly when all that was damaged was a system relay. Relays are very simple to replace and test in other positions or in another car. The control module is often incorrectly diagnosed due to a faulty system relay. The faulty relay may have been moved to a different location after the central electronic module (CEM) has been replaced.

  This gives the incorrect impression that:

  1. The first central electronic module (CEM) was defective
  2. The second central electronic module (CEM) is also defective, but with a different fault.

The relays that control the power supply from the central electronic module (CEM) to other control modules should permit simple fault-tracing. For example, if the diagnostic trouble codes (DTCs) CEM-1A54, (communication with the power seat module (PSM)), CEM-1A55 (communication with the climate control module (CCM)) and CEM-1A56 (communication with the driver information module (DIM)) are stored in the central electronic module (CEM), it is important to check the X-supply relay.

A few tips 

• The monitoring function for the central electronic module (CEM) in VIDA is a good tool for determining whether a relay is engaged or whether an input is high or low. This function, alongside readings using the breakout box, solves many problems related to the function of the control module. Note that only the coil in the relay for the control module is diagnosed, not the relay output
• The central electronic module (CEM) contains PIN codes for the immobilizer function but has nothing to do with the codes for remote controls or memories for power seats and mirrors with memory. (Remote control codes are stored in the upper electronic module (UEM), the power seats are controlled by the power seat module (PSM) and the mirror memories are in the driver's door module (DDM) and the passenger door module (PDM))
• The start relay that is activated by the central electronic module (CEM) has 0 V from the output from the control module in both activated and not activated modes. Do not rule out fault causes based solely on voltage readings. Check whether the relay is clicking or use a test lamp if necessary. The 0 V that is "supplied" by the central electronic module (CEM) in deactivated mode does not engage the relay coil
• The central electronic module (CEM) monitors CAN H (white cable) and CAN L (green cable). This does NOT mean that all CAN problems are related to this control module. The likelihood of the central electronic module (CEM) causing an E001 diagnostic trouble code (DTC) is no greater than this being caused by some other control module. When fault-tracing CAN-related faults in the central electronic module (CEM) or other control modules, measure the resistance in the CAN bus. Check behind the radio for short-circuited wiring, check the CAN cables for the supplemental restraint system module (SRS), the transmission control module (TCM) and the engine control module (ECM) as well as the connectors
• NOTE:

  Do not use an electrician's screwdriver or other tool when checking the connectors, as this may damage the connector. Male terminal P/N 9441394 (Female terminal P/N 9442486) can be used to check the affected connectors.

• NOTE:

  If diagnostic trouble code (DTC) E001 is permanently stored in a control module and the wiring is OK, a good tip is to cut the power to the control modules that have stored E001 (except for the central electronic module (CEM)) and check whether the code disappears. Then restore the power supply to one control module at a time until the code recurs. If it is possible to communicate with the car in ignition positions 0 and 1 but not in position 2, look for a control module that is powered via the ignition switch (+15) to find the defective control module.

• In order to isolate the OBD2 connector from the rest of the CAN network in the car, the central electronic module (CEM) contains four diagnostic relays. This means that the control module "opens" communication with both the low speed and specification networks via hardware. The command for opening communication is given over the communication cable from the VIDA cart to the central electronic module (CEM).

  This means that if VIDA can communicate with at least one control module on the low speed network and one control module on the high speed network, the relays are working properly. Defective communication cable communication between VIDA and the central electronic module (CEM) means that both networks are inaccessible. Where there is no communication between VIDA and the car, and the car is functioning properly, check the cable to VCT2000

• The central electronic module (CEM) is involved in the alarm function, but so are other control modules. For cars after model year 01, VIDA can be used to check the cause of the alarm. Verify correct function of door locks, the passenger door module (PDM), the driver's door module (DDM) and the upper electronic module (UEM) using the monitoring function in VIDA, for example
• NOTE:

  There is a considerable difference between "there are no diagnostic trouble codes (DTCs) in a control module" and "the control module is not reporting any diagnostic trouble codes (DTCs)". When diagnostic trouble codes (DTCs) are read from all control modules and there is a permanent CAN problem, the information can be misinterpreted. If a control module is not listed, it does not necessarily mean that the control module does not contain diagnostic trouble codes (DTCs). It may also mean that the control module is not responding.

• The central electronic module (CEM) only sends the temperature of a serial cable to the seat heating module (SHM). If the serial cable reports good voltage values, it is unlikely that the central electronic module (CEM) is the cause of any problems with seat heating.