Checking the Power Supply and Ground Terminals Power Supply, Central Electronic Module (CEM)

Checking The Voltage/ground Terminal, Central Electronic Module (CEM)

Checking the power supply and ground terminals Power supply, central electronic module (CEM)





The central electronic module (CEM) is powered directly from the battery via five power inputs on the control module.

The control module is also powered at three inputs from the ignition switch.
These inputs are active on different occasions, depending on the position of the ignition key.

Direct supply from battery (30)
1. Power input 1 terminal #E21 (#21) is supplied with power via a fuse from the battery. This power input is used for example to power the motherboard in the central electronic module (CEM). The power supply to the internal electronics is redundant with power input 2. The following relays and control modules are also powered via this input:
- Driver door module (DDM)
- Passenger door module (PDM)
- Left rear door module (LDM)
- Steering wheel module (SWM) and siren control module (SCM) (accessory)
- Relay for back-up (reversing) lamp
- Relay for courtesy lighting
- Power supply to the data link connector (DLC) and the stop lamp switch.

2. Power input 2 terminal #E22 (#22) is supplied with power via a fuse from the battery. This power input is used for example to power the internal electronics in the central electronic module (CEM). The power supply to the internal electronics is redundant with power input 1. The following are also powered via this input:
- Fuel pump (FP) relay
- High beam relay
- The power socket in the cargo compartment.

3. Power input 3 terminal #E20 (#20) is supplied with power via a fuse from the battery. The following are powered at this input:
- Right rear door module (RDM)
- Sun roof module (SRM) (motor)
- Relay for increased X-supply relay
- Power supply to driver's side power seat motors
- Power supply to passenger side power seat motors.

4. Power input 4 terminal #E23 (#23) is supplied with power via a fuse from the battery. The following are powered at this input:
- Relay for the infotainment system
- Climate control
- Media player
- Carphone
- Antenna units
- Audio system.

5. Power input 5 terminal #E18 (#18) is supplied with power via a fuse from the battery. This input is not used at present.

Supply from the ignition switch
1. Power input terminal #A30 (#30). X-supply. The power supply is activated when the ignition key is in position I. The following are powered at this input:
- Heating the rear view mirror
- Sun roof (function)
- other units in the passenger compartment that require X-supply
- signal as indication to the central electronic module (CEM).

2. Power input terminal #A17 (#17). 15-supply. The power supply is activated when the ignition key is in position II. The following are powered at this input:
- Monitoring the collision safety system
- Monitoring the exterior lighting and the back-up (reversing) system.

3. Power input terminal #A42 (#42). 50-supply. The power supply is activated when the ignition key is in position III. At this input, an indication is transmitted to the central electronic module (CEM) and on to the engine control module (ECM) that the ignition key is in position III. This means that the engine control module (ECM) will activate the starter motor if everything is OK.

Checking the power supply
Check the power supply cable from the battery to the engine compartment fusebox. Check for an open-circuit. Check for loose connections. Check for contact resistance. Check for oxidation. Also check that the fuses for the power supply to the central electronic module (CEM) are intact. The voltage must be approximately battery voltage.

Check the wiring from the engine compartment fusebox to the central electronic module (CEM). Check for an open-circuit. Check for loose connections. Check for contact resistance. Check for oxidation. Connect the breakout box to the central electronic module (CEM). Measure the voltage at the following terminals on the central electronic module (CEM):
- #E21 (#21) and ground
- #E22 (#22) and ground
- #E20 (#20) and ground
- #E23 (#23) and ground
- #E18 (#18) and ground. This terminal is not used at present.

The voltage at all measurements must be approximately battery voltage.

Ground terminal, central electronic module (CEM)
The central electronic module (CEM) has two ground terminals. Both the terminals are grounded in the right-hand A-post.
1. Ground input terminal 1 #A14 (#14) is signal ground and is connected to the ground terminal in the car body
2. Ground input 2 (terminal #A28 (#28) is power ground and is connected to the ground terminal in the car body.

Checking the ground terminal
Check the ground lead between central electronic module (CEM) terminal #A14 (#14) and the ground terminal 31/84. Check for an open-circuit. Check for loose connections. Check for contact resistance. Check for oxidation. With the ignition on, the drop in voltage between the ground connection for the central electronic module (CEM) and the ground terminal must not be greater than 0.5 V.

Check the ground lead between central electronic module (CEM) terminal #A28 (#28) and the ground terminal 31/10. Check for an open-circuit. Check for loose connections. Check for contact resistance. Check for oxidation. With the ignition on, the drop in voltage between the ground connection for the central electronic module (CEM) and the ground terminal must not be greater than 0.5 V.

Remedy as necessary.

Other information
- Connecting the breakout box
- Signal specification
- See the relevant wiring diagram for information about circuit and ground terminals in the bodywork
- Checking wiring and terminals

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