Master Tech Kevin Hardy at Autologic HQ supports a technician working on a Mercedes-Benz 212 displaying various symptoms and warning lights.
Case Study for Mercedes 212, 207, 204 & 218 from production period 12/2011 to 03/2012
A customer reported various symptoms affecting a Mercedes-Benz 212, including different warning lights on in the instrument cluster.
Symptoms can vary depending on the model range of the vehicle. The warning lights could come on, some models might have various broken functions including the wiper washers, and for some models it can even result in the engine not being able to start, or not allowing you to select a gear. We have found that re-cycling the ignition may fix the issue temporarily.
FIXING THE FAULT
1 Carrying out a full vehicle scan will show most control units with numerous CAN communication faults. A typical fault read will give CAN faults in:
All reporting CAN issues will be stored faults. The common theme across all models is the codes in the rear SAM and SRS – this will give you a diagnosis point to the problem.
In the rear SAM you will have a few rear light codes, and the following:
These codes are important as it will indicate the premature closing of the 30G relay in the f32 fuse box. The other sign of this being the cause are the codes in SRS:
SRS: Airbag. Part no. 212 901 42 04. 3 Faults:
In 100% of cases, these codes will be stored in the SRS module if the 30G feed is shut off prematurely.
CAUSE OF THE PROBLEM
Defective no-load current relay in engine compartment pre-fuse box F32 (this is the jump point fuse box under the bonnet).
WHAT IS 30G?
30G is a 12Volt permanent live circuit which is shut down after a vehicle has been locked for six hours. This results in 60% of the control units receiving a hard reset (power off and then on again at vehicle restart), this has the effect of less vehicle problems.
2 The rear SAM checks after 75 minutes and will find a drain of >50mA.
3 It will then give a 5-minute warning to the other control units to shut down.
4 After the other control units have shut down, it will then shut circuit 30G by triggering the shut off, a control relay in F32 (front electrical pre-fuse box) without waiting the 6 hours.
a If the voltages/quiescent current is within spec, the rear SAM will go back to sleep
b The system will always try to ensure the vehicle is able to start
c The battery sensor has a diagnostic facility to help diagnosis of current drain faults
5 It’s important to disconnect the battery to replace the f32 fuse box.
Note: Remove the battery sensor in the correct way or this will cause issues after you’ve finished
a Procedure for disconnecting the battery (battery sensor):
b Procedure for connecting the battery sensor:
ELECTRICAL CONNECTION FOR B95
Red +30 from battery positive Blue/white LIN wire to rear SAM
c When disconnecting the battery
Note: no steps are required for initial start- up of B95 battery control module
6 Once the battery is correctly disconnected, the f32 fuse box can be replaced with a new part from Mercedes-Benz (modified).
The old f32 has a design fault and will shut off without the command from rear SAM.
7 The battery will then be put back online after the replacement.
8 Clear codes as necessary.
9 Road test the vehicle on job completion.