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Is there a problem with rear SAM in C Class 2003 W203?

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The vehicle in question is a 2003 CClass diesel W203.

The rear brake light remains consistently on when the ignition is turned on.

Additionally, I am receiving ESP and BAS alerts on the dashboard, as well as a notice that the brake lights are on, prompting me to go to the repair.

A significant number of fault codes are seen on a scanner in relation to ESP and CAN communications.

The scanners also exhibit a persistent malfunction on the back SAM, namely B1048, circuit 54, resulting in a short circuit to B+.


Upon the cessation of vehicle operation, the rear SAM enters a state of dormancy within a mere minute, although maintaining a current consumption of 30mA throughout this sleep phase. Although it seems too high, the alarm module is connected to the rear SAM, suggesting that it may be within the expected range.

Based on my research, I suspect that the ESP fault codes associated with CAN communications may also be linked to issues with the rear SAM. Therefore, I am curious: is a 30mA parasitic drain at the rear SAM a typical occurrence, or does it indicate that the short circuit B1048 fault diagnostic is a legitimate fault, implying that the rear SAM should be sent for testing?

I would much appreciate any advice. I am endeavouring to aid a companion.

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Have you attempted a circuit reset of 30 units?
Deactivate the battery power of the SAM module for a brief duration.

I would be examining the brake light switch, since a defective switch has the potential to result in both the lights being trapped on and the ESP lights being illuminated.

Is a 30mA pull causing battery depletion? Typically, the CAN system carries around 800mA while it is in an active state.

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Thank you for your prompt response.

Unacquainted with the process of doing a circuit 30 reset, which involves temporarily deactivating the SAM. I assume that the technique, or more precisely, its impact, is significantly distinct from just detaching the battery for a little while.

I lack the means to get a wiring schematic. I have conducted a search specifically to get information on the brake switch, in order to determine the most effective method for testing it. The presence of three wires on the switch first led me to question if the brake switch functioned as a potentiometer, measuring the rate of pedal force application. However, after seeing other photographs of Mercedes brake switch wiring on the Internet, such as those depicting 5/6 wire switches, I now conclude that it is a straightforward switch. In the case of a 2-wire switch, the process of testing would be readily apparent, even in the absence of a wiring schematic. An attempt was made to detach the connection; still, the brake lights remained illuminated (and the gearbox was securely locked in Park). However, this outcome did not provide any significant information, since it is likely that disengaging the switch would have resulted in the activation of the brake lights anyway. I would really appreciate your guidance on the optimal testing method for the switch, considering its 3-wire connection, or on how to imitate the switch at the junction. In the absence of a technical handbook or data, the owner would really appreciate any recommendations for acquiring wiring schematics that are of high quality, lucid, and precise.

I appreciate your confirmation on the 30mA sleep current. The battery has seen depletion on two recent occasions, which seems to be a cause for concern. The owner saw the activation of the lights throughout the early hours. The alarm siren did not sound, however, he claims that there is never a siren when the alarm is activated. However, we are closely monitoring the situation and collecting further information about any clear trends.

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Circuit 30 operates on live power, therefore necessitating the detachment of the battery.
However, by only removing the power from the SAM, the failure may be resolved, thereby isolating the problem.

The brake light switch is equipped with separate wires for the lights and the ESP/abs, among other components. It can be easily tested with a multimeter. When the switch is pushed, it creates an open circuit, but when it is not pressed, it acts as a resistance.

An alternative approach would be to detach the switch, fully extract the central component using a ratchet mechanism, and thereafter reconnect the sensor. This procedure may potentially resolve the issue at hand.

However, brake light switches are inexpensive.

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hat seems to be a strategy. I express my sincere gratitude. Are you referring to the concept of a "open circuit when pressed" or a "closed circuit"? Based on your description, it seems that the circuit is ground-side-switched. This means that when the pedal is pushed, one of the three wires will be connected to the ground, and the other two wires will also be connected to the ground, thereby completing the circuit. However, as you have said, doing the test will be rather straightforward, given my understanding of the rationale for the presence of three cables. The testing of the item is expected to be conducted on Sunday or a later date. I will provide a report then.

It is anticipated that the design would be such that the removal of the switch will not result in any leakage of braking fluid.

Thank you so much for your help. If it were my own vehicle, I would not hesitate to do the task without seeking guidance and experience from others. However, I am reluctant to cause damage to someone else's automobile while attempting to assist them. It seems that they already have enough issues with their vehicle without my involvement.

Surprisingly, there was a scarcity of information on the Internet about the "circuit 30 reset". While it is widely acknowledged as a legitimate method and terminology accepted by Mercedes, there is a noticeable lack of information when searching for "what is a..." or "how to do a..." on the Internet.

Thank you one again, having knowledge of the purpose of those three wires has been very beneficial. I anticipate returning after the weekend.

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I am willing to assist if possible.

Are you inquiring if the removal of the sensor will result in the absence of fluid? This observation suggests that we are examining distinct sensors.
The one I am referring about is located at the pedal located over your feet.
When examining the switches and sensors located at the servo and brake reservoir, it is seen that none of them are responsible for activating the brake lights.

Additionally, it is important to note that the malfunction of the alarm is most likely attributed to the siren itself, maybe due to corroded terminals or damaged wiring in close proximity to the siren (specifically, the n/s/f wheel arch liner).

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"That suggests that we are examining distinct sensors."
The one I am referring about is located at the pedal above your feet.

I am quite pleased that I inquired! Initially, I inquired about the ease of accessing the switch located at the pedal. However, he provided reassurance that the switch was really situated on the brake cylinder/servo, rather than at the pedal. It is possible that he did not see the sensor and instead made an assumption that the stop-light switch was the sensor responsible for sensing brake-fluid pressure. This observation maybe elucidates the lack of efficacy in deactivating the brake lights when withdrawing the connection.

It brings to mind the informal slogan of the RAF: "Never make assumptions, verify!"

I will instruct him to engage in physical exertion and locate the pedal switch.

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