Rear SAM Problem - C Class 2003 W203?

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New Member
Nov 17, 2021
The car is a 2003 CClass diesel W203.

The symptoms are rear brake light always on (with ignition on).

Also getting ESP and BAS warnings on dashboard, along with warning that brake lights are on, drive to workshop.

Quite a few fault codes come up on a scanner relating to ESP and CAN comms.

Scanners also show a permanent fault on the rear SAM is B1048, circuit 54, short circuit to B+.

Now, with the car shut down, the rear SAM goes to sleep after just one minute, but is still drawing 30mA in this sleep state. That seems a little high to me, but then the alarm module is attached to the rear SAM, so perhaps it’s normal?

From my searches, I believe that the ESP fault codes relating to CAN comms could also be related to problems with the rear SAM, so I wonder: is a 30mA parasitic drain at the rear SAM normal, or is it indicative that this short circuit B1048 fault diagnostic is a genuine fault, suggesting the rear SAM should be sent away to be tested?

Any guidance would be much appreciated: I’m trying to assist a friend.
Have you tried a circuit 30 reset?
Basically remove battery power from SAM module for a few minutes.

I’d be having a look at the brake light switch, a faulty switch could cause both the lights stuck on and the esp lights on.

30mA draw sound good to me, is it draining the battery? Normally more like 800mA or so when CAN system is awake.
Thanks very much for replying.

Not familiar with a circuit 30 reset, removing power from the SAM for a few minutes. I presume the procedure, or more accurately, its effect, is quite different from simply disconnecting the battery for a few minutes?

I don’t have access to a wiring diagram. I have searched for one, especially to get some info on the brake switch, to figure out how best to test it. There are 3 wires to the switch; that made me first wonder if the brake switch was a form of potentiometer (for rate of pedal force application data), but seeing other Mercedes brake switch wiring images on the Internet eg 5/6 wire switches, I now presume it’s a simple switch. If it were a 2-wire switch, testing would be obvious even without a wiring diagram. We did try disconnecting the connector; however, the brake lights remained on (and the transmission locked in Park), but that didn’t tell me anything because perhaps disconnecting the switch would have put the brake lights on, anyway. If you can guide me on how best to test the switch given its 3-wire connection, or how to simulate the switch at the connector, that would be a great help. Alternatively, any suggestions for obtaining good, clear, accurate wiring diagrams would be most welcome; the owner has no technical manual or data.

Thank you for your reassurance on the 30mA sleep current. (The battery has drained on 2 recent occasions; that appears to be an alarm thing - the owner noticed the headlamps on in the morning. No alarm siren sounded, but he says there never is a siren on alarm activation. But we’re keeping an eye on that, gathering more info on any definite patterns.)
Yeah circuit 30 is live power, so basically disconnect the battery.
However if you remove the power from the SAM only and it cures the fault then it kind of isolates the problem.

Brake light switch has a wire for the lights and then one for the esp/abs and what not, it’s simple to test with a multimeter, open circuit when pressed and resistance when not.

You could also remove the switch, pull the middle part all the way out (on a ratchet)
then refit the sensor, that might also sort your problem.

Brake light switches are cheap though.
That sounds like a plan. Thank you very much. Did you really mean ”open circuit when pressed” or perhaps “closed circuit“? Because, from your description, I’m thinking that it’s a ground-side-switched circuit ie one of the 3 wires will go to ground, and when the pedal is pressed, each of the other 2 wires will then be grounded, completing the circuit. But as you say, it’ll be easy enough to test, now that I know why there are 3 wires. I think it’ll be Sunday or later till I can test it. I’ll report back then.

I expect the design is such that no brake fluid gushes out when the switch is removed.

Many thanks for your guidance. If it were my own car, I wouldn’t be so hesitant to dive in without others’ advice and expertise, but I don’t want to break someone else’s car trying to do them a favour; he seems to have enough snags on the car without my adding to them.

By the way, there wasn’t much on the Internet about “circuit 30 reset” - just enough to realise it’s a genuine Mercedes-recognised procedure and terminology, rather than snake oil, but there’s surprisingly little when you search for “what is a…” or “how to do a ……”

Thanks again, knowing what those 3 wires are for has been a great help. Hopefully, I’ll get back after the weekend.
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No problem happy to help if I can.

You’re asking if no fluid will come out if you remove the sensor? That makes me think we’re looking at different sensors!
The one I mean is at the pedal above your feet sort of thing.
If you’re looking at any switches/sensors at the servo/brake reservoir then non of them operate the brake lights.

Also wanted to mention that the fault with the alarm is likely to be the siren itself and possibly corroded terminals or broken wiring at or near the siren (n/s/f wheel arch liner I believe).
That makes me think we’re looking at different sensors!
The one I mean is at the pedal above your feet sort of thing

So very glad I asked! At the start, I asked him how easy was it to access the switch at the pedal, but he assured me there wasn’t one there; it was on the brake cylinder/servo. Now, I suspect he never looked and merely presumed the sensor he could see detecting brake-fluid pressure was the stop-light switch. And that may well explain why pulling that connector did nothing for turning off the brake lights.

Reminds me of the unofficial motto of the RAF: “Never assume, check!”

I’ll will tell him to get his knees dirty and to go and find the switch on the pedal.

(So, perhaps that sensor on the brake cylinder/servo is indeed a potentiometer-type sensor to acquire data on the rate of brake application?)

Just shows: without a wiring diagram, you’re really on a hiding to nothing.

I’m so very grateful for your help: you may well have saved hours of barking up the wrong tree.
He assures me he did check the brake pedal area, and, whilst there is an obvious space where a brake stop-light switch would go, none is fitted to his car. So, who lnows? 😄

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