How the Mercedes W220 S-Class Rear SAM Module Causes Electrical Faults
The rear SAM (Signal Acquisition Module) on the Mercedes W220 S-Class is a secondary power distribution and signal relay unit mounted in the boot area, usually behind the right-hand trim panel. When it fails, drivers rarely identify it immediately because the faults it produces — a dead rear wiper, a fuel pump that cuts out, rear lighting failures, or a boot lid that will not release — appear to be separate, unrelated problems. In reality, they all trace back to one module losing control of its output circuits.
What the Rear SAM Controls on the W220
Unlike the front SAM, which handles the majority of engine-bay and interior distribution, the rear SAM on the W220 manages circuits at the back of the car. Its responsibilities typically include:
- Fuel pump relay activation
- Rear window wiper motor supply
- Boot interior lighting
- Rear washer pump
- Tow hitch electrics (where fitted)
- Rear-left and rear-right door control communication
- Parking sensor module power (on equipped vehicles)
Because the rear SAM sits on the W220 CAN bus and communicates with the front SAM, EZS ignition module and other control units, a failing rear SAM can trigger fault codes across multiple unrelated systems in the diagnostic log — which leads many workshops to chase individual output faults rather than the module itself.
How the W220 Rear SAM Fails in Practice
Water is the primary cause. The W220 boot area is vulnerable to ingress through deteriorated boot seal rubbers and cracked tail-light gaskets. When moisture reaches the rear SAM housing, it attacks the solder joints on the main relay outputs first. Corrosion builds up gradually, so the faults appear intermittently before becoming permanent. For comparison, see Mercedes W220 S-Klasse SAM-Modul Fehler Häufige Ausfälle und Reparatur.
The second common cause is age-related relay failure. The rear SAM contains several relays soldered directly to the board. After fifteen or more years, relay contact resistance increases and switching becomes unreliable. This is what typically causes the fuel pump circuit fault — the relay closes but passes insufficient current, creating hard starts or stalling under load.
Overloading from aftermarket accessories — particularly tow bars wired without dedicated fusing — can also burn the tow electrics output traces, taking nearby circuits with them.
Fault Codes Associated With the W220 Rear SAM
Using Star Diagnosis or compatible tools, the following codes regularly appear when the rear SAM is at fault. Note that the exact code set depends on the vehicle’s build specification and equipment level.
| Code | Beschreibung |
|---|---|
| B1011 | Rear SAM — internal supply voltage fault |
| B1000 | Control module — general internal fault |
| B1048 | Fuel pump relay output — short or open circuit |
| B1060 | Rear wiper relay output — circuit fault |
| U1120 | Lost communication with rear SAM on CAN |
| B1070 | Boot lid release output — circuit fault |
U1120 appearing in the front SAM, EZS or instrument cluster log is a strong indicator that the rear SAM has stopped communicating entirely — either due to total power loss to the module or internal CAN transceiver failure.
Diagnosing the Rear SAM Before Replacing It
Before ordering a replacement, carry out a few basic checks. First, locate the rear SAM in the boot and remove the trim panel. Inspect the connector and housing for signs of water damage — white residue, green corrosion on pins, or visible moisture. If the connector looks clean, check the 12V supply and ground to the module with a multimeter. A module that is receiving correct power but producing no CAN communication is almost certainly internally faulty. One that has lost power supply entirely may simply need a fuse or ground strap repair.
The fuel pump circuit specifically can be isolated by connecting the fuel pump directly to battery via a jump wire to confirm the pump itself is functional. If the pump runs fine on direct power but not through the SAM relay output, the module is the source. A related diagnostic guide is Was passiert, wenn das CAN-Gateway der Mercedes W220 S-Klasse ausfällt.
OEM Part Numbers for the W220 Rear SAM Module
The W220 rear SAM has several variants depending on model year, engine and equipment specification. These part numbers represent the main production runs and should be checked against the vehicle’s actual data plate before purchasing:
- A 220 820 27 26 — early W220 production, non-Parktronic
- A 220 820 35 26 — mid-series, standard equipment
- A 220 820 55 26 — with tow hitch preparation
- A 220 820 62 26 — later production with Parktronic output
- A 220 820 43 26 — S600 and AMG variant
Matching the exact OEM number matters with the W220 rear SAM because module variants control different relay outputs. Fitting a non-Parktronic variant to a car with parking sensors will result in the sensor system losing power. Always cross-reference via the 17-digit VIN before buying a used unit.
Affected Vehicle Range
The W220 S-Class was produced from 1998 to 2005. The rear SAM fault described here is most commonly reported on vehicles from model year 2000 onwards, particularly the 2002–2005 facelift production. Engines affected include the S280, S320, S350, S430, S500 and S600 as well as the S320 CDI and S400 CDI diesels — the rear SAM is not engine-specific but equipment-level specific. Long-wheelbase W220 variants share the same rear SAM family.
Used OEM Replacement: What to Check
A tested used rear SAM from a donor W220 is a practical solution if the replacement unit has been verified to power up and communicate. When sourcing a used unit, confirm the donor vehicle’s equipment level matches your car — particularly tow hitch wiring and Parktronic. Request the full OEM part number from the donor. Avoid units pulled from accident-damaged vehicles where the boot area suffered impact, as SAM connectors can develop hidden pin damage.
The rear SAM on the W220 does not typically require coding after replacement, unlike the front SAM or EZS module. In most cases it is a plug-and-play swap, though a full system scan after fitting is advisable to confirm all output circuits are functioning correctly and to clear any residual communication faults from the network log.
Häufig gestellte Fragen
Can a faulty W220 rear SAM cause the car to not start?
Yes, indirectly. The rear SAM controls the fuel pump relay on most W220 variants. If the relay output fails internally, the fuel pump will not receive power and the engine will crank without starting. This is one of the more confusing symptoms because the front SAM and EZS appear to be functioning normally. For additional context, read Kauf eines gebrauchten Mercedes W220 S-Klasse EIS-Zündmoduls: Worauf Sie achten sollten.
Does the W220 rear SAM need programming after replacement?
In most cases, no. The rear SAM is not a coded module in the same way as the EZS or instrument cluster. However, fitting an incorrectly specified unit — for example, one without the Parktronic output — will result in missing functions. A post-installation scan is always recommended to verify all circuits are active.
Where exactly is the rear SAM located on the W220?
The rear SAM is mounted inside the boot, behind the right-hand side trim panel. On most production vehicles it is a grey or black rectangular unit with one or two multi-pin connectors. Removing the trim panel exposes it without the need to lift the vehicle.
Can the rear SAM be repaired rather than replaced?
Relay-related failures can sometimes be repaired by a competent electronics technician — the relays are through-hole soldered components and can be desoldered and replaced individually. Water-corroded PCB traces are harder to repair reliably. If the corrosion is superficial and limited to the connector area, cleaning and re-pinning the connector may restore function. Severe internal corrosion usually warrants replacement.
Which fault codes in other modules point to a rear SAM problem?
U1120 appearing in the front SAM, EZS or instrument cluster fault log is the most reliable pointer. It indicates the rear SAM has dropped off the CAN network. Multiple unrelated B-codes across rear lighting, wiper and fuel pump outputs appearing simultaneously also suggest the rear SAM rather than individual component faults.
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Zugehörige Diagnoseanleitungen und Originalteile
- Was das Xenon-Scheinwerfer-Steuermodul der Mercedes W220 S-Klasse leistet
- Was entleert die Batterie in einer Mercedes W220 S-Klasse über Nacht
Verwandtes Produkt: A0325458232 Mercedes-Benz W220 C215 SAM-Steuergerät vorne links 05045116 – Original, gebraucht & geprüft
Die Kategorie durchsuchen: Karosseriesteuergerät, SAM und Komfortsteuergeräte
