A 2012 Mercedes Sprinter 906 II rolled into a Swedish workshop recently with a single complaint: "motoer lampa lyser" and a permanent P2600 fault code. When you scan an OM651 2.2 CDI engine and get a P2600 open circuit code, the challenge is figuring out which of the coolant control circuits actually failed. The engine control module (CDI) monitors current draw continuously, and a loss of circuit continuity turns on the check engine light immediately. When we look at our own data for this specific code on this engine, the diagnosis splits into four distinct paths.
The Auxiliary Coolant Pump Failure
The most likely culprit, ranked at 45 percent probability in this diagnostic run, is an internal electrical break in the auxiliary coolant pump (M13/5). On the OM651 engine, this electric pump circulates coolant through the heat exchanger and the turbo and engine circuit during idle and after engine shutdown. At around 240,000 kilometers, the electric motor carbon brushes tend to wear out completely, the commutator burns, or the stator winding suffers a thermal break.
When the circuit opens, the CDI sets the P2600 code. Proving this requires testing the pump directly rather than just replacing it. The required steps are a resistance measurement of the pump internal motor winding, followed by a voltage and load test at the pump connector while commanding the actuator via a scan tool. Finally, you can perform a function test by applying a direct 12V power supply to the pump on the bench or in the vehicle.
Switchable Coolant Pump Valve Circuit
The second path, showing a 35 percent probability, points to an open circuit in the solenoid valve for the switchable coolant pump (Y133). The OM651 uses a mechanical coolant pump with a vacuum-controlled impeller shroud that restricts circulation during the warm-up phase. This vacuum system is electrically regulated by the CDI through the Y133 valve.
In the Mercedes-Benz DAS or Xentry system, an internal coil break in this solenoid generates P2600 or P2600-13 (Switchable coolant pump control circuit open). A break in the coil wire results in a constant check engine light. To confirm this, you must measure the resistance across the Y133 coil, check the voltage at the feed pin with the ignition switched on, and run an actuator test of the regulating valve via your diagnostic tool.
Harness Damage and Connector Oxidation
Ranked third at 25 percent is damaged wiring or an oxidized connector in the coolant pump circuit. The Sprinter 906 engine bay is exposed to heavy vibration, road salt, and moisture. On the OM651, capillary action (often called wicking) of coolant or moisture ingress into the auxiliary pump or valve connector is an established issue. This creates heavy copper oxidation, visible as green verdigris, which eventually breaks the circuit. Wires can also chafe against engine brackets or chassis sheet metal in the wheel well region, resulting in a complete break of the control or supply wire.
The diagnostic steps here start with a visual inspection and a drag test of the terminal pins in the connector. From there, you need to perform continuity and resistance measurements of the wiring harness between the pump and the engine control module. It is also necessary to check the transition resistance of the ground connection to the chassis.
Power Supply Relays and Fuses
The final scenario, ranked at 15 percent, involves a defective relay or a blown fuse for the coolant pump supply. Power to the electric coolant pump is protected by a dedicated fuse and routed through a relay in the fuse and relay center (SRB). If the fuse blows due to a temporary short circuit or a seized pump motor, or if the relay working contacts are heavily sooted or burned, the circuit breaks entirely.
Because the control module detects the lack of voltage and load, it logs the P2600 code. You can rule this out quickly with a visual and electrical continuity check of the circuit fuse, followed by testing the relay coil and contact resistance while operating the relay.
Approaching the Repair
When this code appears on an OM651, do not assume the auxiliary pump is dead just because it is a common failure. Grab your multimeter and perform the resistance checks on the M13/5 pump and the Y133 solenoid coil first. Taking ten minutes to verify the electrical continuity at the component level will tell you exactly which part of the circuit has failed, preventing you from ordering a water pump when the actual fault is a corroded wire or a failed vacuum solenoid relay.
Reference pages for the codes in this article
- P2600, what we measure for this code across real diagnostics