What Tests Confirm an N55 Valvetronic Motor Is Bad With 135808 and 135A08?

A 2016 BMW 435i owner asked, “What specific tests should be performed to confirm the motor is actually bad?” after faults 135808 and 135A08 appeared even though the N55 started, idled, and drove normally. The direct answer is to test the actuator, its integrated position sensing, the harness, and the mechanical load as one system before replacing the motor again. Those two faults identify a Valvetronic feedback problem, but they do not prove which component caused it.

What 135808 and 135A08 are telling you

The owner reported 135808, described by the scan tool as a Valvetronic servomotor or position-sensor electrical malfunction, and 135A08, described as an implausible Valvetronic position signal. The car had no unusual noise or power loss, and the previous owner reportedly installed an original-equipment motor about 10,000 miles earlier. That combination makes a test-first approach especially important: a recent motor can still fail, but the repair history does not clear the connector, wiring, DME side, or eccentric-shaft mechanism.

BMW’s N55 training material explains why these faults overlap. Valvetronic III uses a brushless motor to turn a worm drive and move the eccentric shaft. The position sensor is integrated into the servomotor rather than mounted separately on the shaft. BMW also describes five Hall-effect sensors inside the assembly. Three identify motor position, while two provide the finer feedback used to calculate eccentric-shaft movement.

That layout means “position sensor” and “motor” are not independent bolt-on parts on this engine. The DME commands the motor and judges the resulting movement through signals from the same actuator assembly. A bad actuator can corrupt the feedback, but so can a poor connection between the actuator and DME. Mechanical resistance can also leave commanded and actual movement out of agreement.

Normal driving does not cancel the evidence. BMW’s functional description says that if actual eccentric-shaft position cannot be detected, the system can move toward maximum valve opening and control air with the throttle in a regulated emergency mode. A car may therefore feel normal during a short drive while the DME has recorded a real electrical or plausibility fault.

The test order that prevents another parts guess

I would ask for the fault report before anyone clears it. The useful record includes whether each fault is current or intermittent, how often it has occurred, the operating condition when it set, and any related supply-voltage, motor-phase, or position faults. A single stored event after a weak start calls for a different test path than a fault that returns immediately on every key cycle.

Next, inspect the whole accessible circuit. The actuator connector should be fully seated, with straight terminals, secure terminal tension, and no oil, water, or corrosion. The harness should be checked along its route for pinching, rubbing, previous repair, stretched sections, and damage near both ends. The Reddit thread that supplied the owner question immediately raised connector contamination and the actuator-to-DME harness as alternatives to another motor. A separate Bimmerpost discussion of the same two faults also pointed owners toward checking the cable and both plugs before condemning the actuator.

A visual check is only the first pass. A wire can look intact and still fail when current flows or when the engine moves. The shop should test the relevant power, ground, motor-phase, and sensor circuits using the correct wiring information for the car. The test needs to find voltage loss and poor terminal contact under realistic load, not only continuity with the circuit disconnected.

The third step is controlled scan data. A BMW-capable diagnostic session should compare commanded Valvetronic position with actual position while the actuator is asked to move. I want to know whether the DME issues a command, whether the reported angle changes smoothly, whether the signal drops out, and whether the fault returns at a repeatable point. A fixed or implausible position value narrows the search, but it still needs electrical checks before anyone labels the DME or actuator as the cause.

Pico Automotive published a Valvetronic case in which the fault wording suggested an open motor phase, yet milliohm testing found a winding imbalance rather than a simple open wire. The same investigation found an abnormal voltage on a sensor-ground path, leaving the technician to distinguish between harness damage and a control-unit protection state. The lesson is practical: the fault description is the DME’s interpretation of electrical behavior, not a direct view of the failed part.

Separating actuator, wiring, and mechanical faults

The evidence should sort into one of three patterns.

Actuator evidence: The motor phases are not balanced, the internal position signals are corrupt, or the actuator fails a controlled movement test while its supply, ground, and external wiring remain sound. Because the position sensing is built into the N55 servomotor, a confirmed internal sensor failure supports replacing the actuator assembly.

Harness or connector evidence: The signal changes when the connector or loom is moved, voltage drops across a loaded connection, terminal tension is poor, or continuity fails between the actuator and DME. In that case, another motor will not repair the circuit. The connection has to be repaired and retested.

Mechanical evidence: Electrical command and feedback remain credible, but current rises or movement slows, stops, or becomes noisy at a repeatable shaft position. That result points toward resistance in the worm drive, eccentric-shaft gear, bearings, or related valvetrain. BMW’s N55 material shows that the motor drives the eccentric shaft through a worm mechanism supplied with engine oil. FCP Euro’s repair guide documents wear at that interface and stresses that diagnosis must match the running condition before parts are replaced.

Mechanical inspection is not a casual first step. Access reaches into the valvetrain, and the eccentric shaft is spring-loaded. I would leave that work to a BMW specialist with the correct service information and tools. The aim of this article is to define the evidence the invoice should contain, not to turn a fault-code reader into a reason to open the cylinder head.

What I would approve on the repair order

Before approving a second Valvetronic motor, I would want the shop to write down these results:

1. The complete BMW fault text and status for 135808, 135A08, and any related faults.
2. The supply-voltage history and the condition in which the faults return.
3. Connector and harness inspection results at the actuator and DME ends.
4. Loaded electrical test results for the relevant circuits.
5. Commanded versus actual position behavior during a controlled test.
6. Motor-phase balance or equivalent actuator evidence if the shop has the equipment.
7. Mechanical movement findings only if the electrical path has passed.

A replacement recommendation should point to a failed test. “The code names the motor” is not enough, especially when a motor was fitted recently and the car has no obvious drivability complaint. If the shop cannot show whether the fault is inside the actuator, in the harness, or in the mechanism it drives, the diagnosis is incomplete.

The safest conclusion is narrow: 135808 and 135A08 justify prompt diagnosis, but neither code authorizes a motor by itself. Prove the circuit, compare command with movement, and then inspect mechanical load. That sequence gives an owner a defensible repair decision whether the final cause is the actuator, a connector, the harness, or the eccentric-shaft mechanism.

Sources

[1] Reddit owner question: N55 Valvetronic Codes 135808 and 135A08

[2] BMW N55 Engine technical training manual

[3] BMW Valvetronic III functional description

[4] Pico Automotive Valvetronic diagnostic case study

[5] Bimmerpost N55 Valvetronic owner discussion

[6] FCP Euro N55 and S55 eccentric-shaft replacement guide