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If the yellow exclamation comes on and stays on, there’s a chance you’ll lose power too. Check the rear reluctor rings. If it has cracked or fallen off, it will bring up an error. This is the most common cause for the light to be on.
However, 3 other options that will bring on the light and reduce power are:
Faulty lateral movement sensor.
Faulty or poorly aligned steering angle sensor.
Wheel rotation sensor fault.
MB Star diagnostics should tell you what fault is causing the error light and loss of power. For the wheel rotation sensor fault, check the wiring from the sensor all the way to the ABS pump. It is becoming more common for the front sensor cables to wear and short circuit.
Why The Power Loss?
The reluctor rings on all 4 corners, the yaw sensor and the the steering angle sensor create the data that the car uses to determine that it’s going in the expected direction. When the wheels slip, the car thinks one or more of the wheels are skidding.
When the car is cornering and loses traction, the lateral G force (read by the yaw sensor) suddenly goes to 0 and the car knows it’s sliding sideways.
Taking data from the yaw sensor, the car can determine which direction it is heading. It cross references that data with the info from the steering angle sensor. If the car is going straight but the wheel is turned, the car has lost front end traction. If the car is turning but the wheel is straight, the car has lost rear wheel traction.
In all of these cases, the car will brake the wheels that have deemed to have lost traction. The hope is that the braking will allow the wheel to regain traction. Whilst that is happening, the ECU or GDU also limits the amount of power to the engine by feathering the throttle body and the clutch. The last thing the car wants is you adding more power in a panic and making the skid worse.
But I’m Not Skidding
There lies the problem. If you have an issue with a sensor, the car will be given false information which makes the car incorrectly determine that it has lost traction. So the throttle and clutch are over-ridden, reducing power.
It could also be that the sensor is saying one thing but the car is physically doing something else. If the tracking is out enough, the car will be cornering when the wheel is pointed straight. This will confuse the car but it’s an obvious one to notice as the steering wheel won’t be straight.
Power Loss But No Light
It’s likely to be an engine sensor putting the car into safe mode. Nothing to do with traction control. However, with this type of safe mode, you almost always get the check engine light on. Your best option is to view live data on MB Star. With the car at stand still, check the yaw sensor output to make sure it’s not outputting incorrect data to give the impression that the car is cornering or slipping.
You can also check the steering angle position with the wheels straight on. The angle shouldn’t be too far off 0.