Mod Details
PremiumNo Difficulty
Mod ID1366
Creditevilution
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Linkhttps://www.evilution.co.uk/mod/light-on-with-power-loss.htm Copy to Clipboard
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.
Faulty Lateral Movement (Yaw) Sensor
It’s a calibrated hardware sensor that’s bolted firmly to the car. They can have component failure which causes de-calibration and, because of the location, they can also suffer from water ingress.
So it’s worth checking for moisture under the carpet and consider fitting a replacement. However, also make sure that the sensor is firm bolted down and facing the correct direction. If it’s rattling around, it can cause spurious signals.
Steering Angle Sensor Issue
The steering wheel angle sensor on the 450 and Roadster are very sensitive and don’t like being disconnected. If the sensor is moved whilst the steering wheel is removed from the car, there’s a good chance that the sensor will fail. Some OBD diagnostic units will display the angle being reported to the car from the sensor. If the wheel is dead straight and the sensor is more than a couple of degrees out, it’ll show an error.
If no angle is shown or the angle doesn’t change when the steering wheel is removed, it’s definitely dead.
Wheel Rotation Issue
More often than not, it’s a rusted and split reluctor ring on the rear of the car. In 25 years I have only ever heard of 2 cars failing on the front reluctor rings.
The 450 had rusted front reluctor rings but the 451 was more interesting as it doesn’t have physical reluctor rings.
They have a circular magnetic array built into the bearing. As I’m sure you know, heat will demagnetise magnetic objects depending on temperature and magnet type so, if the bearing gets too hot and reaches the Curie temperature of the magnets, they’ll cease being magnets. Then the car can’t read the wheel speed. If you leave the car to cool down, the magnetism may return enough to make it function again but eventually, you’ll have to change the bearing assembly.
Also check the wheel rotation sensors work correctly.
Also check the tyre pressures and also make sure the car is running the correct tyre sizes on all 4 wheels.
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 gear box controller (TCU/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.