Audi trunk latch failure analysis.

This is a trunk latch actuator from an Audi station wagon. When 12V is applied the motor will spin the white gear which is attached to a spring encased in the red plastic. The gear is also attached to a rod which is threaded into a second shaft which will be drawn in and release the trunk latch. What makes this interesting is there is no limit switch, the actuator is very minimal.

The other interesting factor is why the actuator needed to be replaced. Most of the time it worked perfectly, however on occasion it would fail the unlock the trunk. The failure to open increased in hot weather. What was happening to the motor was more obvious when it was out of the car. When 12V was repeatedly applied the motor would first pull very firmly and quickly but with each repeated actuation it would become weaker and weaker. This is obvious in the videos below.

Inside the case the brushed dc motor is attached directly to the terminals and there are no other electronics inside. The car must switch the power to the actuator on for a certain length of time to unlock after which it turns off and the spring returns the actuator to its resting position. If the switched signal is too short the actuator won’t work, however if it is too long the motor will stall. The only method the car has for feedback is that there will be a switch that shows when the trunk is open or closed, however if something is holding the trunk closed or it becomes stuck then this will not work. To ensure that the motor doesn’t stay stalled and burn out there has to be a fail-safe (which there is!).

Inside the motor housing there is a large PTC thermistor! When the motor becomes stalled the temperature in the thermistor will rise quickly due to the excess current until only a nominal current is drawn by the actuator. When the actuator is released the temperature in the thermistor drops rapidly and the motor can turn with full torque once again. I believe there are two possibilities to why this actuator stopped behaving as designed. The first is that the thermistor drifted out of spec and its resistance increased too much at lower temperatures. When the car was hot the thermistor resistance was too high to allow sufficient current to turn the motor. The other option is that as the motor brushes wore their resistance increased, so for the same voltage applied to the actuator the motor had less torque. This combined with higher heat and a slight increase in thermistor resistance meant the motor just stalled. If I had a second actuator to compare I could determine the culprit.