One of my TowerPro MG946R digital servos is faulty in a way I haven't seen before: if it receives continuous PWM signal that cyclically performs clockwise and counterclockwise movements, after a while it starts to rotate uncontrolled only clockwise and doesn't stop at all, going over its maximum angle limit, causing gears to crash with their own guards, blocking the whole gear mechanism. If I try to restore it, disconnecting everything, disassembling it and putting the servo gears in the original correct position; trying again, same issue happens again exactly after the same number of cycles (14-15).

Considering that:

  • It operates at 5.5V (specs voltage range is from 4.8V to 6.6V), with 50Hz pulse;
  • PWM signal is correctly provided;
  • There isn't any interruption of power, control signal flux or loose contact in the connection;
  • The faulty servo for few cycles works correctly and if I power off the servo between a cycle and another the error doesn't happen;
  • The problem happens both with and without load;
  • Other servos of the same model with the same configuration work as supposed;

What issue could cause this problem?

The servo will be replaced but I'm curious to know the technical reason of this issue.


1 Answer 1


The most likely failure is that the internal position sensor has failed or behaving erraticaly.

  • 1
    \$\begingroup\$ After the same number of cycles every time? And powering down fixes it? It seems unlikely its the pot if that is the case. \$\endgroup\$
    – DKNguyen
    Sep 14, 2021 at 22:45
  • 1
    \$\begingroup\$ Exactly, the weird thing here is that it doesn't behave erratically, actually it's very accurate in its faulty pattern: for a while it works exactly as supposed then suddenly goes crazy, and always in the same way. \$\endgroup\$ Sep 14, 2021 at 23:10

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