ESC init problems with Pixhawk and Tmotor Air 40A


We are building a new quadcopter, the relevant parts for the problem are listed bellow:

Motors: Tmotor U3
ESC: Tmotor Air 40A (no BEC, 600Hz)
FC: Pixhawk 3DR
TX/RX: Futaba 10J / Futuba 2008SB, connected in SBUS
Firmware(s) tested: 3.2 & 3.2.1

Note: We have had the Pixhawk since the end of 2013 and flew it very successfully before on another RIG.

The problem is quite simple, we mounted motors and ESC, we made the ESC calibration using a squid cable connected to the RX throttle. Up to here, no problems, motors are in sync and all rotating in the good direction.

Then we connect them to the Pixhawk, after some basic config we intend to arm to check the correct working of the motors. Only 2 ESC are initialized when arming, so only 2 motors run. It doesn’t really matter which ones, as if we change the order of the ESC pins on the Pixhalk, other 2 ESC got activated and spin their motors. We’ve been testing different RC_FREQ without success. To describe the problem exaustively, we should add that sometimes, 3 ESC (randomly, or so it seems) are initialized, yet one of them quicky un-init before we can arm.


  • We’ve read somewhere (on this forum) that the PWM output of the Pixhawk are 3.3v instead of 5V, could this be the problem?
  • Is it because our Pixhawk has an older revision (We don’t really know which one btw) ?
  • Did anybody reading this got this combination of FC/ESC to run?
  • If not, should an incompatibility be listed?

Anyone with a beginning of an answer would help big deal!
Thanks a lot for reading.
Best regards,