How to Test Bad ESC (Not Responding or Stuttering)

by Oscar
Published: Last Updated on

Unless you are super tech savvy, it’s almost impossible to fix a dead ESC. Anyway it wouldn’t hurt to learn how to test and troubleshoot bad ESC for common issues, and understand which component might be damaged.

Before we start, please check out my beginners’s guide about ESC’s. It covers all aspects including ESC firmware, protocol, technology, as well as hardware anatomy, it will give you a deeper understanding about ESC’s in general.

ESC not responding

Normally when you plug in the LiPo battery, the motors should beep. But if nothing happens, there are a few possible causes:

  • BLHeli firmware is corrupted – possibly from a failed firmware flash, you can try re-flashing BLHeli via C2 interface
  • The voltage regulator or MCU are fried (or the pins are shorted by solder blobs? If that’s the case these components will overheat and get much hotter than the good ESC)

Motor stuttering

The first thing you should check, is whether or not the motor screws are touching the windings, and make sure the motor wires are all connected to the ESC correctly and that all the solder joints are good. See my tutorial about how to solder.

If that’s not the issue, then it could be bad MOSFET.

Bad MOSFET

There are 3 phases in an ESC, each motor pad corresponds to a phase. If one of those phases is damaged, your motor will stutter (won’t spin up full speed but rather it just twitches).

Usually it has to do with bad MOSFET – the 8-pin surface mount chips on the board.

Damaged or faulty MOSFET is one of the most common causes for bad ESC. When it happens, the “drain” and “source” in the MOSFET are either shorted, or the MOSFET is simply dead.

MOSFET Pin-out; Left: normal MOSFET; Right: “metal” MOSFET

How to Check

Look for sign of physical damage on the MOSFET, such as burn mark and hole.

Damaged MOSFET (holes on them)

If they look okay, then you can check the continuity between power positive and motor solder pads, and again, check power negative and motor solder pads. Repeat for all 3 motor pads. If you are getting continuity in any of these tests, then your ESC is probably damaged.

A MOSFET has 3 types of terminals, gate, source and drain. MOSFET on ESC typically have 8 pins, of which 7 pins are either source or drains, and 1 pin is the gate (see pin-out diagram above). Conventionally source and drain are located on the opposite side, and the gate is normally at the corner.

How ESC MOSFET pins are connected to VBAT, GND and motor output

So base on that we can also just test the continuity of the opposite pins in the MOSFET’s, if they are shorted, the MOSFET is bad. But this is not possible with “metal MOSFET” as the gate and source are both covered inside the heatsink (which is the drain).

“Metal MOSFET” – the outside is the drain

Replacing Bad MOSFET?

If the ESC has a bad MCU or BEC, you can probably replace them, but it doesn’t always work with damaged MOSFET. Because MOSFET can get so hot the damage can occur in the copper layer level. You can still keep that ESC as spare part for repairing other ESC’s.

Anyway, with these tips you might be able to narrow down which MOSFET is broken.

You could probably save $30 by replacing just the MOSFET instead of buying a whole new ESC board. To replace it, you will need a hot air gun and good soldering skills. You’ll also need to know the exact MOSFET used in the ESC (to order replacement). Unfortunately for the most of us, your best option is probably to replace that ESC.

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2 comments

Peter 17th June 2021 - 1:04 pm

Another excellent source of information from Oscar about ESCs and how to determine if you’ve let out the ‘magic smoke’ or not.

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PowerThumb 7th June 2021 - 6:35 pm

Thank you so much for this article!!!

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