How to Test Bad ESC (Motor Stuttering and Not Spinning Smoothly)

Motor stuttering and refusing to rotate is often related to the ESC. It’s one of the more ambiguous issues that plague FPV drones. Both software and hardware problems can lead to this issue, so there are a few things to troubleshoot. Unless you’re exceptionally tech-savvy, fixing a completely dead ESC is usually impractical. However, it’s still useful to learn how to test and troubleshoot common ESC issues to identify damaged components.

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Before starting, I recommend checking out my beginners’s guide about ESC’s. It covers all essentials, including ESC firmware, protocols, technology, and hardware anatomy, to give you a deeper understanding of ESCs.

ESC Not Responding at All

Typically, when you connect the LiPo battery, the motors should beep. If nothing happens, possible causes include:

• Corrupted ESC Firmware: Possibly from a failed firmware flash. If your ESC uses Bluejay or BLHeli_S firmware, you can try
• re-flashing BLHeli via C2 interface to revive it.
• Damaged Voltage Regulator or MCU: Check if these components on the ESC or their pins have been shorted by solder blobs, causing them to overheat significantly compared to a healthy ESC. If this occurs, the ESC likely needs replacing.

Motor Stuttering Troubleshooting

Troubleshooting steps:

• Motor Idle Value: If the motors spin fine in the motors tab using sliders, but stutters when you arm the drone, it’s likely to be motor idle value too low. Set back at the default (5.5%) in Betaflight’s Motors tab and test again.
• Check for Bridged Motor Pads: Ensure none of the ESC’s motor solder pads are bridged. See my soldering guide for tips: https://oscarliang.com/soldering-guide/
• Inspect Motor Screws: Ensure screws aren’t too long, protruding through the motor base and touching windings, causing shorts between motor poles. Check here: https://oscarliang.com/check-motor-screws-touching-winding/.
• Bluejay ESC Settings: If your drone is a tiny whoop with Bluejay ESC, adjust the min/max startup power as detailed in this guide
• : https://oscarliang.com/fix-tiny-whoop-motors-not-spin/

If these steps don’t resolve the issue, you might have a bad MOSFET.

ESC MOSFET Explained

An ESC has three phases, each corresponding to a motor pad. Damage to any of these phases can result in motor stuttering (the motor won’t reach full speed and instead just twitches).

Typically, the issue involves a faulty MOSFET, the 8-pin surface-mount chips on the ESC. A damaged MOSFET may have its “drain” and “source” shorted, or it might be entirely dead.

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

How to Check for a Bad MOSFET

First thing to do is to look for burn marks or holes in the MOSFETs. If they look fine physically, proceed to testing with a multimeter.

Damaged MOSFET (holes on them)

Test continuity between power positive and motor solder pads, then between power negative and motor solder pads. Repeat for all 3 motor pads. If you are getting continuity in any of these tests, it’s an indication of an ESC with damaged MOSFET.

Pinpoint Damaged MOSFET

To find out which MOSFET is damaged you first need to understand the pinout of a a MOSFET chip.

MOSFETs have 3 terminals: gate, source, and drain. Typical ESC MOSFETs have 8 pins: 7 pins are either sources or drains, and 1 pin is the gate (usually located at a corner).

Source and drain are usually located opposite each other. You can test continuity between opposite pins; continuity indicates a damaged MOSFET.

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

Note that “metal MOSFETs” have gates and sources internally covered, making direct testing challenging.

“Metal MOSFET” – the outside is the drain

Replacing Bad MOSFETs

Replacing the MCU or BEC is typically feasible, but replacing MOSFETs doesn’t always work because MOSFET can get so hot the damage can occur in the copper layer level. If you can’t repair it, you can still keep the ESC as spare parts for future repairs.

Using these tips, you can pinpoint the exact damaged MOSFET, potentially saving over $30 by replacing only the MOSFET rather than the entire ESC board. However, repairing an ESC requires professional equipment, high level soldering skills, and knowing the exact MOSFET model. For most FPV pilots, the easiest solution is to buy a new ESC.

Conclusion

Troubleshooting and testing your ESC when experiencing motor stuttering or non-responsive motors can save you time and money. It’s not always possible to repair an ESC, but understanding these diagnostic steps gives you a solid foundation for addressing common ESC problems and keeping your FPV drone flying reliably. Happy flying!