Dewalt 20v DCST922 string trimmer stopped working

[Shug]

I like this tool up to when I was using it in heavy weeds and it bogged down and stopped,  I cleared the weeds from the head of the trimmer that were causing the issue and it restarted.  Then it stopped again same reason, cleared the weeds wrapped around the head then tried to start it, pulling the trigger resulted in the head clicking and slowly moving a small amount, tried again same thing happened. Changed the battery same thing. Removed the head, cleared a very small amount of dried grass from the dismantled head, re assembled and tried to start, no change it wouldn't start. Any help would be appreciated. 

Thanks.

[RDC]

If you pull the trigger and only get a tiny 'bump' from the motor, that's a pretty classic symptom of the motor driver board being bad. That can also show up as nothing happening when pulling the trigger.

 

I'd pretty much guarantee you it has some bad MOSFETs on the thing now, as I just tore one of those down here looking for that exact problem. It'll probably come as no surprise that none of that stuff is even remotely easy to replace. Even if you can get parts, you'd have to replace the entire field/driver assembly of the motor to avoild any soldering or real PITA PCB de-potting, unless you're far enough off your meds and rocker to replace the FETs like I'll be doing to this one.

 

 

 

 

 

[RDC]

OK, replaced the FETs (only the one damaged pair as the other 2 pair were still good) and replaced the 2 Capacitors that are missing in the last posts image, and she was still a dead duck. So I dug a little deeper and found an open Resistor in there as well, one of the 53B, 3.48k. Replaced that and she took right off. After I pull the PCB back off one last time to clean it up and recoat it then she'll be ready to wack some more weeds.

 

[Eric - TIA]

Sweet.  Nice to be able to fix it on your own.

[RDC]

Ran across another one of these that was supposedly 'dead', but after tossing a battery in there the thing technically worked. The motor sounded horrible and it had more grass in it than it probably ever cut, but the top motor bearing was shot, which was making all the noise. Fun part about this one, the marking on the top bearing was 695Z, but the 695Z bearing is 5 x 13 x 4mm, and the one in this thing is 5 x 16 x 4mm. So I had to toss a 625Z in there instead, which is 5 x 16 x 5mm, just 1mm wider, but there is plenty of room for that in there and now this one is good to go again.

 

 

[Tom W]

On 11/15/2022 at 5:38 PM, RDC said:

Ran across another one of these that was supposedly 'dead', but after tossing a battery in there the thing technically worked. The motor sounded horrible and it had more grass in it than it probably ever cut, but the top motor bearing was shot, which was making all the noise. Fun part about this one, the marking on the top bearing was 695Z, but the 695Z bearing is 5 x 13 x 4mm, and the one in this thing is 5 x 16 x 4mm. So I had to toss a 625Z in there instead, which is 5 x 16 x 5mm, just 1mm wider, but there is plenty of room for that in there and now this one is good to go again.

 

 

 

How did you get the black sealant off? Also what did you use to seal the board up again?

 

Thanks.

[sawihel]

On 8/1/2022 at 4:03 PM, Shug said:

I like this tool up to when I was using it in heavy weeds and it bogged down and stopped,  I cleared the weeds from the head of the trimmer that were causing the issue and it restarted.  Then it stopped again same reason, cleared the weeds wrapped around the head then tried to start it, pulling the trigger resulted in the head clicking and slowly moving a small amount, tried again same thing happened. Changed the battery same thing. Removed the head, cleared a very small amount of dried grass from the dismantled head, re assembled and tried to start, no change it wouldn't start. Any help would be appreciated. 

Thanks.

Oki C7350n [539/719] Memory (Additional)

Right in the middle of using it. I’ve used it lightly for about 13 months. It jammed up and would just “click” while other times it would turn “gravely” I took it apart thinking something broke or was getting jammed in the reduction gear. Nope… gears were fine. It was the motor. Black gritty magnet bits stuck to the outer magnets. I’d guess they are inside also causing the intermittent locking or grave feeling. Now, what to do? Test the Dewalt 3 year limited warranty or just buy a new bare tool for $99… (or both)

[Tom W]

On 2/18/2024 at 5:39 PM, sawihel said:

Right in the middle of using it. I’ve used it lightly for about 13 months. It jammed up and would just “click” while other times it would turn “gravely” I took it apart thinking something broke or was getting jammed in the reduction gear. Nope… gears were fine. It was the motor. Black gritty magnet bits stuck to the outer magnets. I’d guess they are inside also causing the intermittent locking or grave feeling. Now, what to do? Test the Dewalt 3 year limited warranty or just buy a new bare tool for $99… (or both)

Try the warranty first.