Makita 9169 9" grinder keeps blowing fuse

[mp10013]

hi guys,
this forum sounds quite informative and i'm going to try answer some stuff on the car problems, being a mech engineer.
my makita 9" grinder keeps blowing the fuse, it starts momentarily then looses all power due to blown fuse. I've checked continuity between plug and switch, all wires ok with a new fuse. the switches function seems to be ok to (continuity appears between relevant wires upon sqeezing switch).
any ideas on what it might be or how to test the windings/stator?

im out of a job at the moment so was hoping to fix it myself if possible.

Thanks

 

[mp10013]

oops forgot to mention brushes look ok too, all well above the wear line.

 

[wotan]

I have a mitre saw that is doing exactly the same thing, I examined the motor and found the fault to be with the rotor, dead short across the windings, blowing the fuse.

Wotan

 

[mp10013]

thanks.
how do i test for a dead short? i have only tested for continuity but thinking about it this test isnt good enogh, wires could still touch *wrongly) but i might still get continuity.
im 25 and not too good on electrical, learning though.

thankyou

 

[regsmyth]

Sometimes the wires can break inside the sheathing where the cord comes out of the handle. You can't always tell unless you strip back a few inches, but sometimes you can see a bit of a kink in the cord.

 

[mp10013]

already stripped back the other day and it was ok continuity came up good to.
just need to understand how to identify a dead short in the rotor and stator

 

[R5GTT]

already stripped back the other day and it was ok continuity came up good to.
just need to understand how to identify a dead short in the rotor and stator

Easiest thing to do is take it to a repair shop and ask them to flash test it for you. This will show if there is an insulation problem with either the armature or field coil. If it fails a flash test then one or both of the above will need replaced - probably at more cost than a new grinder. A flash test is a 2 minute job so I'd expect they'd either do it for free or chagre a nominal £5 or so. If they try to hit you with a charge of £20 or so walk out the door.

 

[mp10013]

thanks for the good advice, appreciated. Will try and get it flash tested asap.

makes sense rotor or armature as got the thing jammed (lesson learnt) probably loaded it up. it only locked up for a second though, probably less, before i let the switch off.