Drill has lost full speed. Faulty trigger unit?

[uart]

I've got an older 24V nicad drill. It's a pretty cheap **** GMC brand, but I've kept it because it's got a 13mm chuck (my newer drills are only 3/8 or 10mm).

Yesterday however it up an died on me. Well not totally dead, it runs as long as you don't try to go full speed. When you depress the trigger it speeds up as normal, until the trigger is fully depressed and then it just loses all power and cuts out.

I'm guessing it's the trigger unit. Just wondering if anyone else has had a similar experience with a failure like this. I always thought they would go out completely when they failed, but this is like a half failure. The variable speed works ok but full speed is totally dead.

Just guessing here, but I think maybe the trigger has contacts that bridge out the variable speed circuitry to provide full power when the trigger is fully depressed, and those contacts must have fired or something. The only thing that seems weird about that is that I would have expected the thing to still work in variable speed mode (without full power) with the trigger fully depressed, but it actually cuts out the power completely. Weird?

Very annoying as I just rebuilt the battery pack on this one and was hoping to at least get a bit more use out of it. Oh well, that's 20 nicad subc's I can probably find a use for somewhere down the line.

 

[pauls_workshop]

Hmmm. Wearing out brushes maybe? Could be a bad resister in the trigger unit too. - Paul

 

[uart]

Hmmm. Wearing out brushes maybe? Could be a bad resister in the trigger unit too. - Paul

Hi Paul. When it first happened the brushes were my first though too. After examining it for some time however it's just too repeatable to be brushes. I mean it goes limp every single time at exactly the same point when the trigger is fully depressed. It doesn't matter if it's under load, doesn't matter if it's no load, it always loses power at the same point every time.

I suppose it's possible that the pot (resistor) could just go open circuit at the point or something.

Has anyone ever heard of my hunch about contacts bridging out the variable speed part and providing power directly to the motor when the trigger is fully depressed? I was only guessing at this, but I know that on a lot of low end drills you really get a noticeable jump in power right at the end of the trigger speed range. I mean it's like they (cheap ones anyway) go smoothly to about 70% power and then just slam straight from 70 to 100%. I may be wrong, but it always made me suspect that the variable speed part got fully bypasses at full trigger.

 

[ravedave]

Hi,
I've fixed a few....
They usually have the variable speed circuit and, just like you thought, close some contacts for full voltage/current from the battery for full speed and power.
The contacts are most likely dirty. They are usually pretty east to take the trigger assembly apart.
Good luck!

Dave

 

[uart]

Hi,
I've fixed a few....
They usually have the variable speed circuit and, just like you thought, close some contacts for full voltage/current from the battery for full speed and power.
The contacts are most likely dirty. They are usually pretty east to take the trigger assembly apart.
Good luck!

Dave

Thanks for the info Dave. I did take a quick look yesterday and I can get access to the trigger unit easy enough. It didn't look like it comes apart but maybe it could be pried open.

Do you need to disconnect the wires or anything to work on them? I couldn't even see how to unclip the wires.

Anyway thanks you've given me some hope. The trigger unit is black plastic and looks like maybe it just all clips together. Does that sound anything like what you've worked on?

TBH I was thinking about trying to pull it (the trigger) apart yesterday, but then I imagined contacts and springs etc flying everywhere and chickened out.

 

[ravedave]

Yeah, most of the triggers are just clipped together with plastic tabs. The wires usually have a release you can trigger with a small nail etc.
Good luck, take pictures of how thing came apart,
Dave

 

[pauls_workshop]

When you pull it apart, clean with some Caig DeOxit Gold spray, electrical contact cleaner. I use it on all my stereo contacts and connections. If your old receiver has a scratchy volume knob, you spray this in behind the knob, turn it around 10 times, and that scratchiness generally goes away. Dirt could be acting like a bad resistor, as it would increase resistance significantly! - Paul