I purchased my TK40 in October of 2009 and have been using it for work (I am Law Enforcement and work exclusively on nights). About three weeks ago, I was working a DUI checkpoint when my light quit working. A co-worker also had a TK40 (I bought mine after seeing his – Flashlight envy!!) and I used his to determine that the problem with my light was the battery holder (We swapped parts to figure out what worked and what didn't). I ordered a new holder to the tune of just over $50.00. 
Then last night, my co-worker's light quit. We did the same thing as before, swapping parts to narrow down the problem. Again it was the battery holder. As he was trying the new parts, I started to fiddle with my non-working battery holder when I noticed the center post was loose. It spun as I messed with it. I found that by turning it (counter-clockwise with the holder being held upright and the contact terminals up) it tightened. I put the batteries back in it and re-assembled my light. Voila! It now works.:twothumbs I did the same with my co-worker's light and his is working again. I've since put a drop of loc-tite on the center post where it connects with the upper end. Hopefully this will keep it from coming undone.
Anyway, should your TK40 quit working, try this fix before calling the company and spending the cash for new parts.
Then last night, my co-worker's light quit. We did the same thing as before, swapping parts to narrow down the problem. Again it was the battery holder. As he was trying the new parts, I started to fiddle with my non-working battery holder when I noticed the center post was loose. It spun as I messed with it. I found that by turning it (counter-clockwise with the holder being held upright and the contact terminals up) it tightened. I put the batteries back in it and re-assembled my light. Voila! It now works.:twothumbs I did the same with my co-worker's light and his is working again. I've since put a drop of loc-tite on the center post where it connects with the upper end. Hopefully this will keep it from coming undone. Anyway, should your TK40 quit working, try this fix before calling the company and spending the cash for new parts.
