Among all the other things going on currently, two of my stock lights are acting a fool.
Battery is dropped in, tailcap tightened down, light switch on… bleh, nothing, or a slight flicker and eventually nothing.
Loosening the tailcap while still on as it reaches about 30% unthreaded the light seems to stabilize and modes can be switched, but any movement of the light will cause intermittent disconnects. Loosen a bit further tailcap ~60% unthreaded and it functions perfectly even with movement.
Have you tried the paper clip trick? Remove tailcap and bridge from neg battery terminal to flashlight body with a paperclip. Light should work if it’s just the switch.
Okay I rechecked everything; with the 26650 DST tube the driver spring is very weak and too much pressure from the battery causes it to duck below the plastic spacer. Should be an easy fix.
The C8 on the other hand, I am genuinely confused. Can a faulty switch make the light variably dim randomly, flicker, and switch modes randomly?
Seems to be the week of flashlight issues, I have a 2x 18650 light that flat out wont function with the stock switch. Bridging turns it on just fine though.
What’s happening with your c8 sounds very similar to when I messed up on soldering a 7135
Hmm. Well it’s fully stock; I’ve probably got somewhere around 3-4 hours of use no drops or anything like that. I haven’t even removed the driver retaining ring yet (because it’s stuck with a ridiculous amount of fujik in the threads).
I guess I’ll see if I can’t rip that sucker off of there and investigate. It has a very annoying pwm whine on med/low so if the driver is the issue, great. 
I had no whine just mode shifting and dimmed output
My light malls Ultrafire C8 Xml-u3s I had 2 out of 12 do that on one of them the pill was loose and flicker and eventually turn off and on the second one it burnt the driver and emitter so on that one I replaced Nanjg 105c driver and a XML2-U21A on a copper sink.
If I tighten the tailcap, the light will not turn on. But if the tailcap is loosened some, it would turn on. Made a loop/ring out of paper clip and drop in the tailcap touching the bottom side. Now the light turns on with tailcap tightened.
I've seen this before with bad connections in the tailcap/switch area or where the cell is too long.
Interesting… what is the issue that the paper clip solves?
I threw an oring into the tailcap, resting on top of the switch retaining insert and it works fine now… :~
Is the o-ring metal or rubber?
My problem was, seemingly out of blue, the flashlight does not come on after I changed battery and tightened the tailcap. Examined the tailcap to see if anything was loose. It was not. Puzzled put battery in, tightened the tailcap and press the switch. No light. So I slowly loosened the tailcap and the light turned on. I could make the light on/off by rocking/wiggling the tailcap. Made a ring with aluminum foil and put it in the tailcap bottom. Light turned on reliably without needing rocking as soon as tailcap made contact with the body. Replaced the aluminum ring with paper clip ring. Some contact must have been somehow lost inside tailcap, but the loop/ring restored the continuity of the contact. Still don’t know what the problem cause is.
It’s rubber, I’m so confused.
I understand the need for continuity to connect the ground from emitter to driver to pill to flashlight body to the switch and finally to the negative on the battery…
I just don’t get why what I did fixes my problem. I’m baffled.
What you’re experiencing is pretty common with all the various battery lengths. I get that all the time. Try your shortest unprotected batteries.
I usually remove that brass cap that covers the tail spring and throw it out. If it has one of those real long tubular springs, cut it down. Actually those are real crappy springs, and best to replace entirely.
Anyway, give it a try. Sometimes it’s the simple solution that best.
Those brass caps don’t recess all the way and keep the tail from completing its circuit, especially with longer cells. They also can dent the hell out of your batteries.
If it’s rubber, then there goes the theory about electrical continuity angle.
And this could not be the battery length issue because the o-ring does not touch the tailcap center brass “nipple”. Maybe someday we will see the “light”.
Pressing the brass cap (and the spring inside) too much must be causing disconnection.
(See the next post .)
Think your theory on the brass cap and the spring is the answer. Both ’unknown00101’s o-ring and my paper clip loop do is not pressing down the brass cap just a hair and maintain continuity. Just a little more pressing must cause the spring inside the brass cap somehow to disconnect the circuit.