Issue With FourSevens MMU-X3R

Hey there. I use my light on patrol as a police officer. Tonight after I finished a traffic stop I realized my light was inside its pouch. When I pulled it out to turn it off I realized it was very dim. I have it programmed to HI / LO so I double clicked and it stayed at LO brightness. Thinking it was an issue of the battery needing charged I charged the battery until my indicator light was green.

I again tried toggling between HI and LO but was only getting LO brightness. I tried the other programming modes with the same result, even when it strobes it does so at LO brightness. I changed my battery out and it behaves the same way with my spare battery. I also tested both batteries and am getting 4.1 volts from the full charged one and 3.5 volts our of the partially charged.

It was not dropped at all recently and was working for the first part of my shift. Both sides of the battery tube things look to be in pristine condition. Any idea on what I might have done to my light? Thanks in advance for any help.

Most probably your cells are shot and you need to replace them. HI is a hefty amount of current and they sound like they just can’t handle it.

I recently pulled/tested some little used, few years old Efest 26650s that should have both been good back in January and they’re toast, happening together.


My spare cell only has one charge cycle on it. I only had it in the light to fully charge it and then it’s sat in my bag as a spare. Any reccomendations in a good 26650 that I can snag to test this out?

Who knows buddy? They could have been sitting unused for 2-3 years and you’re not telling us what cells they are, or what cells FourSevens was even using, if they came with the light.

Efest would probably be easier to source, but these test out well if you can find them here in the States:

Enercig 26650

The key is to just find a new cell, either from online, or a Vape store and see what you see, ruling things, the least expensive way, first.


Well i have a fourseven mmu-x3. It has been sitting on the shelf for like a year and the light works fine. Maybe try cleaning the thread? Or switch the 26650.

I just double checked to be sure and the threads are all clean. I have two 26650 cells for this light. One was purchased with the light and the second I purchased when FourSevens was bought out a few months back. That second cell has never really been used other than to charges one time. Other than that it’s sat in the bottom of my bag.

The chances of two cells dying on the same day seem very remote, to me. There very well may be more going on than a battery issue, but they are recommending the cheapest attempted fixes first. It may take a couple days for people to respond that have had this issue. This forum is full of helpful people and they will most likely figure out what’s wrong.

I believe that light can be programmed for only one mode. Is it possible that it accidentally got re programmed for just 1 mode that happens to be low. Guess I would try making sure the light was programmed for only the modes you need. I don’t have one, just making a guess by reading the specs of the light off a website.

So there are five programmable modes. In order to program the light it needs to be plugged into USB power (which it was not between the time it was working and then stopped). I did check the programs to make sure it hadn’t somehow reprogrammed itself. No matter which program I set the light to it only shines at LO power. Even in the strobe modes it is only on at the LO brightness.

If it is not anything trivial, it looks like you already checked the easy causes, what is leftover is a faulty driver, maybe some component failed in the high mode circuitry. Unfortunately Foursevens is out of business so getting it fixed under warranty may be difficult. What is then leftover is try to get to the driver and look for damaged components, you may just be lucky that the damage is visible.

Well that’s a bummer to hear. I for sure don’t know enough about circuitry to even know what a bad part would look like let alone how to access that without permanently breaking the light. Thanks for at least having a look at the thread.

Perhaps someone who also owns this flashlight has tips? Btw, it looks like a great flashlight, a shame that it stopped working.

Where in the US are you? Maybe someone here that lives nearby, with a good charger and or a different battery could help test it for you. I would if you were anywhere near Midland Michigan.

A healthy cell charged on a known quality charger, is charging up to ~4.20v, not 4.10v. That tells me that a) your charger might be suspect, but probably not because you’ve used it in the past with ‘seemingly good’ results, b) your battery in your volt meter needs replacing, or c) your cells are tired.

The fact that your 3.6v cell wouldn’t work on high, isn’t all that surprising.

But I don’t know that light, or your cells, so I’m just guessing.


Southeast Wisconsin.