E6540 won't boot

I had a similar problem with my E6530.  To troubleshoot, I started removing components one at a time.  I pulled out one of the two memory modules and it still wouldn't start.  Swapped memory modules and still no go.  Pulled out the wifi card, no dice.  Removed the hard drive, nothing.  Pulled the optical drive...Bingo!  Fired right up.  The optical drive, which I had maybe used a couple of times had a dead short causing the laptop to fail to start.  I found a used optical drive on ebay and the machine is good to go.

-Ray

I tried the hard drive and optical drive. No help with either of those. I just moved overseas and don't have much for tools with me, so opening the case is not much of an option at the moment.

You'll need to go inside the system -- with the battery out, hold the power button for 30 sec.  Then remove and replace the memory modules and the drive.  See if it'll then POST.

If it still won't, try one memory module at a time -- if one module halts POST in both sockets, replace it.  If the system will POST with one module in a socket but not with one in the other, you have a bad socket.  See below.

If the system will POST without the drive but not with it, the drive is bad.

Anything other than a bad memory module or drive likely means a replacement mainboard. However, before committing to a repair, realize the system is worth between $50 and $100 US complete and working -- so it's not worth the cost of a major repair such as a mainboard replacement.  You can likely buy a complete used system a few years newer for about the same money -- perhaps even less.

I'm aware of the value (or lack thereof)... but, it's difficult to migrate from a SATA SSD to mSATA, and I have a lot of image editing software and various presets and lens and body profiles on that machine (as well as many other projects). There's also the docking station compatibility factor. 

Frankly, if migration wasn't nearly impossible, I'd happily switch over to my XPS 7450. 

If so, and the drive is OK,  your best option is to purchase a similar E6540 system and use it to make a backup image of the drive.  Keep that in a safe place, and you then have the least costly way to continue using the system.  

If the drive is the issue, you'll have to decide whether data recovery is worth the cost -- it's not inconceivable that a drive that's original to a system of 11-12 years of age is on its way out.

The drive is far from original. It's a 1TB SSD. 

I got some screwdrivers and opened it up. Removing either DIMM doesn't change the behavior. Removing both results in HDD on, battery and WiFi blinking. 

Actually, there are two codes... neither of which show up here (https://www.dell.com/support/manuals/en-us/latitude-e6540-laptop/late6540_om/device-status-lights?guid=guid-2bd338a4-db1f-492f-a464-d6d4c8643e7a&lang=en-us): one is blinking, solid, off, and the other is blinking, off, off. In both cases, the blinking is a much shorter on flash than that seen when the memory modules are removed.

OK. Well, after one DOA motherboard, and a second replacement motherboard that is (probably) good, I am back to the same problem. My guess at this point is that the backlight power supply is probably toast.

Edit: I tried disconnecting the display connector, and the behavior is still the same. So, unless it does this when it can't sense the display, I think maybe it's not the display.

Any other observations on how an E6540 acts when it has a bad fan speed sensor, or a bad temp sensor in the heat sink (if the heat sink has one), or a bad CPU (I know CPUs pretty rarely go bad, but... )?

A couple of other observations, for what they're worth:

1. If I remove the battery, then the unit will stay on, from the power cord... but, still, there's no boot. Screen stays dark, hard drive light flashes briefly, once, every seven seconds, and it just stays like that.

2. With the battery in, the unit turns itself off a couple of seconds after you turn it on. It flashes the hard drive light once, for a longer duration, but still pretty brief, just when it shuts down. Then the battery charge light will come on for about 10 seconds and then shut off. If you hold the power button, the battery light will eventually come on, and will stay lit for as long you hold it, and a few seconds after.

3. If I turn it on with the battery out, as per 1, then install the battery, it will shut off a few seconds later.

4. I removed the WLAN card, in case that was causing problems, but it didn't make a difference.

Just to try something else, I bought a new power adapter. It made no difference.

Next thing I checked was attaching the battery to one of the other motherboards so I could measure it. It's well and truly dead. No voltage from it at all. But the laptop should be able to boot with no battery. Could a bad battery have killed two (or three) motherboards?

OK. Basically the last thing to try. Getting a new battery to see if that changes anything.

New battery makes no difference. But, removing the CPU, does result in a different behavior. With no CPU, all three lights (wireless, battery, and HDD) light up, and the HDD starts flashing, as would be expected with a CPU failure. I wonder if the CPU is shorted and actually pulling down the power supply. 

It could be, but it could equally be a power problem on the board.  It's up to you how many more parts  you want to try, but a complete, working E series system may wind up less expensive than endless parts-cannon purchases.