Toughbook CF-74 won't power up

My CF-74 wasn't connected to mains power for a couple of months and the battery drained to zero. An overnight charge seems to have fully recharged it. The battery status LED near the carry handle is now green after having been orange / red while charging.
Pressing the power button results in nothing (no fan sound, no HDD whirring sound) except that both (ie above the keyboard and also near the carry handle) the green LED Power Indicators flash briefly at the rate of about once every 4 seconds. There is no difference if the battery is fitted or removed, or mains power (via the Panasonic brand 15.6V 8A) "brick" is connected or not.
I have removed the battery, disconnected the power supply and held the the power button down for about 1 minute - no change.
The CMOS(?) battery measures 3V with a DMM.
The HDD was connected to another PC using an external enclosure. It spun-up immediately and Windows Explorer can read it's files OK.
I have removed the HDD, DVD, RAM and PC card WiFi card. Still no joy. I have re-seated the single RAM stick & also tried it in the other slot. No joy.
When the power LED is flashing, if I hold down the power button for about 15 seconds, the power LED stops flashing, which may possibly mean the start circuit is attempting an initially start / stop.
Does anyone know what the power LED flashing every 4 seconds means?

 

Motherboard is confused. Need to try to reset it again.

 

Confused; yep, me too.
"Reset the motherboard" - would you please advise how to do that.

 

Try pull battery, disconnect power supply, remove cmos battery, then hold power button down for a minute or so.

reconnect cmos, plug in, DO not insert battery yet. restart machine, if it boots, go to bios, save changes reboot with battery

 

service manual
http://tim.id.au/laptops/panasonic/CF-74GCDADBM.pdf

reference manual
https://cpt.pc-dlp.panasonic.co.jp/itn/manual/cf74/74mk1-rm-pce0194b_xp-xp-nonlogo-M-p20061667.pdf

 

Thanks Shawn, I've done as you suggested at your reply #4 but still the same. The green Power Led is the only obvious activity, briefly flashing approx. every 4 seconds. I left the CMOS battery disconnected overnight, the power brick disconnected and without the main battery installed. Same, same.
I've read though both of the manuals you linked to (namy thanks), and unless I missed reading something, nothing jumped out at me.
Confusion reigns,

 

Had a cf 52 do this to me. The mb was confused. I messed around resetting it several times until it finally worked.
I thought for sure the mb was toast.
Try letting it set there and flash green for a while, like an hour if needed. See what happens. Nothing to lose at this point.

What I think happened to mine is I had it set for high temp which mean it only charges battery to 80%,
I recharged the dead battery with my external charger which goes to 100%. In my case that set the mb into panic mode as it thought the battery may overheat. So the flashing green is the mb safely discharging the battery some.
This is a bit of guesswork on my part. Your mileage may vary.

 

I see we are having issues attaching images again....ARRRGGGG

 

It rather sounds like the machine is in an aggravated state of suspend or hibernation.

What happens if you attempt to "start" with no ram installed?

I'd set the battery aside for a bit and just use power adapter. Do you have access to another battery?

 

Sheepman - I also suspected suspend / hibernation. The no RAM installed was unsuccessful, as was starting with no battery (ie power brick only)

Shawn - I'm glad to say that your suggestion was spot on and highly successful. I left the CF-74 turned on last night (battery only) with the power light briefly cycling at about every 4 seconds. This morning it was displaying the CMOS boot screen, saying "Select Boot Options". F2 initially didn't work - then I found that I'd dislodged the keyboard connector during dismantling to get at the CMOS battery connector. DOOH !!
Anyways, all is good now - Win 10 is happily running once again.
It does seem that the flashing green light was the mb discharging the battery. Perhaps when a battery is fully discharged as mine was, the full recharge heats the battery and stores that in a memory. The battery was certainly not physically hot on the morning following the overnight recharge.
Shawn, many, many thanks for your patience, persistence and advice. Much appreciated.

 

My Post (#10) mentioned CMOS boot Screen - of course I should have said BIOS Boot Screen,. DOOH x double

 

Groovy