On Monday, December 28, 2015 at 4:17:44 PM UTC-6, Bob Engelhardt wrote:
I have a Milwaukee 12v cordless jig saw. It was working fine then
stopped, dead. Then started again, with no symptoms. It did this back
& forth a few times, then stayed dead. It never stopped while being
used - it just wouldn't start at all.

I took it apart to diagnose & as soon as I put it on the battery, it was
OK! I reassembled it and periodically tested it over a week. Always
OK. Today I put the base/shoe back on & it was dead!

As I took it apart again, I kept checking it, but it didn't come back.
And then, sitting there, it did come back, spontaneously.

It has a small PCB for speed control. The PCB also controls an LED
"work light", which comes on when the battery is connected. Except when
it's dead - making the LED a good go, no-go indicator. And showing that
the problem is more basic than speed-control circuitry.

The PCB has a number of wires soldered in through holes. I have checked
all of them. I have stressed the board with mild twisting; I've cleaned
it with carbon tet; and have warmed it with a hair dryer. Nothing had
any effect.

The board is SMT (with a 48 pin chip!), except for the wires, 1 cap, and
the output FETs, which are TH.

Is there anything else that I can try?

Thanks,
Bob

Might this be a battery issue? You see, a lot of these modern battery packs have a Battery Management System circuit board inside the battery pack, and its job is to properly charge the batteries and shut off the battery when it's drained. If the battery pack and the saw have to communicate with each other, maybe the BMS says it's time to shut off the battery, yet the battery is not yet dead.

You might be able to check by swapping in a different battery pack.

Good luck and Happy New Year.