I have a ramps 1.4 board it was working normally i left it for 2 months and started it back hotend heat up normally, but heated bed is not heating up (board heated bed led not lighting up) there is no output from the board.
What should i check first?

Check you have power connected to the 12v 11amp circuit, this is the power for the heated bed.

Quote
Dust
Check you have power connected to the 12v 11amp circuit, this is the power for the heated bed.

Yes there is 12v out of the fuse and the 12v reach The MOSFET

check the d8 io pin (not the mosfet output) with respect to a real ground (eg power supply gnd, but not -ve on mosfets)

make sure its on when trying to heat the bed

if D8 is on, and the mosfet is off, the mosfet has died.

Edited 1 time(s). Last edit at 01/12/2019 12:11PM by Dust.

Quote
Dust
check the d8 io pin (not the mosfet output) with respect to a real ground (eg power supply gnd, but not -ve on mosfets)

make sure its on when trying to heat the bed

if D8 is on, and the mosfet is off, the mosfet has died.

D8 is on 5v
But output to bed is 1.5v without load and when connected to bed it drops to 0.5 so i think it is a MOSFET

Quote
Dust
check the d8 io pin (not the mosfet output) with respect to a real ground (eg power supply gnd, but not -ve on mosfets)

make sure its on when trying to heat the bed

if D8 is on, and the mosfet is off, the mosfet has died.

Changed the MOSFET still the same all related resistors are ok
What should be the next step

Edited 1 time(s). Last edit at 01/14/2019 12:31PM by Yoko.

that it...

the circuit is just this

mega pin d8 -> mosfet gate -> d8 headted bed -v pin, turns the gnd on and off.
12v 11amp power -> 11amp polyfuse -> d8 heated bed +v pin

you say you see 5v at d8 pin... so the mega is fine
you say you have 12v on the output of the polyfuse, so thats fine
you say the d8 led is not on, this is on output so the -v pin is not being switched to gnd (or the led has died)
you say you have replaced the mosfet...


re check a few things

Unplug the bed, perhaps is a dead short. See if the d8 led now comes on.
Check d8 output +v is 12v with respect to gnd on the power supply
Check d8 output -v is 12v when off and 0v when on with respect to the gnd on power supply
Check your bed thermistor is reading room temperature, if not, software will not turn it on.
Make sure there is a connection from 11a gnd to 5a gnd on the power plug.
Check there is nothing in or under the ramps that shouldn't be there, paperclips, staples, nails, anything metal.
Take a picture of your wiring and show us, might be something obvious you didn't tell us.

Edited 1 time(s). Last edit at 01/14/2019 09:20PM by Dust.

I have removed the bed completely and the issue was the same
While measuring the voltages with the heated bed disconnected i found that the led some times lit up for just a second
I over tightned the screws for the connection and it worked!
I still don't know what is the root cause but it is working now