Debugging RAMPS for heated bed problem (1 reply)

Hi,

Earlier this week I started using my printer but the heated bed refused to work. I googled for hints what to try but couldn't figure out the cause of the problem. Except things to measure from RAMPS (1.4) and they are:
- Voltage on PSU pins: +12V (OK)
- Voltage between PSU - and D8 pins when heated bed is turned off in Pronterface: +12V (OK?)
- The same as above but heated bed turned on in Pronterface: 0V on both D8 pins (no voltage between D8 pins --> no power to heated bed)
- Resistance of the heated bed when detached from RAMPS 1.1 Ohm (OK)
- Voltage between MOSFET pins and PSU-: 0, 0, 5V (not sure if I got it measured properly)
- Fuses seem to be ok (measured resistance close to 0 Ohms)

And the LED on RAMPS doesn̈́'t turn on when heated bed is attached and turned on from Pronterface. But when the heated bed is detached and turned on in Pronteface the LED switches on (and there is voltage between D8 + and -).

Any suggestions what's wrong? The notorious MOSFET? Or the heated bed? The printer was working ok for a couple of years without problems before this happened.

Thanks in advance.