I Can't Get A Break

[printbus]

If you don't have a heat bed relay, I don't think you can move the bed heater to D9. D8 gets 12v power through an 11-amp polyfuse to protect the board (and your house) from a short in the heat bed circuit. This is the larger of the two yellowish postage stamp looking things. D9 gets 12v power through the smaller 1/2-amp polyfuse. You'll open the small polyfuse and shut down your printer quickly if you try to feed the heater directly off D9. Ignoring that problem, there's another. The MOSFET switch for D8 has a heatsink on it. The MOSFET switch for D9 does not. You'd likely overheat the D9 switch by drawing heat bed current through it.

FWIW, when polyfuses are subjected to overcurrent, they go into a high resistance state acting somewhat like a fuse. When the overcurrent goes away, the polyfuse resistance will restore to normal. So, I somewhat disagree with Colin. Since RAMPS has an 11-amp polyfuse, the board layout should be designed to handle at least that without any issue. I don't know if it was or not, but this makes me lean towards thinking the problem was in the wiring to the board.

If you do have a heat bed relay (or do as some have and wire in an automobile 12V relay to use for one), you should be able to swap functionality of the D8 and D9 connectors by swapping the values for the FAN_PIN and HEAT_BED_PIN in pins.h. Double check your pins.h file. For motherboard type 33, HEAT_BED_PIN should normally be defined as 8, hence the heat bed connection to the D8 port.

When a heat bed relay is used, only minimal current related to the bed heater is flowing through the RAMPS board. Also note that when you use a heat bed relay, the on-board polyfuse is no longer in the loop so it can't protect against overcurrent anymore.

FINAL EDIT? The reason why a heat bed relay can heat up the bed faster is that it is more efficient at getting power to the heater. It basically uses a mechanical switch connection that will have less voltage drop than the on-resistance of the RAMPS MOSFET switch. The RAMPS board also has voltage drop in the "on" resistance of the polyfuse. There may also be more voltage drop from traces on the RAMPS board and the push-on connectors that are generally known to be somewhat imperfect. These voltage drops equate to power that never makes it to the heat bed.