Need help diagnosing heatbed issue

The bed seems to have given up and I can't find the answer anywhere.

After I applied insulation to the bottom of my MK2 and ran it a few times it would not go above 90C. I thought I knackered it so bought a brand new MK2a but it made no difference. After reading online I though it might have been a bad polyfuse that can't reset itself, so I shortened it out, but again no cigar. I also noticed that the heatbed mosfet gets very hot (might be normal) so I stuck a heat sink on it anyway but I'm pretty sure 0 difference was made.

Don't know what else to check, please help.

RAMPS 1.4 - Marlin - PID is off

PSU 12.20V when bed off. 11.5V when on
V to bed 10.6V

Edited 1 time(s). Last edit at 12/24/2014 11:12AM by Asalt.

If your bed is standard Reprap the same as mine it has 3 pins:

1. +ve 12v
2. & 3. need to be connected together to -ve 12v otherwise only half the bed is used.

so bought a brand new MK2a but it made no difference.

When you say "made no difference", is the new one actually heating to ~90C like the previous one or nothing at all now?

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Waitaki 3D Printer

The new one will only just about get to 90C too

what is the gage of wire use to connect power to your bed. is there any way you can use two or 4 wires? do the wires get warm?

how about power to the mosfets, do you have more than 1 gnd and positive wire to the leads from the supply. it could be your current is limited by the wiring


do you have the bed with glass on top, and insulated on the bottom side?

Edited 1 time(s). Last edit at 12/26/2014 02:26AM by jamesdanielv.

I read this kind of post all the time and seldom see mention of a volt/ohm/ammeter being used to troubleshoot. I think everyone in this group needs to recognize these are essential tools needed right from the start of a build. Test the power supply for voltage. Test fuses, wiring for continuity. Test heat beds and nozzle heaters for resistance which gives power requirements and proves they work. Ring out wires to make sure they are connected correctly. Check motors for shorts, opens, resistance and polarity. Check switches, sensors, outputs. Run out and get a meter, we'll wait for some useful data to help solve your problem!