Extruder problems

I have calibrated all my motors fine.

Now I am trying to test my extruder by clicking on the switch heat on button in the RepSnapper and in the RepRap console. But there is no heat in the hot end.
What could be causing this?
My thoughts are there is a lose connection in the hot end or thermistor. But when I put my multi-meter across the pins for the hot end there is no voltage. Is this some thing wrong with the firmware or the circuit board it self?
If the thermistor was disconnected, would the firmware assume it was up to temperature and not supply a voltage to the hot end?

I just did a continuity test on the hot end connection it showed 010 (connected (I think)). Unconnected to anything it shows just a 1 but to the left of the screen. I put the ends of the thermistor on the multi-meter and it showed no connection. But this could be because the thermistor is like 100K so there is not a very strong connection.
My multi-meter cost me £2 off ebay. So I don't trust it completely. Although it does show 12V when I put it to my supply.

Is there a way of testing the hot end not through the firmware? Can I just connect the wires straight up to my 12V supply? Or is 12V too much? I don't want to blow anything.

Thanks

Which electronics are you using? Gen3, RAMPS, Gen6, Gen7, something else? That might help clue people into where you should start probing.

As some generic advice, when the micro wants to turn on the heater for the extruder, it will assert one of it's IO pins which in turn is connected to a power transistor and turns on the heater. I'd suggest looking through the schematic for your hardware and find that IO line that controls the power transistor for the heater and use your MM to see if there is any voltage on that line when you enable the heater. That will at least tell you if the micro is trying to switch on the heater.

If there is a voltage reading there, the problem lies between the power transistor and the extruder heater element.

If there is no voltage there, then the micro probably thinks that the extruder is up to temp. In which case the problem lies with the thermistor circuit. Probe the output from that circuit (where it is connected to the micro) and check to see what the voltage is there. Then see if you can compare that with your firmwares temperature table to see if you can figure out what temp reading the micro is calculating.

JB

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I'm using Sanguinololu 1.2 and thanks for your suggestion I will have a look for the schematic tomorrow.