Z Limit Switch Issue

I have a weird issue with Repetier .80. It appears to be read the Z limit as triggered whenever the first extruder heater is on. I am using the Azteeg X3 which wires exactly like a RAMPS.

If I home on Z with the extruder off it works fine. If the heater is on, homing behaves as if the limits switch is already triggered. It moves a tiny bit down then moves up when the switch is not physically triggered.

Marlin works fine in all modes, so I know the wiring is fine.

You Z-Min pin is 18:

#define Z_MIN_PIN          18

if you are sure you have no cross talk (Repetier uses no PWM and has a different frequency for the extruder, so it may be different to Marlin) you must be using that pin for some other purpose as well, also pins.h defines no other function on that pin.

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I'll try just jumpering the pins to see if the controller is reading that as open with the extruder on.