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Ian,
PWM_Max values of 100 (0x64) to 125 (0x7D) equates to a pwm range of 39~49%. I’m assuming needle just remained pegged at full? You could try higher values, but if you forced SOC to 20% (SW2), VR2 should allow adjustment of PWM over the full range up to 100%. With Q1 “on” 100% through the 68ohm resistor, it would be equivalent to 68 ohm from sender wire to earth. 95% duty would be closer to average current produced by 71Ohms. I would think the needle would move to empty, similar to the 71ohm resistor test you did.
Did you confirm PWM pulses on TB1 position 10 when connected to sender wire? If so, did duty change with VR2?
Does the attached look like your setup? It shows either LCD brd or your test trim pots as they would connect to sender wire going to instrument cluster.
The voltage drop across the 71~283 ohm indicates current in the 2~5mA range…definitely not driving a coil! I think this interface is current driven (similar to 4mA current loop).
If everything above checks out but still no needle movement from full, then try adding a large cap from TB1-10 to gnd (100~470uf) .
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