I am trying to adjust the trimmer pot on a Longruner A4988 motor driver using the Vref method but can't get the voltage higher than around 0.2 V. I'm trying to hook it up to a 1.7 A stepper motor with Rs of 0.1 so I've calculated that Vref should be 0.952 for 70% however I can get nowhere close to that number. When I turn the pot it goes back down to 0 V after reaching just over 0.2 V. Does anyone know why this is happening and how to fix it?
Can you move the potentiometer to its middle position (approximately) and then test the voltage relative to GND at both ends of the pot as well as at the wiper. Tell us what the three readings are.
It would also be useful to post an image of your driver and mark on the image the points to which you have connected your multimeter.
The Pololu A4988 web page has lots of useful info, including about setting the current limit - just be aware the Pololu drivers may have different current sense resistors than your module.
This Thread also covers the subject of setting the limit.
...R