bombarie:
I'm using an MCP4162 single-channel digital rheostat with this library: GitHub - dreamcat4/Mcp4261: Arduino library for the Microchip MCP4261It's a really simple controller, the plug has 2 contacts and inside the controller we find a linear potentiometer (10kohm). At 10kohm the car doesn't move and at 0ohm the car goes at max speed.
It seemed straight forward enough to introduce a digital potentiometer/rheostat to replace the slot car controller. I chose the MCP4162 (10kohm).
Use a multimeter. Once you're sure that you've connected everything up properly, and once you have set up the system to operate at some condition that generally represents the one that you will operate at, then measure the DC voltage on one end (terminal) of this rheostat, and then measure the DC voltage on the other end of the rheostat. Also measure the current through the rheostat.
Also, check to see if these levels are ok, and that they don't exceed the ones in the data sheet.