If the light comes on immediately before the wheels move, cleaning the sensors isn't the problem.
An open circuit in a sensor, a lack of power to the control unit, a defective alternator, pump relay problems, and a bad controller are some of the things that can set the light before driving.
If the light comes on at about 5mph and stays on then a sensor signal is likely the problem. Unfortunately your car is too old for self-diagnostics. We use the Bosch LED Tester to test these old systems. It attaches to the harness after the controller is disconnected. It tests in minutes what a good plan and a wiring diagram will take hours. Probably best left to someone with the tool.
Bosch Master, ASE Master, L1
33 years MB technician