I'm no auto electrician but the speed sensing module has a 12v badge on it. Now I'm sure the way a modern car takes it speed readings from the abs sensors and it reads resistance in ohms as I'm sure that's how you check if an abs sensor is faulty. But anyway this isn't helping you out but maybe your speed sensor will work in the same way and although it has a 12v supply maybe the speed is determined by resistance if that makes any sense as I said I'm no expert and may be wrong on all accounts.