Preparing for a retrofit that has DC brushed servo axis drives. Resistance for motor chassis ground to motor power leads for one motor is 4.0 megohms, 0.63 M for the second and infinite for 3rd axis. All axes were tested for function and working fine prior to the test, but I'm pretty sure these readings should all be infinite. Is it possible there is some carbon dust inside from work brushes causing this? is it an issue that needs to be dealt with or can I ignore it? It is a used machine with some mileage on it but in good shape...