'Microprocessor fit' is applied to certain models of WAP-4 that have been retrofitted with a monitoring system that uses computerized circuitry to replace some of the electromechanical relays and switches that were part of the original design of these locos. Systems monitored by the microprocessor circuits include the DJ, silicon rectifier, battery chargers, etc. The advantage of this is that the status of the monitored equipment is displayed on an LCD panel in front of the driver and he does not have to look around and inspect the state of each relay or switch behind him manually. In such cases the microprocessor does not actually control anything ( so fitted and not controlled )