DME

The DME, or Digital Motor Electronics Relay is the main power switch for the ECU, and therefore controls the entire Engine Management System. Without it, there would be no power to the Fuel system or ABS (if fitted).

Overview

The DME relay is a standard five-pin relay that switches power to engine-critical components such as the fuel pump and injectors when the engine is running. It will be placed either on the inner wing of the engine bay (M20 engines) or on the bulkhead (M40/M42 engines).It is a small relay mounted in its own holder alongside the Fuel pump relay and the Lambda heater relay (if fitted).

This allows parts of the ECU to remain powered at all times, while shutting off power to other critical systems when not in use.

Operation

Two pins of the DME relay, 30 and 86, are connected directly to the Battery through an unfused red wire and are therefore hot (constant 12V) at all times.

Pin 85 is connected to the ECU; usually pin 36. When the ignition is turned on, the ECU receives 12V at pin 26. It will then switch pin 36 to ground which will activate the DME relay.

When the relay activates, it will connect pin 30 to both pins 87, supplying live power to the Fuel pump relay, the injectors, the ABS if fitted, and back to the ECU at pin 37.

Testing

Bridge the DME relay by connecting pin 30 to both pins 87. This is to see if power to both the ECU and the Fuel Pump relay has been lost. If the car starts once bridged, it confirms battery power is getting to Pin 30, and that both 87 circuits are working (power to ECU and Fuel pump relay). It doesn't test the circuit that latches the DME relay, which therefore needs to be tested before condemning the relay.

That circuit consists of Ignition supply to the ECU, which latches and sends a ground signal to pin 85 of the DME relay. If pin 86 has a positive supply, the relay should latch. If it doesn't, then the relay is faulty and needs to be replaced.