When the ignition is turned on the DME portion of the relay is engaged. This provides power to the car’s DME which then waits to sense engine rotation. When the DME sees that the engine is rotating it then sends the signal to engage the fuel pump portion of the relay. This provides power to the fuel pump and in turn fuel for the car to start.

This creates a significant safety feature in that if the DME ever sees engine rotation slow below 200 RPM it will stop sending the signal required by the DME relay to run the fuel pump. This will prevent excess fuel from being pumped out of a broken fuel line in the event of an accident.

Now that we know how the DME relay is supposed to work and what it does let’s take a look at some of the problems you will see associated with a DME relay failure.

DME Relay Troubleshooting

The symptoms of a faulty or failing DME relay may present in several different ways. The most common symptom is for the car not to restart after recently being shut down. When the car is allowed to cool off it will then start again as normal.

Another possibility is for the car to shut down unexpectedly while driving. The car will not want to start again immediately but will most likely restart as normal after being given some time to cool off.

In the event of a complete failure of the DME relay, the car may not start at all regardless of temperature or condition.

Since the DME relay is critical to the cars operation it should be the first component checked in the event of any no start condition. The easiest way to check for a faulty DME relay is to simply replace it with a known good unit. A spare DME relay is a must-have item for any Porsche 944 owner’s glovebox.

Jumping the DME Relay 

For testing purposes only, if a known good DME relay is not available it is possible to use jumpers to bypass the relay’s operation. Here you can see a diagram showing the pins on the bottom of the DME relay and relay socket.

Different DME relays may have one of two different numbering systems. Both are provided here for your convenience.

To completely bypass DME relay you will need to jump pin 3 (86) “ignition switch power” to pins 8 (87) “DME power” and 4 (87b) “fuel pump power.” You can do this directly on the socket by using a set of jumper wires or by connecting the pins on the DME relay itself before plugging it in.

Bypassing the DME relay completely should be done only for testing purposes. It not only puts a large current draw on the car’s ignition switch but also creates a significant safety hazard in the event of an accident.