DME RELAY - Exterior

The DME relay is responsible for providing power to the DME computer for start up and operation, and also for supplying power to the fuel pump. It is also responsible for the erratic no start conditions that has made it famous. Typical symptons of failure are sporadic moments where the starter cranks but the engine does not start, while the next moment it starts right up and runs perfectly fine. This is a common problem on both Motronic controlled 911s and 944s. It is not a question of if your DME relay will fail, but when. On the 84 to 89 911s, the DME relay is located under the left front seat directly adjacent to the DME box. On the 90 and up 911s, it is mounted in the fuse box.


DME Relay - Interior

Contained within the relay are actually two relays which operate in sequence of the other. When starting the car, power is supplied to the main DME relay, powering the DME computer. As long as the DME sees the engine turning a minimum of 200 rpms, it switches on the fuel pump relay, powering the fuel pump and allowing the car to start and run. This is designed as a saftey precaution, shutting off the fuel pump in the event of an accident where the fuel system ruptured. Pictured below is the interior of a failing DME relay. This relay, installed as a replacement six years ago, has already begun failing. Note the heat induced discoloration of the red enamel wire coating of the main DME coil compared to those of the fuel pump. Shown in it's actual position mounted on the car's floorboard, note the horizontal mounting position of the heavy copper relays onto the vertically positioned circuit board. Heat, along with the vibration and shock that the heavy relays place on the support joints all contribute to solder joint failure.


DME Relay - Cracked Solder Joints

Examining the back of the board, we can see that most of the failed joints are due to overheated solder joints. In addition, we can see a minimum amount of solder at each joint, most likely for production cost savings. This leaves less material for heat dissapation and structural strength. A few are on their way to complete separation, while most are completely separated. Vibration, movement, and heat from thermal cycling all cause intermittent contact, resulting in sporadic instances of start and no start conditions. The simplest solution is to always carry a spare, resoldering and reflowing the joints in your old or new one with an adequate amount of solder, two to three times the original volume, providing better heat dissipation and virtually eliminating the likelihood of future failure. Click image for a closer view.