Starting in 1983 Porsche started using Bosch Motronic fuel injection on their production model cars. By 1984 this system was used on all models with the exception of the 928. Over the years it has proven to be one of the most dependable systems in the automotive world. However, onto every parade a little rain must fall, and even these systems can fail. When they do fail the first thing that you are going to hear is, “Replace the DME relay”.
This is not the shot in the dark that it may seem. The entire DME system (Digital Motor Electronics) runs on very low voltage and amperage signals. In order to control the high current load of the fuel pump a double relay is used. One side of the relay powers up the computer and the other powers up the fuel pump.
After several years of dependable service these relays can fail. Instead of merely replacing the relay we are going to determine the reason for the failure and redesign the part to virtually eliminate the risk of future failure.
After replacing several of these relays every year, I started wondering why they were failing. Being the curious individual that I am, I started disassembling all of the failed relays and found that about 95% of the failures were due to overheated solder joints in the relays’ printed circuit board. Upon further examination I found that the circuit boards had an absolute minimum of solder at each of the joints. A cost saving measure I am sure, but less solder means less heat dissipation and easier melting of the solder under high current load conditions.
The repair and redesign seemed quite simple. Just re-solder the board using an adequate volume of solder (two or tree times the original volume). The increased volume of solder will dissipate the heat better, virtually eliminating the likelihood of solder joint failure.
(See figure #1 – Before and after repair.)
The DME relays are all fairly easy to remove from the car and repair. The easiest relays to find and remove are ’90 and later 911’s as well as ’85 ˝ and later 944/968’s. They are located in the vehicles fuse box, under the front hood.
(See figure #2 – Relay marked with the colored circle is the DME relay.)
Next in the order of difficulty are ‘84-’89 911 Carreras’. Their DME relays are located under the drivers seat bolted to the floor between the DME and the DME altitude compensator.
(See Figure #3 – Computer/Relay/Alt. Compensator.)
The most difficult to remove are the early 944’s. While they are also in the fuse box, the fuse box happens to be stuffed under the dash leaving little room for my stubby little fingers.
After removal of the relay the black plastic cover can be easily pried off to gain access to the failed printed circuit board. After a quick re-solder and re-installation you will be back on the road. Not only will you have repaired your own relay; you will not have to worry about its failure in the future.
Good Luck
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