Solid State DME Relay (SSR)

It is well known that the DME relay is among the top reasons for no-start and intermittent-start issues. Many of us keep a spare relay in the glove box “Just in case”. The most common failure is a cracked solder joint, which can be repaired if you’re handy with a soldering iron. But that is not the only way these relays fail. Burned coils, burned contacts, bad diode and worn armature are also common. In the best case, fixing one problem is only a stall for a failure down the road. And that is the fundamental problem with all mechanical relays, eventually they will fail. Personally, the number of times I am willing to endure my Porsche not starting due to a DME relay failure is exactly zero times. That is why I designed the first direct replacement Solid-State DME Relay (SSR). The difference that makes Solid-State relays superior is that no fundamental limit exists for how many times it can switch a load on and off.  Whereas mechanical relays will wear slightly each and every time it cycles. Even worse, the life of a mechanical relay is dramatically reduced as the load increases. This is because the tiny contacts cannot withstand the plasma arching that, for example, often results from “hot switching” a failing/seized fuel pump. As contacts wear, its closed resistance increases which results in the entire relay heating up. The additional heat weakens solder joints making them more susceptible to cracking under repeated heating/cooling and constant engine vibration. In contrast, Solid-State relays do not have contacts or any other moving parts. The internal resistance is fixed regardless of how many times it cycles. That internal resistance is so small (thanks to the latest semi-conductor technology) no significant heat is generated under normal operating conditions. Even if the fuel pump stalls and draws just under the maximum current before the fuse blows, F9T’s SSR could switch it on an off until the battery runs dry and its performance would not degrade. The ultra-low internal resistance also means no bulky/heavy heat sinks are required. In fact this SSR is less than half the weight of a mechanical relay. This is an advantage in an automotive environment because it reduces the stress on solder connections by vibrating with the engine rather that against it like the heavy mass of a mechanical relay does.