Like MBDoc said, your 126 body 280 would not naturally use that relay for engine management. It depends on what was done to the car to "Federalize" the car. The 280SEL til 85 was a simple K-Jet system similar to 450SEL between 76-79 in the US. The only electric component is the fuel pump and the warm-up pressure regulator. The fuel pump relay poweres them. Most cars imported had lambda control systems added but they very seldom used MB parts.
Bosch Master, ASE Master, L1
33 years MB technician