In the photos below, you can see the switch pack on the right, and a test jig on the left. The test jig is pretending to be a Gwagen, but it fits on my workbench a lot better. It has three green lamps to show when the switch pack is sending power to each diff lock solenoid. The test jig also has three toggle switches to allow you to simulate the sensor switches in each diff lock. I’d like to thank Sean Philyaw at Eurotruck Importers for finding and generously donating the Mercedes connectors needed for the test jig.

The switch pack only allows you to select lockers in the sequence I - II - III. So the yellow lights will always come on in that order. Also, it only sends power to the lockers in that same sequence. The red light light for the previous locker must be on before any power goes to the next locker. If a red light goes out, the next locker after it in the sequence is disabled, even if the relevant yellow light is on. And finally, if you deselect a locker, the lockers above it in the sequence are immediately deselected too. This modification that we are about to do will remove all three of these restrictions. You will notice later that it requires three changes to the circuit, one for each feature that is being removed.