Performing a fuel pressure test on your TWIN fuel pump KE-jet engine is safer and less hassle with the engine NOT running. The only way you can do that is to find a way for the fuel pump to run all the time while doing the test or any adjustment. When you turn the key on, the pump will only run for a few seconds and then shut off. There are two ways you can get the pump to run constantly. On the 560SL there are three ways. Let Kent explain in the video below:
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First method: Find and remove the fuel pump relay and "jumper" the two pins that power the pump. The banana plug ends on this jumper lead will plug right into the socket holes where the relay is mounted. That is pretty easy on some models like the W126, but on the R107 it is a big pain to get
Second method: Install the three supplied alligator clips on the ends of the banana plugs. Run this single wire from the positive terminal on your battery to the back of your car where the fuel pumps are mounted. Find the positive leads on the fuel pump ( not the brown wire as it is ground). Leave the button off when doing this. Remove the two factory wires. Clip the end with two alligator clips to the two pump terminals. Clip the other alligator clip onto the positive post of the battery. Turn the button on and listen for the whir of the pumps. If it runs you are hooked up correctly. If it does not run, switch it off immediately and make sure you don't have the wires crossed.
PLEASE READ CAREFULLY BEFORE PURCHASING: We will provide general instructions, but we are not able to give specific fuel pump relay locations for each model from 1976 to 1985 ( a couple of examples will be shown ). We will not supply you with the pin numbers you need to "jump" with this wire on your model. There are too many variables. Keep in mind YOU can find this information if you do a thorough and deep internet search. If you can find a PDF of the printed service manual, you will have this information along with the pressure test values you will need when pressure testing your car's fuel injection system.