look in the engine compartment, behind the firewall there are a series of relays next to the windshield wipers, looking at a left hand drive car about a third of the way along on the left you will see a push buton, now go to the middle, you will see a long unit, about 5 inches, it has a harness attached to its right and is the longest relay there. That is the fueld pump relay, a favorite is dry joints in it whcih can be soldered, the symptoms are fuel pumps not activating or irregular running.
Usually next to the fuel pump, back near the tank.
The fuel pump is located in a plastic housing under the vehicle. The housing is located behind the right (passenger side) rear lower wheel frame assembly. Remove the housing and follow the hoses from the gas tank to it.
if its a 116 engine, that is, it has one air intake on the passanger side to the air filter then the relay is on the firewall, on the left looking at the engine is the ecu, big black thick connector on it, next to it directly on the right is the fuel pump relay, maybe marked kickdown, next to that klima, for ac, next to that ovp overvoltage protection. if u have 2 air intakes for 117 engine let me know, I can look in my books for you.
On the side of the water pump (not 100% on this where the lower radiator hose goe in to the housing on the pump)
i wouldnt go near a high pressure fuel system with bio you will damage the pump and injectors
Under the hood on the fire wall just ahead of the stearing wheel up to the right of the fuse box
Torque specs for WATER PUMP Mercedes 420 SEL are: 7.3 diesel and 15 to 18 foot pounds of torque
it is a 410 ga pump shotgun
the rancher 420 does not have a cvt so there will be no cvt intake
in the front its bolted to the front cover on the drivers side the easy way to find it , it has a cover that unscrews so u can add fluid its on the belt drive
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it is located in the trunk of the car