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Pelican Technical Article:

Fuel Pump Testing

Nick Czerula

Time:

3 hours3 hrs

Tab:

$0

Talent:

*****

Tools:

Phillips & flathead & Phillips screwdriver, socket set

Applicable Models:

Mercedes-Benz W211 (2003-09)

Parts Required:

Fuel pump, fuel pump-sealing O-ring

Hot Tip:

Check fuse before digging too deep

Performance Gain:

Properly functioning fuel pump

Complementary Modification:

Replace fuel filter

In modern Mercedes-Benz automobiles the fuel pump is electric and located inside the saddle-fuel tank. The fuel pump creates positive pressure in the fuel lines, pushing gasoline to the fuel rail, where the DME controls flow to the engine via a programmed fuel injection map. In tank fuel pumps have many benefits. The fuel is submerged in fuel keeping it cool, preventing vapor lock and electrical issues. The fuel pump delivers a constant flow of gasoline to the fuel pressure regulator, located in the left side of the fuel tank. The fuel then flows to the fuel rail at a regulated pressure and volume.

A faulty fuel pump can cause poor engine performance and hard starting problems. When they fail completely, you may be left with an engine that doesn't start. If you suspect your fuel pump has failed, you can confirm the fuel pump is receiving the correct voltage when commanded ON by the fuel pump relay using a digital volt ohm meter (DVOM). If the correct voltage is present, but the pump does not activate, it is likely faulty. Use the DVOM in conjunction with a wiring diagram to determine the correct terminals for testing. You can access terminals under the rear seat cushion, below the access panel.

The fuel pump is located in the right side (passenger side) of the fuel tank. There is an access panel below the passenger seat cushion. Even though the fuel pump is located in the right side of the fuel tank, the electrical connector is on the left side, under the access panel for the fuel filter.

Remember that your car may have been serviced before and had parts replaced with different size fasteners used in the replacement. The sizes of the nuts and bolts we give may be different from what you have so be prepared with different size sockets and wrenches.

Protect your eyes, hands and body from fluids, dust and debris while working on your vehicle. If you're working with the electrical system, disconnect the battery before beginning. Always catch fluids in appropriate containers and properly dispose of any fluid waste. Recycle parts, packaging and fluids when possible. Never work on your vehicle if you feel the task is beyond your ability.

Our vehicle may vary slightly from yours as models do change and evolve, as they grow older. If something seems different, let us know and share your info to help other users. Do you have questions or want to add to the article? Leave a comment below. When leaving a comment, please leave your vehicle information.

Remove the engine cover over fuel injectors. See our tech article on engine covers removing.

Pressure testing: Locate the fuel pressure test port at the right side of the fuel rail (red arrow).
Figure 1

Pressure testing: Locate the fuel pressure test port at the right side of the fuel rail (red arrow).

Pressure testing: Remove the fuel test port cap (red arrow).
Figure 2

Pressure testing: Remove the fuel test port cap (red arrow).

Pressure testing: Connect a fuel pressure test gauge to the test port (red arrow).
Figure 3

Pressure testing: Connect a fuel pressure test gauge to the test port (red arrow). Next, start or attempt to start the engine. The fuel pressure should be 3.7: 4.2 bar with the engine running or when cranking. Once you test the pressure, recharge the fuel system by cycling the key or running the fuel pump. Note the fuel pressure. Then allow the fuel system to sit under pressure for 30 minutes. It should hold a minimum of 2.5 bar.

Electrical testing: Start at the left side seat cushion.
Figure 4

Electrical testing: Start at the left side seat cushion. In front of the cushion is a small plastic release lever. Pull the lever up to release the seat cushion latch.

Electrical testing: Fold the seat cushion forward.
Figure 5

Electrical testing: Fold the seat cushion forward. The left side is shown. Repeat this step for the right side.

Electrical testing: With the seat cushion folded forward, remove the carpet from below the seat.
Figure 6

Electrical testing: With the seat cushion folded forward, remove the carpet from below the seat. Pull it up at the corner to detach the clip (red arrow). The left side is shown. Repeat this step for the right side.

Electrical testing: Peel the carpet away from the body and support it out of your way.
Figure 7

Electrical testing: Peel the carpet away from the body and support it out of your way. I have mine propped up with a steering wheel holder (red arrow). Even though the fuel pump is located in the right side of the fuel tank, the electrical connector is on the left side (green arrow).

Electrical testing: Remove the six 8mm access cover fasteners (red arrows).
Figure 8

Electrical testing: Remove the six 8mm access cover fasteners (red arrows). Then lever the access cover up and remove it (inset).

Electrical testing: Disconnect the fuel pump electrical connector by pressing the locking tab while pulling the connector off the fuel pump module.
Figure 9

Electrical testing: Disconnect the fuel pump electrical connector by pressing the locking tab while pulling the connector off the fuel pump module.

Electrical testing: Connect a DVOM across the fuel pump terminals.
Figure 10

Electrical testing: Connect a DVOM across the fuel pump terminals. These are the two terminals on my subject vehicle. The red arrow points to the battery positive feed to the pump. The yellow arrow points to the battery negative feed to the pump. You will be testing voltage across the large wires at the connector. Turn the key ON. The DVOM should read battery volts (around 12 volts). If no voltage is found, check the relay and the fuel pump fuse. With the key ON, the fuel pump will receive voltage for three to five seconds to prime the system, (if engine doesn't start).

Electrical testing: I suggest load testing using a test light and a DVOM.
Figure 11

Electrical testing: I suggest load testing using a test light and a DVOM. Connect the DVOM across the fuel pump electrical connector terminals and take a reading, it should read battery volts when the key is turned ON. Then connect an incandescent bulb style test light to the battery ground and touch the test light probe tip to the positive wire you are backprobing with the DVOM. Your reading should hold steady. A maximum drop in voltage of 0.5 volts is OK. Anything more is a problem. In this photo, voltage held steady. If you have no voltage to the pump, see the following steps for checking the relay function.

Electrical testing: Working in the left side of the trunk, rotate the knobs (red arrow) 90° counterclockwise.
Figure 12

Electrical testing: Working in the left side of the trunk, rotate the knobs (red arrow) 90 degrees counterclockwise. Then open the access door.

Electrical testing: Remove the door from the carpet trim panel.
Figure 13

Electrical testing: Remove the door from the carpet trim panel.

Electrical testing: Next, to the CD changer or CD changer mounting spot, there is a rear electronic panel.
Figure 14

Electrical testing: Next, to the CD changer or CD changer mounting spot, there is a rear electronic panel. Remove the plastic cover (red arrow) by pulling it straight off.

Electrical testing: You now have access to the fuel pump fuse (yellow arrow) and the fuel pump relay (green arrow).
Figure 15

Electrical testing: You now have access to the fuel pump fuse (yellow arrow) and the fuel pump relay (green arrow). First check that the relay has power with the key in the run position. You can also jump out your fuel pump relay (to bypass it) for testing. The vehicle used in this tech article was a 2004 E320 4MATIC sedan with a 6-cylinder engine. The fuel pump relay position may vary depending on year and model. Double check your fuel pump relay location with the latest MercedesBenz repair information. Remove the fuel pump replay. Connect a fused jumper wire between the terminals indicated by red arrows. The inset photo shows the fused jumper in place. This will activate your fuel pump. Check for voltage at the fuel pump as you did earlier. Battery volts should be present.

You can also check the relay circuit using a digital multi-meter (DVOM). Connect the black lead of your DVOM to battery negative and use the other lead to backprobe the terminals. Use the following list to cross-reference your voltage readings. Keep in mind that wire colors and positions may vary depending on year and engine. What you see below if from my 2004 E320 4MATIC sedan with a 6-cylinder engine. Other readings should be similar.
Red arrow: Terminal 30 - constant battery positive
Purple arrow: Terminal 86: switched battery positive, present when key is in ON position
Blue arrow: Terminal 85: DME relay control, close to zero volts when DME turns relay on
Red arrow: Terminal 87 - Feed to fuel pump, battery positive when relay is activated

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Comments and Suggestions:
GMS Comments: Any thoughts on a 2006 E350 with intermittent no start.
Conditions:
12 volts ignition to the fuel pump relay, sometimes no trigger from the ECM to 0 volts when prime and also when crank.
New crank sensor was installed.
Any thoughts as to what would cause a intermittent no 0 volt trigger to FP relay when prime or crank? Thanks Gregg
June 22, 2017
Cash Comments: I did the exact same test you featured here on my 2005 C240 - 2.6 liter I'm getting 12 volts at the plug for about 5 seconds and the light on the Custer is lighting up as you can see in the picture the relay seems to be good at this am in the truck I use a jumper wire and it powered on the plant I'm just not getting any pressure at the Schrader valve three days ago I was getting pressure when I use the pressure test now I'm getting nothing I don't think the fuel pump I can hear it whining I think something else is wrong can you help me also I swapped out fuse and relay with Spare
May 1, 2017
  Followup from the Pelican Staff: Sounds like the fuel pump is faulty. Give The Pelican Parts parts specialists a call at 1-888-280-7799. They can figure out what part or repair kit you need.
- Nick at Pelican Parts
 
Mike Comments: Your write-ups are always so very helpful, and the high resolution photos are excellent too. Thank you so much.
April 28, 2017
  Followup from the Pelican Staff: Thanks for the feedback. Glad we could help.
- Nick at Pelican Parts
 
Ryan Comments: 09 e350 - voltage tested at the pump and got nothing. Jumped the relay and was getting power. Replaced the relay with a new one but still no power at the pump. Is that a DME issue with telling the pump to turn on? Or does that mean the pumps bad?
March 5, 2017
  Followup from the Pelican Staff: If the DME is not commanding the relay ON, there is either a fault preventing it or a faulty DME. Check DME data stream for fuel pump activation to see it states ON. - Nick at Pelican Parts  
svseveren Comments: I have 3.5v at Terminal 85 with the key on.
January 26, 2017
  Followup from the Pelican Staff: What is it with key off? if you ground it using an incandescent test light, does the engine start? - Nick at Pelican Parts  
svseveren Comments: I have an '06 E350. Fuel pump not running. Jumped the relay and got voltage at Fuel pump, fuel pump works, but only when I bypass jump the relay in the trunk left side. There must be something that sends a signal to the relay to open and close... Is that the DME referenced above? What is the DME? I suspect there may be a fuel pump control module or maybe a signal comes from a more broad computer. Any suggestions?
January 26, 2017
  Followup from the Pelican Staff: DME is the engine computer, it likely controls it from inputs from theft the key. I don't have wiring for your vehicle to confirm what controls the circuit. I would grab a repair manual. It will have the wiring.

Give our parts specialists a call at 1-888-280-7799. - Nick at Pelican Parts
 
Joe H Comments: A 2008 e350, the 85 terminal for the fuel pump relay that in the write up above states should be close to zero volts.
December 30, 2016
  Followup from the Pelican Staff: That mans the DME is not commanding the relay on. Could be an open circuit or faulty DME.- Nick at Pelican Parts  
Joe H Comments: If you have battery voltage at terminal 85 with key on does that indicate a problem with the DME, rear SAM, or ecm?
December 29, 2016
  Followup from the Pelican Staff: On what vehicle? What connector? Can you share the wiring for the vehicle so I can inspect the circuit? - Nick at Pelican Parts  
bbirdwell Comments: FWIW, on the E55 AMG version, that relay is for the heat exchanger coolant pump. The AMG fuel pump relay is hidden under the cover on the opposite side of the trunk. There is no access panel; one must pull the entire cover. On the E55 the fuel pump relay is problematic it melts when the fuel filter clogs and should be replaced along with the fuel filter every 60,000 miles.
June 8, 2016
  Followup from the Pelican Staff: Thanks for the additional info and feedback. We appreciate it.
- Nick at Pelican Parts
 
Donny Comments: On a mercedes c180 2001 with the fuel pump there is a plug containing 4 wires 2 very thin 2 heavier .with key oni cannot get a voltage from any wire to earth. Should I get a voltage between the 2 heavier wires? I suspect the pump is the problem but want to know it is gettin power.
May 24, 2016
  Followup from the Pelican Staff: The two larger gauge wire should have battery positive and battery negative when the pump should be running. - Nick at Pelican Parts  
Mitch Comments: would the above tests apply to my 2005 C55 AMG?
March 14, 2016
  Followup from the Pelican Staff: I don't think so. Applies to:
Mercedes-Benz W211 (2003-09) - Nick at Pelican Parts
 

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Page last updated: Sun 6/25/2017 02:45:54 AM