Keep blowing Fuel pump fuse. Help Please!
 

The problem started 10 days ago when I drove into the gas station, filled up and then it wouldn't start, just kept cranking over. Little history, a year ago I changed the fuel pump and filter, also the CPS. 15 days ago I changed all 8 coils and all the sparkplugs. It drove awesome until the gas station incident. I towed it to the indy and he found the fuel pump fuse blown, changed it and it fired right up, I had him change the CPS while he was at it. This morning(5days after first fuse blown) went to start it up and same thing happened, kept cranking and it wouldn't start, found fuse blown again. Would the coils have any relation to what has happened? Would appreciate any help as I have no idea where to start, thanks in advance.

Fuel pump is on a completely different electrical circuit from the coils.

Either the pumps are drawing too much current (possible dirty filter), and/or the relay is bad, and/or there is a short in the circuit. First place to check is under the fuel pumps' cover under the back seat. One of the updates is a piece of foam that goes between the fuel pump harness and the metal cover over the fuel pumps.

Replacing the relay is cheap; be sure to check the socket to ensure it is good and not burned. Checking under the metal cover over the fuel pumps is free; the foam will cost you a very small amount of money.

Checking the amperage draw of the pumps is a bit more difficult; I built a custom piece of test equipment that plugs inline in the circuit so I can measure the fuel pumps' current at each oil change. I see around 25 amps for both fuel pumps combined. And that is on a 30-amp fuse. The relay is rated for 40 amps in a 2005 model.

Thank you very much for the great info, at least I have a place to start now. On another note, is there a reason the fuse blows when the car is turned off?

How do you know they blow when the car is turned off? Most likely blow when starting.

What he said. Seen several blow because it wore through the harness at the cover. Check where the harness actually plugs in, right above that. You can usually see a shiny part on the cover itself where paint has been worn off. You won't blow the fuse when the car is off unless your relay is stuck and constantly supplying power to pump.

Just an update, I had assumed the fuse for the fuel pump had blown again, but that wasn't the case. After leaving the car for 2 days it just started right up again, Went to work and back everything fine. I called the garage who did the work last week and asked them where the fuse for the fuel pump is, and they said it was on the driver side(LHD), so now I know they did not do anything, back to square one where the car wont start and after leaving for a couple of days starts up and drives ok for a few days. I'm thinking as advised maybe old fuel relay?

Relay is cheap and easy to replace. The fuel pump relay is under the panel in the trunk on the side *away* from the fuse box (the side with the fuel tank cap).

Since yours is a 2004, there should only be one relay and fuse under the panel in the trunk. You will have to remove the panel; there is no access port. 2005 models built up to 31 May 2005 have two relays in the trunk; one for the fuel pumps and one for the air pump. Your 2004 should have the air pump relay under the hood over the auxiliary battery.

Remember, what is the fuel pump relay in a non-AMG is the heat exchanger coolant pump relay in the AMG. What is the air pump relay in a non-AMG is the oil cooler radiator fan in the AMG. Do not confuse the non-AMG and AMG fuse and relay designations. The AMG has a modified harness.

FWIW, when my fuel pump relay went out, it literally melted the socket; I had to rewire and replace the assembly.

Edit: if your shop stated the fuse on the driver's side was changed, they are using the W211 non-AMG fuse/relay diagram and they changed the fuse/relay for your heat exchanger coolant pump. Again, the W211 AMG fuel pump fuse is located with the relay on the passenger side of the car.

Well back to square one after replacing fuel relay. Car drove great for 10 days, then I let the fuel level get down to a 1/4 tank for first time since the last time it wouldn't start and behold I drove home to pick something up real quick, went to start it up 5 mins later and it wouldn't start again. No codes showing, nothing. I am stumped.

For now keep a fuel pressure gauge connected to the Schrader valve on the fuel line; this will allow you to monitor the fuel pressure. Should be in the area of 80 psi.

When the fuel pumps and filter/regulator were replaced, did anyone ensure the fuel return lines were brought from the filter/regulator side back to the pump side? If they were not, fuel will pool in the filter/regulator side of the saddle tank and the fuel pump side will go dry.

Thanks again, I don't know how to check the return lines myself, but I poured 3 gallons of fuel, still didn't start, I can hear the pumps prime, but still no go. I'm having towed to Mercedes specialist in the morning, will update with what it turns out to be.

Another update, the shop said they hooked it upto the DAS and it was indicating high resistance in filter, side. They pulled the plug out and basically one of the pins is totally fried, I just replaced both sender and pump a year ago, now I'm stumped what would cause this high load so soon, would appreciate any thoughts, thanks.

https://cimg6.ibsrv.net/gimg/www.mbw...09b2e55688.jpg

You buy gasoline from crappy stations right after they fill the tanks and the dirt has floated back to the top. Excess draw from being clogged. You sure the filter was changed at the dealer? I call BS. Should take years to be clogged enough to generate enough resistance to burn the plastic, PARTICULARLY on the filter side that isn't high load.

Youre absolutely right. They are saying either the pumps were not original or the plug was not seated properly, either way they want to change both left and right with the updated ones. I guess thats what you get when you go to someone who doesn't specialize in MB's. Expensive lesson!

When I had my fuel tank/system replaced under the extended warranty, I blew my fuel pump fuse within 5 miles, apparently the tech did not use enough insulation on the wiring and had to re-do the fix. Could be the same issue :nix:

Well, this is separate from the fuel pump fuse. The fuel filter sender has its own fuse, unrelated to the twin pumps themselves.

Not separate. The pic he's showing is the power supply connection for the pumps. The filter/sender does not have a fuse.

I'm sorry, I looked at that photo and initially thought it was the filter side for some reason.

Ive seen the pins in the harness become a little loose as well, the high resistance will come from the actual connection, any time I remove the connector I like to put a test pin in to make sure they are still snug. If they feel a little loose you can tighten it up with small screw driver or pick. See it on C class a lot as well

You need to check the fuel pressure at the injector rail when the car fails to start, but you can hear the pumps.

At key "on", the fuel pumps energize for one second and then turn off. Fuel pumps remain energized at engine start. Fuel pressure at idle should be in the 80-85 psi range (5.5 to 5.9 Bar).

It turns out when the pumps were changed last time, they did not include the wiring harness kit.

What kit?

Here is what I installed in my car. The adapter cables were required due to updated filter and pumps. If you install the old filter and pumps, you will not need the adapter cables.

211-470-00-00 Fuel Sending Unit (dual pumps)
211-470-64-94 Fuel Pump Assembly (actually filter and regulator)
211-471-05-79 Fuel Sender Seal x 2
211-440-05-07 Cable Harness adapter for fuel sending unit
211-540-07-00 adapter cable to fuel pumps (one pump has reversed polarity compared to old pump assembly)
203-997-04-81 foam cushion (mounts on metal cover so the fuel pump harness cannot short to the cover)

This is what I needed to replace the melted relay and socket:
Hella socket 84949011
Terminals for socket. 10-12 ga plus 14-16 ga, 0.250” (6.3mm)
002-542-72-19 relay