bbirdwell

I received another email from a forum member who, despite having had the fuel tank assembly replaced under warranty at the dealership 3000 miles ago, suffered the dreaded "melted fuel pump relay". This got me to thinking what can be done to improve reliability a bit.

I located some "Carbon Conductive Assembly Paste". This is used to prevent corrosion and to improve conductivity/connectivity by filling in the microscopic areas around the leads and connectors.

I pulled the 30-amp fuse to find the leads badly corroded. Remember that corrosion means resistance and voltage loss and heat generation. I cleaned the fuse leads with 600-grit paper, applied the carbon compound, and reinserted in the fuse holder. I also pulled the fuel pump relay, inspected it (no corrosion or signs of overheating), applied the carbon compound, and reinserted in the relay holder.

FWIW, I've sketched out a schematic for a modified fuel pump relay setup that uses 3-relays versus the single OEM relay. I believe this will overcome the overloaded stock relay while complying with the current sourcing limitations of the stock ECU. I'll be posting that in a few days for feedback prior to building and installing.

Elhonaz

nice. wheres the fuse located?

bbirdwell

Passenger side trunk on a left-hand drive car. It's the 30-amp fuse. 2004 and later have another relay plus a 40-amp fuse for the air pump co-located next to the fuel pump relay and fuse.

Hulk

Did water get in there?

DavesMeanE's

This is hilarious - just fixed a few of over the last few weeks, and talked to another guy on the phone 20 minutes ago about it!!

Good info here!

Thanks

Dave

bbirdwell

I cannot find any signs of water infiltration but it does bother me seeing the corrosion.

tlambert

Thanks for posting. Good reminder, I'm going to pull mine and check it out.

Dublinoh

This is something E63 owners should do? Fuse and relay's location would be appreciated if true. Thanks

bbirdwell

Unknown to me but if you pull the cover off of the passenger side of your trunk and find relays and fuse, it would be much appreciated if you would share it.

Dublinoh

I can see two fuses 40 amp but don't see relays, I don't have time to open it up. This is the passenger side hinge trunk area. Please excuse the picture quality...

bbirdwell

Looks like you're on the trail. PM me your VIN and I'll see if I can find some info from the schematics and grounding diagram.

EDIT: Disregard. Your E63 has two each 40-amp fuses in the trunk (seen in your photo). The one with a Black/Grey wire leaving it is the fuse for the two each fuel pump controllers (not relays). The other fuse is for the air pump. Both 40-amp fuses are fed by a 150-amp fuse (#80?) in the rear pre-fuse block located near the spare tire. Your E63 uses two each Pulse-Width-Modulated controllers (one for each fuel pump). Looks like Mercedes figured out a way to get around the relay issue...

Won't hurt to check the fuse leads and ensure they are free of corrosion.

MACEDON

subbed

shardul

iTrader: (1)

Refreshing to see some good info being posted.

Hulk

Shards you are right

I think before people starting hacking away at their fuel system, they need to do maintenance and check voltages and amps under load along with relays etc

bbirdwell

Okay, I thought about the pros/cons of going to a three-relay system. I've decided to instead go with a higher-capacity relay.

Thoughts:
- Prior to 1 June 2005, E55s used one connection from the rear SAM Motor Electronics to activate one relay. The relay coil resistance indicates the relay needs about 0.3 amps from the rear SAM Motor Electronics to trigger.
- On and after 1 June 2005, the E55 used two connections from the rear SAM Motor Electronics to activate two relays (one connection per relay). This implies the drivers in the rear SAM Motor Electronics can only support the 0.3 amps (plus safety margin) each relay requires. Having two connections implies connecting two relays to one activation line would draw too much current (~ 0.6 to 0.7 amps to trigger two relays from one line) and could potentially burn out the driver chip in the rear SAM Motor Electronics.
- The fuel pump relay in the pre-1 June 2005 E55 is rated at 40 amps resistive, 35 amps inductive. The fuel pumps draw approximately 27 amps through this single relay. Call it 67% of rated load carried constantly. (FWIW, I measured the temperature of my relay at 148 degrees F on the outside of the case today.)
- The fuel pump relays in the post-1 June 2005 E55 are also rated at 40 amps but only have to pass through approximately 13 amps each. Call it 33% of rated load carried constantly.

- I located a 70-amp resistive rated relay that uses 9.5mm connectors versus the stock 40-amp relay 6.3mm connectors. So...I need to remove the connectors from the stock relay socket, insert the 6.3mm trigger/ground connectors into the new socket, cut the 6.3mm power leads from the fuel pump circuits, crimp and solder the 9.5mm connectors onto the power leads, insert 9.5mm connectors into the new socket, then insert the new 70-amp rated relay. Now I should be looking at 39% of rated load carried constantly.
- Proposed relay has a slightly higher coil resistance that indicates approximately 0.25 amps current required from the rear SAM Motor Electronics to trigger the relay. So...rear SAM Motor Electronics should be safe.

Parts ordered today. I'll see about making time in the next 2-3 weeks to adapt the larger relay (I don't have to deal with the bean counters trying to save $2 to $5 per car) and post results. If this doesn't work, I'll fall back on the three-relay design.

I'm still bothered by having a 30-amp fuse on a circuit that draws 24-27 amps. The E63 stepped the circuit up to a 40-amp fuse. I'll have to look at the wire gauge to see if it can support a 40-amp fuse without melting insulation. That would be a "bad thing".

EDIT: for the fuel pump relay, the rear SAM Motor Electronics does not source current, it "sinks" it; i.e. it connects the circuit to ground. See the schematic for the relay later in this thread.
Additional EDIT: It is the Motor Electronics that activate the relay(s), not the rear SAM. I misread the schematic. My bad.

quan

Great information! please keep us posted of your findings!

seanol

bbirdwell,
This may sound dumb but can you use a 30 amp relay triggered by the ECU to trigger two more 30 amp relays to support the load? The first relay would only need the lower amperage and you could just switch battery power for the input to the next two.

May be less hassle.

bbirdwell

Not dumb and great minds think alike. Attached is the sketch I made of how to do exactly that. Nonetheless, I'm going to try a single high-capacity relay for now. My IR thermometer is showing the *outside* of the plastic case on the stock relay as running approximately 70 degrees F above ambient (160 degrees F give or take a degree). I would hazard the internal temp is higher. I'll install the larger relay and compare temperatures. If they remain high, I'll go with the three-relay setup.

seanol

Great! Very interested in your results and look forward to the data.
Thanks again for all you do,
Sean

Max.H

Very helpful and informative posts!

bbirdwell

I re-looked this. Turned out there was a water leak on the driver's side where the trunk lid gasket was not seated properly. I would now say that over the years, water would leak into the trunk and hot Texas days would turn the trunk into a sauna with extremely high humidity.

Your post was on my mind and, after two days ago driving for an hour and a half through a downpour (average speed 33 mph on 70 mph highways), I opened my trunk and noted a piece of cardboard on the driver's side carpet was damp (the 1/2" of water in the spare tire well was also a hint). Comparing the gasket between the wet and dry side revealed where the driver's side gasket was not seated. The gasket lip should be under the sheet metal, not on top.

Looks like I'm going to be pulling all of the fuses and relays this weekend to check for corrosion as well as re-seating and sealing the trunk gasket.

Oh yeah, the cylinder 1 coil failed this morning so I had to limp the car home. Picked up a new coil at the dealer so I'll be swapping that out as well. <sigh> Think I might end up putting one of these into my "spare parts" bag for road trips.

ben73

Although my car has no issue with the relay, I have been considering doing the same.

Shame it's more difficult to upgrade the current carrying capability of the connector on top of the fuel pump itself.

Hulk

I think for the pumps to work optimally, maintenance is key. Pumps, wiring, relays etc. that is a must before thinking to change the fuel system

Nice hunting birdman

bbirdwell

Replaced the stock 40-amp rated relay with a higher-capacity 70-amp rated relay yesterday. In the photo you can see the difference in power connector size between the two relays' terminals. The copper connectors crimped and soldered without a problem.

I used a Floesser 2270 relay; equivalent relays are:
Mercedes: 000 542 97 19
Audi/VW: 443 951 253J, 8D0 951 253, 4H0 951 253
General Motors: 90226846

The old relay I could remove and insert using just my thumb and forefinger; the new relay required placing multiple fingers under the socket and using the heel of my palm to insert it. Certainly a much more secure connection.
Again, I used the conductive carbon paste.

I'll monitor case temps this week and see if this brought them down.
FWIW, the wire gauge appears to be 12 gauge.

Dublinoh

[QUOTE=bbirdwell;7167100]I re-looked this. Turned out there was a water leak on the driver's side where the trunk lid gasket was not seated properly.

An other potential leak source is the seal around the tail light especially when using high power sprayers at car washes.