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My 06 ML500 would crank and not start. Troubleshooting revealed that the fuse #57 was blown. However, I had a lot of trouble getting the fuse out of the rear fusebox, and it looked like the fusebox had melted around the fuse. Once I replaced the fuse, the car ran again. But, I had exactly the same problem after another couple of months use. I think I have a problem greater than a blown fuse. I've ordered a new rear fusebox to deal with the melting. Any ideas on what may be going on, the underlying problem?
Picture showing fuse #57 Detail showing the melting around this fuse
Your in tank fuel pump / filter / sender unit is probably getting clogged, causing the fuel pump to draw more current (trying to push through the clogged filter) and overheat and blow the fuse.
Kent is right, you'll be buying another fuse box, if you don't fix the problem (melted fuse box is a symptom). I would also check the wire insulation too!
It's too bad the fuse doesn't blow before a short occurs from a melted fuse holder. High resistance = heat = melted plastic = possible fire.
These in tank electrical components always give me cause for concern.....If it's melting the fuse box......How hot is the pump getting (sitting in gasoline!)?
Changing the fuel pump and sender is a PITA on our rigs. I've done 2 in my CLK, 2 in my E55 and 1 in the C320 and they were easy compared to the ML500
I've seen burnt contacts generate enough heat to melt and burn out relays so replacing the fuse box with burnt contacts on the fuse is something thay should be done. Measuring the amp draw would be a good next step.
Thank you for the reply. That makes perfect sense. I wish I would have thought of that (and asking for help when it blew the first time) and not replaced the fuse and drove it for another couple of months until it blew again. Live and learn - hard way. I won't make that mistake again.
Originally Posted by Pmarino
Your in tank fuel pump / filter / sender unit is probably getting clogged, causing the fuel pump to draw more current (trying to push through the clogged filter) and overheat and blow the fuse.
Thank you for the reply and the analysis. Never seen this problem before so didn't see that. In hindsight it makes perfect sense. I'll get fixing the disease, and the symptoms.
Originally Posted by lkchris
The purpose of a fuse is TO MELT when a short circuit occurs. This protects the circuit in question, possibly preventing a fire.
You about 98% have a problem with your fuel pump creating an electrical overload.
it’s generous to think 2% you have a problem with fuses and fuse box.
Thanks again. Yes, I know how difficult it is. I actually already bought a replacement fuel pump when it blew first time as I diagnosed no fuel pressure on the rail. I've seen that problem before on a old E420, plugged fuel filter in that case, easy fix.
I found this YouTube video:
I thought the fuel pump was bad at first, hence the replacement fuel pump ordered. But, when I found the blown fuse, I thought that was my problem. In the video, the fuses are good, and pump is getting power, but not working.
I thought I just got lucky. I see my mistake now. I won't make it again. I don't mind learning from mistakes. I'd prefer not to. I am glad my mistake didn't result in the car burning down and killing someone.
Originally Posted by Pmarino
Kent is right, you'll be buying another fuse box, if you don't fix the problem (melted fuse box is a symptom). I would also check the wire insulation too!
It's too bad the fuse doesn't blow before a short occurs from a melted fuse holder. High resistance = heat = melted plastic = possible fire.
These in tank electrical components always give me cause for concern.....If it's melting the fuse box......How hot is the pump getting (sitting in gasoline!)?
Changing the fuel pump and sender is a PITA on our rigs. I've done 2 in my CLK, 2 in my E55 and 1 in the C320 and they were easy compared to the ML500
Thanks for the help, and suggestion. I'll replace the pump and fuse box. I'll google to figure out how to measure amp draw. That is not something I've done before. I do have a decent multimeter.
Originally Posted by Tsumi
I've seen burnt contacts generate enough heat to melt and burn out relays so replacing the fuse box with burnt contacts on the fuse is something that should be done. Measuring the amp draw would be a good next step.
I had a chance to work on this today and have some more facts, and a few more questions. Using everything I learned in this thread, and the videos above, I removed the plate covering the fuel pump, passenger plate. I pulled the connector to test the power. The wife kindly cranked the car while I measured voltage. A shocker. I was expecting 12 volts, but I got 7.6v.
1. I am wondering if this is likely a poor connection in the fusebox, the melting. I do understand that is NOT my problem.
2. If it is the case that that the pump is dying because it is straining to pump through a clogged filter, isn't the filter in the other compartment behind the driver's seat? Should I be pulling that plate and changing that filter? Or, is there another filter in the pump that will be replaced when I replace the pump that will take care of this problem?
Here is what I tested:
The fuse box should easily and happily accomodate current flow up to and beyond the fuse's rated capacity without ill effect. That your fuse box is melting tells me that there's a crappy connection (resistance) that's generating heat as current flows through it which results in a melted fuse socket. My $$$ is on a bad connection in or around the fuse socket.
Corollary to the bad connection is that your pump isn't seeing as high a voltage as it would like to operate properly. Lower voltage means higher amps, compounding the problem and blowing the fuse.
Last edited by rapidoxidation; Jul 24, 2022 at 10:47 AM.
Thanks for the response. Makes sense to me. To start, I think my plan then is to install the new fuse-box when I get it and check the voltage at the socket to see if I am at 12V and go from there accordingly.
Originally Posted by rapidoxidation
The fuse box should easily and happily accommodate current flow up to and beyond the fuse's rated capacity without ill effect. That your fuse box is melting tells me that there's a crappy connection (resistance) that's generating heat as current flows through it which results in a melted fuse socket. My $$$ is on a bad connection in or around the fuse socket.
Corollary to the bad connection is that your pump isn't seeing as high a voltage as it would like to operate properly. Lower voltage means higher amps, compounding the problem and blowing the fuse.
What's your fuel pressure values? What's the fuel pump relay (and the board) look like?
[QUOTE[color=#333333]2. If it is the case that that the pump is dying because it is straining to pump through a clogged filter, isn't the filter in the other compartment behind the driver's seat? Should I be pulling that plate and changing that filter? Or, is there another filter in the pump that will be replaced when I replace the pump that will take care of this problem?][/QUOTE]
Yes, you have to access both saddle tanks, Passenger and driver side. You'll have to use a piece of wire or a string to fish the hose connections from one side of the tank to the other. Just remember to attach it befrore remove the old one's.( I use bailing wire).
Meh.....Maybe? I'll take the bet (i love internet diagnosing )
Originally Posted by rapidoxidation
The fuse box should easily and happily accomodate current flow up to and beyond the fuse's rated capacity without ill effect. That your fuse box is melting tells me that there's a crappy connection (resistance) that's generating heat as current flows through it which results in a melted fuse socket. My $$$ is on a bad connection in or around the fuse socket.
Corollary to the bad connection is that your pump isn't seeing as high a voltage as it would like to operate properly. Lower voltage means higher amps, compounding the problem and blowing the fuse.
Using Ohm's Law (volts = current x resistance and power = volts x amps), and assuming the pump is using 5 amps on a good day... he's seeing 7.6 volts at the pump, meaning there's a loss of about 6 volts somewhere at 5 amps... That's about 30 watts of power lost, more than enough to melt a fuse block due to a bad connection. Ball park figures for sure, but the hypothesis fits the problem.
https://www.underhoodservice.com/fue...-current-draw/
[QUOTE[color=#000000]3. The fuel pump circuit must be energized to properly test it. Energize the fuel pump relay and power the fuel pump circuit. The majority of fuel pumps run for only a few seconds once the relay is energized (only long enough to prime the system) and until an RPM signal is generated.][/QUOTE]
https://automotivetechinfo.com/wp-co...p-Circuits.pdf
[QUOTEThere are three possible causes for a blown fuse here: the wire after the fuse is shorted to ground, the fuel pump control module is shorted to ground internally, or the fuel pump has increased its current draw to the point that it exceeds the fuse’s amperage limit.]As a fuel pump’s commutator and brushes wear, it tends to draw more amperage than the system is designed to handle.[/QUOTE]
Sorry for the delay in providing an update, I only get to work on my cars on Saturdays. (As an aside, IMHO, the only way to own a used Mercedes is to have more than one. You cannot own only one as your daily driver and not go bankrupt using the dealer or an independent mechanic. So I have several and drive the ones that work and fix the ones with problems when I can.) Here is a progress report. The replacement fuse box I purchased on eBay, a used part from Lithuania. (There are a lot of good reasonably priced parts from there on eBay, and the sellers are reasonable). So I had to wait for it arrive. When it did, it enabled to see the connectors and plan its replacement. I got the fuse box out and noticed the pin to the fuel pump fuse was oxidized in the old unit. The pin in the connector was too, heat. (see the picture). I carefully cleaned this connector with a brass brush, and reassembled. The car started first time. However, when I checked the voltage, I had only improved it to 8.5V. So with the car running I moved it into my shop and started work on what I understand is the underlying problem, the fuel pump AND/OR the fuel filter. I bought replacements for both of these and have so far replaced the fuel pump, the fuel filter is next. (BTW, with the fuel pump replaced, the car starts and runs.) I have a question about the fuel pump and the fuel filter replacement, but I will start another thread for that.
[QUOTE(As an aside, IMHO, the only way to own a used Mercedes is to have more than one. You cannot own only one as your daily driver and not go bankrupt using the dealer or an independent mechanic. So I have several and drive the ones that work and fix the ones with problems when I can.)][/QUOTE]
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Full Pump Fuse #57 Failing, Heat? 06 ML500
High resistance = heat = melted plastic = possible fire.
( I use bailing wire).
(i love internet diagnosing 
) 
