Fuel pump relay keeps burning out - W209/W203/W211
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Joined: May 2017
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W209 CLK55 - 2003
Fuel pump relay keeps burning out - W209/W203/W211
I have a 2003 CLK55 with around 120k miles on it. It has an issue where the fuel pump relay runs hot and burns out. In the last 18 months I have replaced it (I think) 5 times.
My first thought is that this is a bad fuel pump that is drawing too much current. Before I swap it out, I wanted to ask:
- Is a failing fuel pump a common problem given symptoms, age and mileage? I put 203/211 in the header because I suspect they are similar in tank systems
- Is there an easy way to test? I have a multimeter but don’t want to pull the pump just yet.
- Are there any other things that could cause the relay to burn out? Common issues?
The one piece of information that might be relevant... when I first got the car, the relay seemed to get stuck in the on position and that ran the fuel pump continuously for a while until the battery ran down. I realised it was an issue and pulled the relay before replacing it. Assume in that situation it was just pumping fuel to the engine and back through the return line. I wondered whether that could have been the original cause.
My first thought is that this is a bad fuel pump that is drawing too much current. Before I swap it out, I wanted to ask:
- Is a failing fuel pump a common problem given symptoms, age and mileage? I put 203/211 in the header because I suspect they are similar in tank systems
- Is there an easy way to test? I have a multimeter but don’t want to pull the pump just yet.
- Are there any other things that could cause the relay to burn out? Common issues?
The one piece of information that might be relevant... when I first got the car, the relay seemed to get stuck in the on position and that ran the fuel pump continuously for a while until the battery ran down. I realised it was an issue and pulled the relay before replacing it. Assume in that situation it was just pumping fuel to the engine and back through the return line. I wondered whether that could have been the original cause.
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From: Republic of Texas
'99 and '05 E55 AMG
You should check out the W211 AMG sub-forum. The E55's built prior to MY2006 are problematic when it comes to relays.
https://mbworld.org/forums/w211-amg/...t-not-cps.html
Clogged filter and old pumps are the common cause of melted relays. Read the links in the thread linked above. For my car, I ended up installing a 70-amp relay and I replace filter/regulator and pumps every 60,000 miles (not cheap but cheaper than being on the side of the road). I also measure the current draw on the fuel pumps at every oil change, when new each pump was drawing 13.1 amps, at 60K miles one pump was pulling only 12 amps and the other was pulling 14 amps. You'll have to build a custom tester to measure the current draw; I have a schematic in the W211 AMG section.
https://mbworld.org/forums/w211-amg/...t-not-cps.html
Clogged filter and old pumps are the common cause of melted relays. Read the links in the thread linked above. For my car, I ended up installing a 70-amp relay and I replace filter/regulator and pumps every 60,000 miles (not cheap but cheaper than being on the side of the road). I also measure the current draw on the fuel pumps at every oil change, when new each pump was drawing 13.1 amps, at 60K miles one pump was pulling only 12 amps and the other was pulling 14 amps. You'll have to build a custom tester to measure the current draw; I have a schematic in the W211 AMG section.
Thread Starter
Senior Member
Joined: May 2017
Posts: 257
Likes: 31
W209 CLK55 - 2003
You should check out the W211 AMG sub-forum. The E55's built prior to MY2006 are problematic when it comes to relays.
https://mbworld.org/forums/w211-amg/...t-not-cps.html
Clogged filter and old pumps are the common cause of melted relays. Read the links in the thread linked above. For my car, I ended up installing a 70-amp relay and I replace filter/regulator and pumps every 60,000 miles (not cheap but cheaper than being on the side of the road). I also measure the current draw on the fuel pumps at every oil change, when new each pump was drawing 13.1 amps, at 60K miles one pump was pulling only 12 amps and the other was pulling 14 amps. You'll have to build a custom tester to measure the current draw; I have a schematic in the W211 AMG section.
https://mbworld.org/forums/w211-amg/...t-not-cps.html
Clogged filter and old pumps are the common cause of melted relays. Read the links in the thread linked above. For my car, I ended up installing a 70-amp relay and I replace filter/regulator and pumps every 60,000 miles (not cheap but cheaper than being on the side of the road). I also measure the current draw on the fuel pumps at every oil change, when new each pump was drawing 13.1 amps, at 60K miles one pump was pulling only 12 amps and the other was pulling 14 amps. You'll have to build a custom tester to measure the current draw; I have a schematic in the W211 AMG section.
I’ve had a look at the W211 posts and the fuel system does seem to be slightly different, two pumps for a start. The set up of relays also looks to be different, probably related to the fact that the kompressor motor needs more fuel.
i don’t see any evidence that suggests the NA motor has the same issues, only one post in the CLK55/63’foeum that implies that a new pump fixed it.
I am very tempted to open things up to see what I am dealing with and simply replace if I find the car is on its original pump.
Thread Starter
Senior Member
Joined: May 2017
Posts: 257
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W209 CLK55 - 2003
To add, the fuel filter is less than 10k miles old (2016).Edited to say: this was a fault with the rear SAM in the end. Replaced and problem solved.
Last edited by breeze247; Jun 2, 2019 at 05:18 PM. Reason: Solution
