Fuel Pump Relay Failure

1979-1991 280SE, 300SE, 300SEL, 380SE, 380SEL, 380SEC, 420SE, 420SEL, 420SEC, 500SE, 500SEL, 500SEC, 560SEL, 560SEC
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Model you own: w126
Location: Western Sydney

Fuel Pump Relay Failure

Post by Sanford1946 » Wed 10 Jan, 2018 1:01 pm

My 420SEL is usually totally reliable so it came as a very rude shock when it wouldn't start on the morning of Boxing Day last year. Diagnosis went like this: engine spins on starter but no go; turn key to just ignition and there was no humming noise at all?? Usually I can just hear the fuel pump running so I got under the car, clipped up the multimeter to the fuel pump and sure enough, when the key was turned there was zero voltage at the pump. Running a jumper lead to the fuel pumps had them both spinning happily but sadly that doesn't really allow the car to be driven.
I pulled the fuel pump relay out; it is easily found being right next to the fuse box and behind the climate control relay. Being in the middle of the holiday break, the forum sponsor (always my first call for parts) was shut. A trawl through the internet sales site showed that relays of European origin were "thin on the ground" – to put it mildly – and they were expensive. The alternative relays available from China were around $40, cheap but in my view of dubious reliability. A search through this forum showed that terminals 7 and 8 in the relay socket can be bridged to run the fuel pump but that makes the pumps run all the time – regardless of the ignition key. On the American Benz forum I found this link which outlines resoldering the printed circuit board:
http://www.benzworld.org/forums/w126-s- ... older.html

Now my knowledge and experience of electronics is limited to marveling at the fact that all those tiny components do the things they do. My diagnostic skills end at the point where if I can see melted plastic and smell burning I chuck it in the bin. My relay showed neither of those ills so I followed the advice in the link above and took a look at the circuit board through a powerful magnifier. Sure enough the two pins that control the flow of electricity to the fuel pumps (they go into pins 7 and 8 in the relay socket) were definitely suffering from "dry joint syndrome" where the soldering at the circuit board had cracked away from the pin terminals. After some very careful soldering (high grade 60/40 solder, super-cleaned circuit board, cleaned tip on the iron, steady hand etc) the solder around the pins became nicely rounded bulbs of solder with no cracks.
Before I put the hopefully repaired relay to the test I grabbed another relay (an OVP relay I had pulled to bits previously) and cut it up so that I had a two-pinned plug that I could use as an emergency bridge between pins 7 and 8; that plug now sits in the fuse box as shown in the photo.

I tried the emergency plug, the pumps could be heard running and the car started instantly. I put the emergency bridge in the fuse box and put the resoldered fuel pump relay into its socket. With hope abundant I turned the key to "ignition on" and the pumps ran! Another turn, the starter spun and the engine fired into glorious 420SEL V8 life!!

So there you go. If at some time in the future I have any other relay fail on me, the first thing I will do is check the soldering because now I know I can fix that problem. Sadly, if there is melted plastic and an acrid smell of burning I guess it will be the bin.
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Once upon a time there was perfection.
Then came the W126

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Re: Fuel Pump Relay Failure

Post by gazman » Wed 10 Jan, 2018 1:41 pm

Well done . . . Always nice to have a win
1989 560 SEL King of the road

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Re: Fuel Pump Relay Failure

Post by Ivanerrol » Wed 10 Jan, 2018 10:02 pm

Sanford1946 wrote:
Wed 10 Jan, 2018 1:01 pm
After some very careful soldering (high grade 60/40 solder, super-cleaned circuit board, cleaned tip on the iron, steady hand etc) the solder around the pins became nicely rounded bulbs of solder with no cracks.
I'm assuming that would be 60%lead and 40% tin. :wave:

One of the issues with all these electronic components and circuit boards is the "unintended consequence" of lowering the lead content of solder. All very noble - but once the lead is in there it's there for life.
NASA have lost more than one extra terrestrial vehicle because of circuit board failure in the harsh environment of space - entirely due to solder with too low lead content.

Now you have sorted out that FPR it should soldier on for another 25 years quite happily. :dance:
W211 - E240
W204 - C280
W202 - C200, C180, C180
W126 - 380SE , 380SE (Ex SA Import), 560SEL
W124 - 300e, 260e (ex Japan)
W111 220s (Indonesia) 4 speed manual column shift
W123 230
W116 450SEL
W140 420SEL
W210 E240, E240
W209 CLK 240
W201 190e 2.6 (ex U.K.)

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Re: Fuel Pump Relay Failure

Post by CraigB » Wed 10 Jan, 2018 10:26 pm

Great thread! Thanks for sharing that experience. Love the get out of trouble repair.
Craig Baulderstone

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