W123 280CE fuel pump relay

[Llewelyn]

got an annoying intermittent problem with the car cutting out, seems pretty certain it's the pump relay that's causing it. I made a classic get-you-home bodge today by connecting the pump wiring in the boot to a feed from the rear fog light circuit, but that's hardly a long-term solution. Having done that and set off up the road, within a couple of minutes the car ran as normal without feeding power to the pump, so obviously the relay decided to play ball again.

Now, the relay appears to be a clever one that senses engine speed and also has a rev limiter function... and it's quite possible that there's a dry joint in it or something. I could easily investigate that if I could find the bloody thing. It's a 1981 car, RHD, if that makes any difference.

 

[jaymanek]

Its above the bonnet pull handle inside the car... So you need to lower the dash trim.

 

[Alex Crow]

Usual position is as Jay says above, but a few W123 cars have them mounted next to the battery under the bonnet.

A dry solder joint is a strong possibility, and a good DIY for the competent.

 

[neilrr]

Is it? I thought it was in the engine bay around the bottom of the windscreen area!

 

[BoyracerAU]

Definitely inside near the base of the a pillar if it's at all like my W123 230E.

 

[jaymanek]

Only very early cars had the fuel relays in the engine compartment.. the shiny silver ones that are universal across the other relays.

 

[Alex Crow]

Only very early cars had the fuel relays in the engine compartment.. the shiny silver ones that are universal across the other relays.

No, some of the 123 had the later type electronic relays fitted in a holder near the battery.
I do agree it is most likely to be below the glovebox against the side, most are, just wanted to pint out the other possible location.

 

[Llewelyn]

Yes, I found it. Also found a guy in America selling a complete MB manual as pdf download, at a reasonable price too.

Took the cover off the relay and found sundry bad solder joints - I've resoldered them and we'll see if it works - I've left my classy bodge in place for now but will disconnect that if it's solved.

resoldering (the whole board) worked wonders on the cruise control module, so I'm hopeful for this one, especially as I could see the problem joints in this case. This isn't the only such module I've seen with this problem: there's a well-known fault with mid-90s landrover discovery rear window module, which also has relays on a circuit board: I have a suspicion that the repeated mechanical shock of the relay operating, over a number of years, may make the joints fail.

 

[jaymanek]

Ive re soldered quite a few of these now and never had problems since... however ive had a couple that were just dead....