LP fuel pump not working

F

fritz

Member
My 2006 118 wont start.. after some great reading here on the forum I suspected the "in tank" LP fuel pump. I disconnected the supply fuel line that runs from the tank to the filter and turned the key on... no fuel comes out of the line. OK... then I read how to jump the relay, so I jump the #30 and #87 pin and wala diesel fuel comes shooting out of the line into the bucket immediately!!! I quickly unplug the jumper wire...!!! OK ... I hook the supply line back up to the filter and tighten the clamp, then I jump the relay again and quickly try to start the engine... it starts immediately... I let it run for a few seconds but seems to run perfectly. I'm cautiously assuming it's the relay and the actual pump is OK. I ordered a new relay and will update after I get it installed.
Thanks to all for this great Forum.
 
Nautamaran

Nautamaran

2004 140” HRC 2500 (Crewed)
Nice work.
If you have a volt meter you may wish to confirm the control signal is reaching the relay socket?
You should see (switched) 12 volts across the control pins 85/86 on the relay socket with the key “on”.

-dave
 
F

fritz

Member
well thank you guys... This Forum deserves all the credit... I will read up on the S108... thank Vic... will also put a meter on the relay socket..
thanks again.
 
autostaretx

autostaretx

Erratic Member
You can simply swap the starter and pump relay to see if the pump then works.
... per Vic (see below)

--dick
 
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2

220629

Well-known member
autostaretx said:
You can simply swap the starter and pump relay to see if the pump then works.

--dick
Click to expand...
Nope. They look similar, but are quite different.

The starter relay is higher capacity and has special large blades. The fuel pump relay is the standard amp configuration.

Depending upon the options list, there may be a number of non-essential 87 relays under the driver seat which could be swapped to verify operation. Eg. - Horn, Electric Cooling Fan.

https://sprinter-source.com/forum/showthread.php?p=221060#post221060

:cheers: vic
 
autostaretx

autostaretx

Erratic Member
Aqua Puttana said:
Nope. They look similar, but are quite different.

The starter relay is higher capacity and has special large blades. The fuel pump relay is the standard amp configuration.

Depending upon the options list, there may be a number of non-essential 87 relays under the driver seat which could be swapped to verify operation. Eg. - Horn, Electric Cooling Fan.

https://sprinter-source.com/forum/showthread.php?p=221060#post221060

:cheers: vic
Click to expand...
Auto parts stores have the single pole, double throw relays for $6

--dick (who needed one for a non-Sprinter project)
 
F

fritz

Member
ok.. I got the new relay , plugged it in and the truck started right up. only thing a bit strange is the new relay is black and not yellow like the original and the number is different... 4RD 007 790-00 on the old one... and 4RD 007 794-02 on the new one. Both Hella 12V ... Also the second row of numbers is scratched off completely on the new one...??? Looks like someone took a pocket knife or some kind of scraper and carved them off the top of the relay.. kinda strange but I guess I trust Europarts.
Thanks again to all for this wonderful Forum.
 
autostaretx

autostaretx

Erratic Member
To slightly translate that chart: the 794 has a 100 ohm coil BUT also has a 680 ohm resistor in parallel with the coil... (net resistance: 87 ohms, which does match the other relay)

The job of that resistor is to "eat" any back-EMF spike when the relay is de-energized (the collapsing magnetic field and moving armature induces the spike). This is a "good thing", and MB frequently does that.
The 794 also has a higher "resistive/lighting" load rating for its contacts.

As for the scraped numbers ... there's a tiny chance that Hella uses the same plastic cover for two models, and just scrapes off the "other" number. Or it could be a counterfeit relay. (altho counterfeiters usually try to look "more pro")
update: i fully agree with RJV's take on the matter.

--dick
 
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R

RJV

Active member
fritz said:
Also the second row of numbers is scratched off completely on the new one...??? Looks like someone took a pocket knife or some kind of scraper and carved them off the top of the relay.. kinda strange but I guess I trust Europarts.
Click to expand...
One of the parts I recently installed(I think either the water pump or thermostat), which was ordered from Europarts-SD had some kind of marking(assuming a number or logo) ground down to flat with a Dremel or similar. I had read here somewhere on this site that someone else had the same issue and I believe the general theory is that the parts are OE/OEM. When they are manufactured they are done so with the Mercedes logo/part number so that the parts maker can supply Mercedes. I don't believe that legally any parts can be sold with a Mercedes marking unless sold by an authorized Mercedes dealer. Whether the parts are made with the extra markings to save the OEM the effort of a 2nd production run of the same parts without the markings OR if it's because the parts were returned from Mercedes for some reason--I'm not sure. But I agree that Europarts-SD knows their stuff and I wouldn't worry much about it.

I can't be sure that this is the case with your new relay but it sounds likely.
 
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