W111 3.5 fuel pump dead

[sjefke]

So here I was driving home from work when suddenly my coupe started dropping in speed. A quick look at tach showed 0 rpm, then 2000+ then 0 and when I hit the clutch, engine was dead. Rolled into parking lot and engine cranked but would not fire. Checked distributor lead, cleaned points, but symptoms seemed to be no fuel because engine would sputter just a tiny bit, so sparks were flying and injectors were opening.

Disconnected fuel hose, and yep, no fuel squirting, just some seeping. Tapping on fuel pump under car did not work and when traffic died down, I also did not hear the short humm from the fuel pressurization when engaging the ignition key. Of course this is all in my nice clothes so crawling under car to check electrical was not really an option (but I was tempted).

First time it let me down on the road since I bought it. AAA tow truck was able to back onto our hilly drive way and roll the car into the garage, so I can get to work on it.

All seems to have the symptoms of a bad fuel pump, but given that tapping also did not produce any results, I will first start with checking whether the pump gets any electricty at all (bad leads).

Anybody have any specific thoughts on what common fuel pump failures are for this model (W111 280se/c 3.5)?

Is there a relay/fuse for it? If so where? (I know it is in the manual, but asking you all is a lot easier )

If pump is bad, can these pumps be rebuild (by me)? I may just order a new one anyway, but love to rebuild the old one just to have a spare.

Thanks,

Bert

P.S. Never realized how heavy that car was until we tried to push it in a parking spot. Steering is a nightmare too without p/s.

[Tony H]

Hi Bert,
Sorry to hear of your experience. Not sure if this is to obvious but did you connect a 12v source directly to the fuel pump to see if it runs? I took one apart from a 108 4.5 which is very similar and they seem bullit proof. I think they are VERY expensive but you might be able to adapt one from a 4.5 if you can find a used one. The motor is crimped together but if you find it bad I see nothing to lose by taking it apart. You might be able to replace the brushes and seals and put it back together.
Good luck

[GGR]

You should start by checking the relay. It is round aluminum color somewhere in the engine bay. Mine is front left just behind the injection calculator, but my car is an 220 transformed into a 3.5. Your calculator must be in the front right, so look into that area. Once you located it take it out. Turn the start key to contact position and then establish a contact in the relay socket between where plugs 30 and 87 go. If you can't hear your pump runing, pull a wire from battery directly to the pump. If it still doesn't run, check earth at the pump. If it still doesn't run, dismantle the pump, repare or replace.

If you can hear the pump running when you bypass the relay, then dismantle the relay, clean the contacts and make sure it works properly (mine was dirty and for some reason contacts were not close enough to enter into contact. I had to bend a bit one of the arms). If the relay is still not working after repair or replacement, it then becomes a bit more complicated, as it is controlled by the calculator.

[alabbasi]

Bert

The 3.5 and 4.5 pumps are the same. As a matter of fact, all fuel pumps used on D-Jet cars should work and that would include VW, Porsche and Volvo.

These pumps look exactly the same with the exception of the electrical connections which i think on the other cars is a spade connection.

Tony suggest the right first step which is to connect a 12v source to it and see if it runs. You can use a jumper box and two screw drivers to make the connection.

I've found that python rebuilds these pumps and that rebuilt ones can be had for about $250 or less. But I have not seen a rebuilt MB unit (just porsche, vw and volvo).

Check with advance auto parts as they normally stock these and they have a huge warehouse just off I20 near Austell.

If the pump does not run, it may be seized. The do come apart easily but you'll need new o-rings to seal it back up. Basically the pump comes apart into three pieces, the first piece is the cover where the impeller is located and then the second part is where the motor is.

[alabbasi]

Looks like advance has the new fuel pump for $210. Looks exactly the same as the Bosch unit but made by Airtex. May be rebuilt.

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Fuel-Pump-Airtex_18770072-P_52_R|GRPFUE2AMS_936379858___

I could not find it when looking for your specific model but it came up for a 1974 450SL. Sometimes you have to chop and mix to get to the part you need but that my friend is a deal.. a steal.. the sale of the century (quoting lock stock and two smoking barrels)

[Tony H]

The 4.5 pump is not the same as a 3.5 pump. The 3.5 pump has 3 connections on the front and the 4.5 pump has one connection on the front and one connection on the side of the motor. This is based on the fuel pump I removed from a early D-jetronic 450SL at the pick and pull compared to mine so I could be wrong. I will try to look at both tonight.
Tony

[sjefke]

Guys, thanks for the pump pointers. I was planning to do exactly what you suggested, but work has gotten in the way (how do you guys have all that time to work on your cars???) plus we are leaving for vacation tomorrow.

Hopefully this afternoon I can crawl under it and test the voltage. Having worked with electric fuel pumps on MGs, I have my suspicions that the cause is outside the pump, because a tap with a hammer usually will cause some reaction from the pump unless wires have broken internally.

Tony, please check how 4.5 and 3.5 pumps differ. If it is "just" a flexible hose or electrical hookup, it should be OK, but I'd still like to know the difference.

Pictures would be nice.

Thanks,

Bert

[sjefke]

Update: I was able to crawl under the car and just as I thought: pump does NOT get voltage. No readouts from any leads. Plopped a battery under the car, touched the pump poles and, voila, as the French would say, pump started humming.

So, now it is back to getting the wiring diagram out and tracing it back. Relay is next stop.

And Tony is correct: 3.5 pump has 3 hoses on top side, electrical leads on the side. Pictured 4.5 pump from Al is different (but does not mean it would not work). I'll post picture later.

I'm not sure I can get it fixed before going on vacation, but repair cost just dropped significantly I may still look for a spare pump. It's an easy thing to change at the side of the road but can leave you completely stranded.

Bert

[Tony H]

Bert,
Glad to see it's not the fuel pump. I guess you don't need the pictures at this point but I already took them so here is a 3.5 fuel pump. It has 3 ports labeled S D R presumably suction, discharge and return. Here is a partially disassembled 4.5 fuel pump but you can see that there is one port on the front and one port on the side. As Al pointed out I'm sure any fuel pump with the correct pressure and flow will work if you figure out how to connect the return line.
Tony

[sjefke]

Tony, I have fuel pump envy. Attached is a picture of mine. I want your shiny new one

I think I know where the relay is, but will start a new thread for confirmation.

Bert

[Tony H]

The "shiny new one" came from a 3.5 coupe that someone threw a lot of money at trying to get it to run. They eventually gave up and it ended up at a dismantler. The one ON my car looks just like yours. Nothing that some walnut shells can't take care of. BTW I have the factory wiring diagram if you need I could scan it and email it to you tomorrow.

[sjefke]

Unbelievable what people throw away!

I have the wiring diagrams too. I looked at workshop manual as well as service manuals, but could not find fuel pump relay position.

Bert

[GGR]

Check also the fuse, it's N°4. If it looks good turn it in its fittings, sometimes cotact gets bad there.

[sjefke]

Originally Posted by GGR

Check also the fuse, it's N°4. If it looks good turn it in its fittings, sometimes cotact gets bad there.

Already did. Voltmeter shows power on both sides of fuse.

Thanks,

Bert

[AgSilver]

Check out Mallory http://www.malloryperformance.com/

Their pumps never fail, they are better designed and far less money. It takes very little effort to modify the installation. There are pressure ranges for all applications.