1972 280se 4.5 fuel pump die today - PeachParts Mercedes-Benz Forum

guinea13 · #1 01-19-2012, 10:28 AM

Hello everyone. I have recently become the proud owner of a 72' 280se and have been going through little by little. Driving it and finding little issues to fix. Now I have run into a big one which I can't seem to figure out.

I have been trying to figure out a misfire issue in the 4.5 engine and have been tracing points, fuel injectors, fuel pressure, etc... and never finding the issue. I have even stumped a Mercedes mechanic with this one. Today I went to drive my 280 to work and the pump just turned off. I am thinking that this is not a coincidence. The fuel pump was working great pushing out about 32 psi. I am not sure if it is the relay of if the misfire/pump issue is due an ECU issue.

Any thoughts on what is could be or where I could get a spare ECU?

sjefke · #2 01-19-2012, 12:35 PM

Start with the connections on the pump and see whether you get power at all. If you don't get power (12V), check the fuse. If you do get power at the pump, then the pump's grounding circuit (relay + ECU) may be problem. Make sure the pump is not internally toast. Having taken two apart in the last two weeks, they seem to be built to last, so you may have an external (wiring) problem.

You can connect a switch and ground wire to the appropriate plug of the relay to manually ground & close the relay. If that gets the pump humming again, at least you can drive. If you suspect the relay, you can swap out the master relay with the fuel pump relay. They are both the same and interchancgeable so if the fuel pump relay is bad and you plug the master plug on the fuel pump relay, nothing should work.

If relay is correctly working, a big test is to see whether the ECU does the required 0.5 second fuel pump activation if you switch the car on contact. If you don't hear anything, it could be ECU not giving ground signal to relay.

I had a similar fuel pump issue (but no misfiring, etc) and it was the ECU after all. Werminghausen also had problems with the fuel pump on his 300SEL3.5. Search for 3.5 fuel pump and/or ECU with my user name and you should see the threads with all the details on how to check and trick the fuel pump in working.

Good luck,

Bert

guinea13 · #3 01-23-2012, 05:57 AM

Quote:

Originally Posted by sjefke

Start with the connections on the pump and see whether you get power at all. If you don't get power (12V), check the fuse. If you do get power at the pump, then the pump's grounding circuit (relay + ECU) may be problem. Make sure the pump is not internally toast. Having taken two apart in the last two weeks, they seem to be built to last, so you may have an external (wiring) problem.

You can connect a switch and ground wire to the appropriate plug of the relay to manually ground & close the relay. If that gets the pump humming again, at least you can drive. If you suspect the relay, you can swap out the master relay with the fuel pump relay. They are both the same and interchancgeable so if the fuel pump relay is bad and you plug the master plug on the fuel pump relay, nothing should work.

If relay is correctly working, a big test is to see whether the ECU does the required 0.5 second fuel pump activation if you switch the car on contact. If you don't hear anything, it could be ECU not giving ground signal to relay.

I had a similar fuel pump issue (but no misfiring, etc) and it was the ECU after all. Werminghausen also had problems with the fuel pump on his 300SEL3.5. Search for 3.5 fuel pump and/or ECU with my user name and you should see the threads with all the details on how to check and trick the fuel pump in working.

Good luck,

Bert

No voltage at the pump, the relay clicks. I checked poll 86 and there is 12 volts. I checked poll 85 and there is no ground. I went and checked the ECU on line 19 and there is ground? I have no idea.

Does anyone know how (which polls) to jump the relay to run the fuel pump. I need to get the car in the garage for now to work on it.

sjefke · #4 01-23-2012, 10:26 AM

You should have voltage at the fuel pump when you put the car on contact, independent of relay and ECU. The relay closes the fuel pump ground circuit. The ECU closes the relay's circuit by providing ground to the relay. So if you have no voltage at the pump at all, you should check the fuse (no 5 I think, but I may be wrong). Try connecting the plus of the fuel pump to another 12V source and see what happens.

Good luck,

Bert

guinea13 · #5 01-24-2012, 09:06 AM

There was no voltage at the pump. Fuse was good, which made me think there was somthing wrong at the fuse box and I was right. No power was getting to the fuse box (only on number 4 fuse). When I jumpered number 5 fuses power over to 4 the pump came right on. Now I get to figure out why I do not have power to that fuse. GREAT, I get to trace wires.

guinea13 · #6 01-24-2012, 09:30 AM

Well figured it out. Did a little research and found this post:

Quote:

Originally Posted by Tomguy

Often the cause is corrosion on the single small cable on the (+) terminal at the battery (or that it's just come right off). If that's fine, you can get to the relay by pulling out the ECU mounting board. It's a pain in the you-know-what, so I would check that terminal first.

Snipped the cable and reconnected and wolla it works. I wonder if the misfire will stop now. Only a test drive will tell. I am not going to bet the farm but we shall see.

sjefke · #7 01-24-2012, 10:01 AM

If it is that little cable directly from the + terminal, that one goes to your master relay as far as I recall, and cleaning it could solve some other problems too.

Glad you figured it out. Took me a while too the first time.

Bert

guinea13 · #8 01-24-2012, 11:22 AM

It did!!!! Seems like the misfire went away from the short drive I took. Transmission is back to the way it was about 5 months ago also. I took both cables off the power and cleaned them. Helped alot. Figured it would fix the heater which stopped all of a sudden but it did not. I will save that for another day.