At the top of all of this, sparks + fuel + oxygen = Boom.


Dig around for a "How Bosch CIS / K Jet works" manual. Some of the Bosch factory sites have these for free. It will answer lots of questions.


Accumulator is just there to hold pressure during hot soak / allow for quick starts. The only failure mode is the diagram fails allowing fuel to leak to the rubber hose side ( it is "t" ed into the rubber hose leading to the fuel tank.

Car should at least start even with a leaking accumulator.

Old fuel can lock up a pump, how long was this car sitting? You can pull the pumps and test each one for operation without hooking up a fuel supply, just be ready for fuel to squirt out. Running for a few seconds is OK.



+ 12 V to the + Pos post , ground to the - Neg post

Fuel dist needs pressure to work, see the Bosch book. Injectors on a CIS system don't pulse, they constantly spray with volume adjusted by air flow flap / fuel distributor.

See Bosch book.

Check valves are there to prevent fuel pressure loss when engine is off. They could stick closed but I've never encountered that, they can leak and lose pressure with engine off.

In general, fuel pumps are made many ways, roller vane , flat vane , geo rotor, screw, turbine. Late 60's Bosch D jets were usually roller vane. The 97 E320 pump I tried to rescue was a rotary screw.

Old fuel is a mess, you might have to rig up a external fuel tank, fresh fuel mixed with carb cleaner and hot wire the pump to let things circulate for a while. ( engine off ) just be sure to disconnect injector lines and route them to a can so you don't hydra lock the engine. You can also blow air through the injectors then use spray carb cleaner in the inlet to let them soak.

Lastly, the diagram in the fuel distributor fails, someone on e bay is making rebuild kits with detailed instructions.

We managed to hook the battery charger to each pump and they both run. I guess it is time to do a current draw test. I need to find out how to do this and what the specs are for these pumps. They are both Bosch....one is part#0 580 254 950
014 made in Germany
688040021

The other is part#0 580 254 950
014 made in Germany
688070009
Car is 1988 420sel

Really great info there. I wiill try to find that manual. Thanks.

The car I am working on has only been sitting since the problem happened with the lifters which required a cam change. After the cam change, I backed it out and when it was put into drive, started bogging everytime I gave it some gas. I hobbled it to where it sits now. I went back the following day and it would'nt even start. I had a thread going, where i thought i had made a mistake with the cam swap, but am now thinking it is a fuel issue.

Sorry for the confusion, but i had two other 420 sel's parts cars, that i got spare pumps out of, in case the one we are working on had a bad pump. My spare cars sat for a very long time. The spares have all been removed from their respective cars....however, each pump still had fuel in them that poured out. I now sit with 6 pumps total sitting on the bench, including the pumps from the car in question. I tested both pumps from the car we are working on...hooking it up to a battery charger, and they run.
I would like to do a current draw test that the o.p was talking about.

Both pumps are the same, the 688070009 ish number is a date code or something like that. No idea what 014 is.

The pump will have a low amp draw if you test out of car since it isn't pushing against anything.

Do a amp draw test when everything is put back together, they should have about the same draw. Pumps generally fail in a no run mode, sometimes they shear the coupling between the motor and pumping element. In that case the amp draw will be low and motor speed fast.

Firstly I tested the pumps that came out of the car we are trying to get running again, which both happen to run with power applied. 1st got 1.1 and the other got 1.2.

Then i tested 2 of these spares that had been sitting with old fuel. One DID turn on with power applied and went from 1.3 to 5 and as the numbers increased the pump slowly shut off at 5. I disconnected and applied power to the second spare. I got no function, so I did a current draw test on that one and got 6.98. I could feel it vibrate but no function.

What does this mean? I don't know since i don't have specs for the pumps...but through testing these 4 pumps...it appears that a higher number is no good and the pumps that came out of the car are in fact good????

I used the multimeter setting 10a and tested the pumps directly to the terminals on the pumps themselves since they are all out of the cars on the bench.

Anyone know what numbers these pumps should be giving if they are good?

The 1.1 amp readings for a no load pump would be what I'd expect.

The 5 amp unit is probably pushing through gummy fuel and the other one locked solid.

Just to be sure of the hookup, the meter is set to amps, positive test lead on meter side moved to "amps " socket, one wire lifted from pump, one meter lead hooked to pump, other to the wire you just removed. For a digital meter, polarity does not matter, you may get a negative reading. For an analog meter , if the the needle drops below zero, reverse the test leads.

I hooked up a battery charger with some small alligator clamps, to do the test. The black lead off the battery charger went to the negative terminal on the pump. The red wire off the battery charger went to the red lead off the multi-tester and the black lead off the multi- tester went to the positive post on the pump. The black is considered the common wire for this test?

I found these procedures on youtube, from a guy testing model airplane electric motors. Use caution working with fuel and sources of ignition. I did notice that although the pumps were dry...they still smelled up the place for the few seconds they were run....and there are sparks upon initial start up. Vaporized fuel coming out of that pump with a spark could probably cause an explosion, which ironically is what causes these machines to work, right...so don't go blaming me if you get blowd up.

The red lead off my particular Digital meter needed to have the peg at the multi-tester end, moved to the 10a dc post, as was previously mentioned.

I did briefly test them with no load since they were not pushing any fluid from them, so do these pumps get the go ahead to put back in?.

Wondering what numbers I would get moving fuel through? Would they be the same?

What to test next? If the car "should" start with a bad accumulator?
I wonder if I could test the accumulator by pouring fuel through one end to see if fuel comes out the other side, hence to check if the diaphragm is ruptured? that's easy enough to check?

Maybe the wiring harness back there is damaged? I obviously know where the pump end of the wiring harness is...but where does the other end go? I see it goes up inside the car.

On the w126 cars...does the fuel pump wiring harness enter the car behind the rear seat or behind the fuel tank in the trunk?