I know this a very straight forward procedure of connecting a direct power source to the pumps and if they go on they work and if they don't they're broke.

But...is it possible for there to be a gray area?

My case in point, I picked up a pair of used pumps that should work but when I connect direct power to them they trigger but don't run, almost like a broke solenoid in a way. They also do the same when installed in the car using the key the start.

Could this be a result of the direct power source I'm using not being powerful enough or the connection being weak? I used a jumpstarter to test with.

I was able to test the older pumps of the same model the same way and one did run the other didn't.

It's looking like the pumps I picked up are bad...

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

I ussally test them with another vehicles battery.But it does not show the ones with shorts.I just if over a 100,000 miles wear,replace them.

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1999 w140, quit voting to old, and to old to fight, a god damned veteran

They are bad ! fuel pumps can run but not deliver enough fuel. You can measure the current on the fuel pumps if they draw more current (amp) then specified they also bad. If you only use 12v on the pumps it is safe to test them directly from a battery but don't let them run for long they get hot without the fuel cooling them. You always need to be sure you got a good connection on anything electric.

The govt mandated E10 content in gasoline can cause the comutator bars on the motor to corrode if they sit too long.

Try a couple of batteries in series to get more voltage, Pos of battery A to the pump pos, neg of this battery to the pos of battery B, neg of battery B to pump neg.

If this does not work, add another battery to the string. Don't go over 48V unless you use insulated gloves , remote switch as this is the limit to work bare handed.

I recently used 120 VAC house power to jump start a pump in a tank and got it running but a lower voltage should work.

With the pump out of the car, you might be able to use compressed air to get things turning.

I'm going to take a look on Tuesday and see if I can get them working.
They were working in the car they we're parted from so it's a possibility the ethanol dried in the pumps between the time they were taken out and now.

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

So unfortunately I've still had no luck getting them to run. I pulled the whole fuel pump/fuel filter assembly off the car and took it all apart, separating the pumps. I tested them at that point and they did the same thing as before, they trigger but don't run.

I then sprayed lubricant in them and let them sit for a couple hours and the same thing happened, they trigger but don't run. I tried testing them 3 different ways by connecting them straight to the battery, using a battery charger with a start function, and using a jump-starter.

No luck using any method

Guess they are bad

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1992 Mercedes-Benz 400E
2002 Mercedes-Benz ML500

Why does this 1988, 420sel have 2 fuel pumps? What would be the symptoms if one of the two pumps was not working?

We have removed the entire fuel pump(s) assembly. I know which is the fuel filter and which are the fuel pumps...but what is this other smaller fat can, that the steel lines connect to?

I would like to paint these brackets and get new bolts. Everything was very rusty. What is a good product to use to avoid these parts rusting so badly? I will get new bolts and cover them in anti-seize, anyone have a good paint in mind that I can get in spray bomb form?

I did bypass the relay and could hear fuel at the fuel distributor but am not sure if there is enough pressure there to fire the engine. It sounds like it is working but I cannot smell any indication of it firing. Checked the ignition and seem to be getting spark. Could it be that the fuel pumps are working but not building enough pressure to flow into the combustion chamber. If this is possible...what components should I be checking? I have several spares, but they are used...In total, I have 6 fuel pumps to choose from, and it would be nice to use the best 2 pumps to re-install.

Now that they are removed from the car...i would be interested in knowing what the value‘s are to be testing the fuel pumps out of the car?

Two pumps are used on CIS / K jet injected cars because they need high fuel pressure ( 100 PSI max I think ) The pumps are in series, first pump supercharges the second.

The smaller fat can is a accumulator, it holds pressure when the engine is off. This reduces vapor lock on a hot engine. The small hose that leads back to the fuel supply hose is the non pressurized side of the diaphragm. On older cars they was open to atmosphere so the results when it failed are predictable.

If one pump on a 420/560 sel dosent run it can stall or bog, and usually pump noise will be loud or growl, if the front pump dosent work hard starting stalling occurs, if the 2nd or rear pump dosent work it may still run but lacks throttle responce,bogs, test amp draw normal about 3-5 amps , bad 6-8 amps each, make sure positve battery post small red wire is tight, if its loose fuel pump relay wont turn on, good pumps turn on as soon as power is applied

Thank you, 97 SL320.

So, is it possible that one of the fuel pumps is working enough to cause an audible fuel swishing sound at the fuel distributor....but, because the OTHER (2nd) fuel pump is not working and therefore not building up enough pressure (100 psi) to reach the proper psi to fire the injectors?
I get a small squirt of fuel when the fuel-line fitting is cracked, after presurization, but not much.

I guess that might mean that at least the accumulator is working then?

And maybe one fuel pump is bad? ...then...,one pump is good, which is why it sounds like it's working? I can test them individually,now that they are out of the car. I will wait until some of the fuel fumes clear before sparking up the battery charger. Stale fuel really stinks!

I can see now how each post on the pump is marked pos. and neg.

So, 12 volts to the pos. post.?

Does that fuel distributor need to be at a constant pressurization to work, then? Is that right? I don't know?

Does the accumulator keep the pressure constant during the pulsing from the injectors constantly firing off some of the pressure?

Then would the fuel pressure regulator keep certain that only 100psi goes to the distributor?

Is the f.p regulator the small can (similar to the accumulator, but smaller) up next to the fuel distributor itself?

I was also reading about these check-valves located at each end of the pumps....I wonder what would happen, if one of those check valves stuck closed? How are they tested?

What is the purpose of the check-valves at the fuel pumps?

Is there suppossed to be 2 of those c\valves then?

I can see how they are piggybacking one pump, to the other, "supercharging the psi, as you said above". It is obvious the way they are plumbed, makes sense, now that you mentioned it.

What are the insides of these fuel pumps made of? Could we bench test them and run some marvel mystery oil through to wash the varnishy fuel out of them?

Sorry, that last post took awhile to write and I did'nt see this until after the last post.

Your explaination of this sounds about right. Before this car quit running...it would stall as soon as we put it in gear. Now, it won't start at all. Cranks but no go. I can hear something working when the key is turned on...but not the usual whine that it used to do, when it ran.

Could it be that I have had a fuel pump go out on me right after a camshaft swap, and my problem has been the fuel pumps the whole time?
I'll test those pumps tomorrow.

How do I do a current draw test. I have a multimeter, but which setting?
Thanks, again.

If you suspect poor or bad pump, test pump output you ned to get over 1 litre of fuel in 20 seconds, disconnect fuel line at fuel distributor bridge f/p relay
with line in container 1litre in 20 seconds if not , find out why, the fuel regulator,accumulator, or leaking imjectors will cause hard start after car shutdown, the cis system relies on correct pressure to work correctly , bosch
has an excellant book explaining how cis works , recommend reading book
to understand this system, its reliable and easy to work on , testing takes time , you need a gauge to diagnose , but here will try and talk you thru.

Thank you. Can I use marvel mystery oil or some light oil like air compressor oil? Maybe diesel? I don't really want to use gasoline. The pumps have been removed from the car already.

I guess, if I use something other than gasoline then the flow test for 1 litre per 20 seconds would'nt work cause of viscosity? Would diesel have about the same viscosity for about 20 degrees celcius ambient temperatures? 68 farenheit.