How to diagnose a bad fuel pump relay - 91 560 SEL
 

My car stalls intermittently in park or in gear. I jumped pins 7 and 8 on the fuel pump relay and the problem disappears. I wanted to make sure that I am not missing anything as I do not want to misdiagnose the problem. Is this jumper test on the fuel pump relay a definite indication of a bad relay or are there any more tests that are needed to make sure there is no misdiagnosis of the problem?

I say it would be ,or run power source directly to fuel pump

When I diagnose a problem like this I find the cost of the part ($70+/-). Then I look at how complex the system is. I say to myself, which is cheaper, the part or the labor cost to diagnose the problem. I then do which ever is less. I can say that the fuel pump relay system is more complex than it looks. It requires an o'scope to do it right.
Now If I had the car, I would put a relay in it. Actually, I would resolder the circuit board first. I had the problem on my 500SEL about 10 years ago, resoldered, and still going.

Paul

three spares i always have in my trunk fpr klima ovp

problem not resolved yet. I replaced the fuel pump relay with a new one, but the still the same problem.
*with the relay installed, it would idle fine (voltage at fuel pump=13.3V) for a while then goes to rough idle (voltage at fuel pump=12.5V) and finally dies. Both fuel pumps also continues to run while idle is rough
*Without relay installed and with pins 7 and 8 shorted, the car idles smoothly. I had the car idle for an hour and with no problems.

I appears that some other part upstream supplying the fuel relay maybe causing the problem. Any ideas?

Is this a simple fuel pump relay on-off or is it a kind of klima relay where you have some speciality function like klima control (air-conditioner cut off) or emergency cut off that's why the car idle when you bridge the relay you bypass the safety system. If it is a multi purpose relay try to find out what function the relay has.

I may as well add a set of fresh brass tipped fuses to replace all the eroded ones causing intermittent faults

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One important signal to the FPR is TD(RPM). When the engine starts running rough, what is the tach doing? The TD signal comes from the Ignition Control Unit which also feeds the tach, Idle control unit, and probably some more things.

Paul

I had similiar problem , was the ezl was bad, was intermittantly losing voltage from ezl to f/p relay= the td signal, with voltmeter on ezl td signal found low voltage, changed ezl with known good one and problem gone, i too had changed f/p relay as that usually is the fix.

problem resolved finally. It was a bad O2 sensor. It was very challenging to diagnose because there were not error codes. No Check Engine warning light. After many hours of going through the schematic diagrams, it became apparent that the cause was in the path that included the o2 sensor. The FPR and the coolant sensor were ruled out.