Mercedes W140. Unknown issue. What's wrong?

[alexS320]

Hello 500eboard users,

I own a 1994 Mercedes W140 S320, M104.994 engine with HFM injection.

VIN: WDB1400321A225744



Recently I connected my tester to vehicle's 38 pins OBD I port, and I could delete all the errors I had except for one stubborn error: Fuel pump relay open circuit/short circuit.


The car starts and runs fine, but me and my dad wondered what's the reasoning behind this error. Then I went in the trunk to check the fuel pump relay (K27) at the fuse box, and I saw this workaround made by someone in the past:




If I take off the alien yellow and red wires altogether with their yellow 20A fuse, the car cranks but the fuel pump won't start at all. These seem to be helping vehicle's fuel pump to start.

With key in position 2, workaround taken off, and fuel pump relay disconnected, I could only receive 9V on relay pin #6 (red/blue wire). Theoretically, connecting the fuel pump relay too must show an even lower voltage than that, not good. Module N3/4 isn't sending the correct command? Could it be shorted in the past by some biodegradable wiring harness?

My immobilizer is OK, it closes and opens the car as it should. No green/red flashing door lights.

My ignition switch seems good too. There are 2 fuses on my Base Module (N16/1). One always has 12V, the other one only has 12V with key in position 2.

Fuel pump & its relay are functional.

Engine upper & lower wiring harness, ETA wiring harness, MAF wiring harness have all been replaced already.

Then I proceeded to open up module N3/4:






Any suggestions?

 

[alexS320]

A similar issue was debated here 1 year ago, but fortunately for the owner he only needed a new fuel pump relay. Mine is in good shape, so it's something else causing this.

Thread 'Help with jerry-rigged Fuel Pump Relay wiring'

Jul 10, 2025

Hi all.

Recently purchased this car 1995 E420 - I know its the E500 forum, but this is a similar setup and I'm talking to a knowledgable and active group.

The person whom I bought the car from said that his friend replaced the fuel pump... Implying there was some fuel issues prior to selling.

Anyway, the SYMPTOM is I can hear fuel circulating in the fuel lines under the hood when the key is on the "ON" position. It keeps going, it doesn't just prime and turn off (what I assume is normal).

The rear seat was loose, so I discover this. Turns out, that's where the fuel pump relay is...

• sytruong
• Replies: 15
• Forum: Electrical, CAN and Wiring

• 

@gsxr @gerryvz @sytruong is my N3/4 module shot?

 

[alexS320]

This is how my wiring is configured to the fuel pump relay:

Pin 87 (#1 Red wire) - Permanent 12V

Pin 30 (#3 Red/Black wire) - goes to fuel pump

Pin 85 (#5 Brown wire) - ground

Pin 86 (#6 Red/blue) - with key in position 2, and fuel pump relay disconnected, it only gives 9V. Car cranks but pump won't prime and go, without the workaround.



Then I checked two W140 M104 HFM diagrams for the wiring order inside fuel pump relay socket. Diagrams are for years 1996 and 1997 because I couldn't find any diagram for 1994. Are there any reversed pins on my socket?

 

[gsxr]

You'll need to look up the HFM schematics and figure out what is supplying +12v to socket #6.

If it's anything like the M119 V8 engines, this terminal only gets power for ~2 seconds when the key is turned on, to prime the pumps. Then the pumps should also run during cranking, and with the engine running. But the pumps will not run continuously with the key on and engine off.

 

[alexS320]

You'll need to look up the HFM schematics and figure out what is supplying +12v to socket #6.

If it's anything like the M119 V8 engines, this terminal only gets power for ~2 seconds when the key is turned on, to prime the pumps. Then the pumps should also run during cranking, and with the engine running. But the pumps will not run continuously with the key on and engine off.

I followed module N3/4's pin #29 through the circuit board with a multimeter (set on buzzer) and it led me to a Siemens BTS 611 integrated circuit. I don't know its exact condition yet, I still have to do more testing today. It can be replaced, or even repaired in some cases, but I don't know whether it's conditioned by something else.

Yes, similar to M119 V8 and many other engines out there, M104 should prime the pump briefly when the key is turned on too, then turn the pump off until the engine is started. BUT in my case the pump runs continuously with key on and engine off until I take the key out. That workaround showcased in my original post was connected to an ignition-hot wire that wakes up the pump altogether when key is in position 2 and engine off.