Fuel pump relay 500 SE

[Svein R]

Hello guys, My car is a 1992 500SE european model.

I parked the car in my garage and everything was fine, the next day when I came to drive it out the garage it wouldn`t start at all. When I switch on the ignition there are no buzzing noise from my pump, I have followed all the troubleshooting procedures from Mercedes workshop manual and I came to the conclusion that it was my LH module that was faulty. I sent my LH module to a company here in Norway that is supposed to be good at car electronics, they have tested the module and found it to be fine.

My pump and car starts fine when I bridge the relay socket, I have also replaced the relay, but when I turn the ignition on it wont engage the relay, do you have any ideas of what the problem could be?

 

[gsxr]

Svein, do you have ANY other spare LH module from another M119 to test with? Doesn't have to be from a 140 nor a 5.0L engine. If you have one, swap that in for testing.

Based on your description I'm not convinced your LH module is working properly, despite testing as "fine"...

 

[Svein R]

I have tried another LH module and it’s the same

 

[gsxr]

I have tried another LH module and it’s the same

Hmmm. That is odd. First check all fuses in the GM/BM module. Maybe even swap in a different BM/GM if you have a spare.

Next up is checking the terminals / wires between the relay socket and the LH module. Relay K27, terminals 86 and 87. Also verify there is +12V at Z7/15 being sent to the relay socket (terminal 86 on the relay). Here's the schematic:

https://www.500eboard.co/forums/misc/Manuals/Servicev3.2/W124/w124CD1/Program/ETM/24075002.pdf

 

[Svein R]

Okay thanks, but a couple of questions just to make sure I understood you correctly. When I turn on my ignition, which pin should there be 12V + on the LH module? Which again powers terminal 86 on my relay socket?

 

[gsxr]

From the schematic, it appears that when the ignition is turned on, there should be +12V sent from the BM/GM to connection Z7/15 (inside the wire harness somewhere) which then goes to LH module connector #1, terminal 36.

The LH module then grounds terminal 25 to energize the relay.

Also see the LH pinouts here:

https://www.w124performance.com/docs/mb/M119/M119_LH_pinouts.pdf

My guess is that the 12v is missing at the relay terminal 86 (socket 4)... again, just a guess. If so, you need to figure out why.