OK so here goes. I have owned a 300E-24 Saloon since I was 18. Some 14 odd years later the same problem has re-surfaced for the 3rd time. (I found 2x old MAS relays tonight from this car) It kills the fuel pumps or rather they become very intermittent to the point where it won’t run. Out of the blue recently it did it again and wouldn’t start. I couldn’t hear the fuel pumps so I knew the age old issue had cropped up again.
These little relays are expensive from MB (If they are still available I haven’t checked) and expensive used too. I am reluctant to dive into another used unit as it may have the same fault waiting. Sooooo.... I opened the current one up tonight and the circuit board inside was super clean with no issues visible to my eye. I jumpered the fuel pump pins and the car fires up fine only that the engine speed tachometer does not function.
Now – if I wind the jumper wire around the fuel pump pins on the MAS relay the car starts up just fine again and the Engine tachometer does work again as normal.
See pic below for what I mean.
What I am thinking now is wiring in a new fuel pump relay which will function independent of the MAS relay to bypass that fault inside the unit. This would be a replaceable fuse topped relay with it’s own grounds and ignition feed. Do any of the CIS experts see a reason why this might be a very bad idea?
I know – buy a new one you cheap $$@# I will later in the year maybe but my fleet is draining me currently!