Could anyone explain the workings of the OVP in relation to the fuel pump relay please. My 1995 W124 e220 always starts on the button until yesterday..... When I turned on the ignition I didn't hear the fuel pump prime and I think, but may be mistaken, that the dash warning lights were slightly dimmer than normal and it wouldn't fire up due to the fuel pump not running. Had to go out so couldn't look any further but read up on fault and it pointed to fuel pump relay & or OVP.That evening put key into ignition and it started first time ! Just to check I removed both relays pulled them apart but couldn't see anything obvious, no cracked solder, burning etc.Re fitted and still ok but cannot trust it so have ordered a new OVP from Mercedes. Now how does the OVP work with the pump relay ? Does it switch the supply to the pump relay or monitor the relay whilst running ? I suspect it monitors after starting as the pump relay has a 30A fuse in it & the OVP a 15A in it.The pump defiantly wasn't running when it wouldn't start but runs fine now so I don't think the pumps at fault as they usually work or don't. Any ideas please ???
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grober
MB Master
The fuel pump relay supply is independent of the OVP which normally supplies the engine ECU, ABS unit and fast idle control unit afaik . The fuel pump relay is 2 stage-- however first priming stage is activated via the ignition switch on before cranking position. Once the engine is turning the relay receives an engine running signal from the ECU/or instrument binnacle to allow the relay to continue to supply continuous current to the pumps. I suggest you bridge the main fuel pump relay contacts temporarily to see if the car runs - this should help track down the fault. You might have an ignition switch problem.
