m103 mas relay problem
 

hi,the fuel pumps on my 1990 300sl 12v will not run when turning ignition on

or cranking.i removed the mas relay and jumped pins 1-2 and car fired up

no problem.i gave the relay to a friend who is expereinced in electronics etc

and on opening up the relay he has found no visible problem.

does the mas relay require some kind of signal as well as 12v to fire the fuel pump.

would it be a tdc signal or maybe the crankshaft signal needed to be sent to the mas module to tell it to run the pump.

i also noticed when i bridged pins 1-2 with the mas fitted or not the car runs but the rev counter does not move.

any help would great thanks.

The pump should run for a few seconds when the key is turned on, so RPM doesn't factor into that. Make sure the MAS is powered up with the key on, ignition switches give trouble in those cars and may be keeping the MAS off line.