Does the MAS relay die suddenly or have a slow death?
 

Hello: I've been having starting issues with the M103 in my 1990 300SE. It's been intermittent, but today has been really bad. I cleaned the relay and its contacts and the engine ran for about 5 minutes until it shut off again.

I'd like to know if the MAS relay dies over time or if it just dies suddenly. The relay in one way or another seems related to my current problem. I want to try to properly diagnose it before I just buy another relay. By the way, I've also been having problems with my a/c alternately blowing cold and warm air.


Thank you for your replies.

So I jumped pins 1 and 2 and the car starts and runs, albeit rough. Does that confirm the relay is bad? With the jumper in the engine runs at a rough low idle, around 500-700 rpm. Is that because of the MAS relay not being installed at the time? In other words, does the MAS also control engine idle...?

Also, can I drive the car at all while the pins are jumped and the relay is out?

The mas died in mine suddenly when I accidentally shorted the fuel pump trying to fix a fuel leak.

Well, that certainly would have been a quick death for it. However, it was aided by you. I'm talking about its death under normal circumstances like natural causes.

Normally it has a gripper and dies.

I'm going to need a translation on that one. A "gripper"?

I just messed with this on a '91 300SL. The ECU controls the idle, but if it thinks the MAS has gone missing, it probably goes into a limp mode, fully retarding the timing and cutting off all power to the idle air valve. The MAS also powers the AC compressor.

Grabbing one's chest and keeling over from a heart attack. :D Good o'l fashioned kicking the bucket. :D

Thanks guys, but it seems mine was dying a slow death. There were earlier symptoms that I recognize now. I did jump pin #'s 1 and 2 to get the car going. I've ordered a rebuilt one as I want something as important as that relay to be reliable.