What is the MAS relay Function?
I'm wondering what effect adjusting this might have on my 1991 190e 2.6.

Thanks guys!

Perhaps you should try adjusting it and see what happens!

Just kidding. If we're talking about the same MAS relay (the shiny metal one with the knob on top), it's responsible for fuel pump activation/cut-out, as well as A/C compressor control. The knob on top just locks it into the electrical connector.

Also, it provides current for 02 sensor heater, controls air injection pump, checks engine coolant temp, controls start valve and compares engine speed and AC compressor speed. It is also aware of the vehicle speed and when the kickdown switch is tripped.

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2012 E350
2006 Callaway SC560

And your wallet takes a beating when you have to replace one of these "jewels".

As stated above, this silver box controls many functions and if any of them go south, you get to replace the MAS. I prefer the segregated systems that the '88 and '89s had. MB switched to this in '90.

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"