Speedy:

Pressure switch for the overboost protection system. Opens a solenoid and vents the boost signal to the aneroid to cut the fuel off when boost pressure is too high (and one of the reasons that aftermarket boost controllers don't always make a big difference in performance!)

Should be set for 0.9 atm (13.8 psi), factory max boost pressure.

Don't disconnect it to get more boost, you can instead blow something up!

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!