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Old 09-07-2007, 06:51 PM
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2.5Turbo 2.5Turbo is offline
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Join Date: Feb 2006
Location: Tacoma, WA
Posts: 900
There's a pressure switch on the intake manifold that closes the circuit to a switchover valve next to the vacuum modulator (round blue thing on the driver's fender). When the pressure exceeds a certain amount, the switch closes and grounds the solenoid in the switchover valve, venting the pressure that is headed to the ALDA to the atmosphere. This reduces the fuel enrichment and therefore the dangerous boost pressure.

That's theoretically how it's supposed to work. However, with these cars running on 17 years old, the switch and the valve get plugged up with soot and oil, rendering them inoperative. The switch should be normally open with the switchover valve normally sending pressure to the ALDA.

I think in 92 they eliminated the overboost circuit and ran the line directly to the ALDA with a T fitting that went to the pressure sensor by the battery. I don't know if they eliminated the pressure switch or not. This put all the overboost protection in the hands of the pressure sensor and EDS. I guess they figured that with the wastegate normally open, the possibility of something going awry and holding the wastegate closed (on a stock, unmolested setup at least) was extremely slim.

1990 300D 2.5 Turbo -155k
2000 E430 - 103k
2014 Jeep Grand Cherokee Ecodiesel 4x4 - 11k
2014 VW Passat TDI SE - 7k Bro's Diesel
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