Over boost control - PeachParts Mercedes-Benz Forum

chomamma · #1 09-07-2007, 09:11 AM

How is overboost controled in a 2.5 other than wastegate?

2.5Turbo · #2 09-07-2007, 05:51 PM

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.

chomamma · #3 09-07-2007, 09:54 PM

Doe the switch look like a big brass nut screwed into the side of the intake with an electrical connector on it?

Also there are two vacuum hoses coming out of the two differnt vacuume valve above the air intake. One is suppose to be attached to EGR the other some other to some other butterfly control. I may have mixed up which hose goes to which device. Do you know which valve controls the egr? What does the other control do on the intake tube? Thanks.

chomamma · #4 09-07-2007, 11:49 PM

Then really you still have boost, just no fuel to go with the pressure?

2.5Turbo · #5 09-08-2007, 12:11 AM

Yep, that big "brass nut" is the pressure switch. The only other thing on that end of the manifold is the boost pressure line to the ALDA and pressure sensor.

I'm not sure which one goes where as my car is 2500 miles away at home. I know there are two vacuum transducers between the airbox and the expansion tank, but I can't remember which one is which. Somebody else should chime in shortly with the answer. The flapper/butterfly valve between the turbo and the EGR is there to make the engine draw more EGR gases when needed. It closes off the intake somewhat which makes the engine draw more gasses through the EGR.

When no pressure gets to the ALDA (like when the overboost system is activated), the fuel is limited to the performance of a NA engine. Some boost will be built with no signal to the ALDA, but there isn't enough fuel being burned to raise the boost to unsafe levels. I haven't actually done any tests, but I would think only a few PSI are built with no boost signal to the ALDA.

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