Overboost Cut Off Switch

[the_good_fellow]

what are those two wires connected to it for?(83 300SD). Is it related to the Kick Down switch? thanks!

[psfred]

One wire is +12V, the other goes to the sensor switch on the intake. The overboost protection valve (the switch is on the manifold) vents the boost sensor pressure line that goes to the ALDA and cuts the extra fuel off if the boost pressure goes too high, usually when the wastegate actuator fails.

This prevents blowing the engine up. Don't disconnect or bypass it except for testing.

Do, however, check that it isn't open all the time (poor performance) and that the boost line isn't plugged.

Peter

[the_good_fellow]

Thaks! Peter

At first I thought it was spring calibrated valve. Is the sensor switch on the manifold an "on/off" type or a variable type? Does it shut the fuel supply off slowly or instantly?

[psfred]

The switch is on/off. When closed (excessive pressure) it opens the switchover valve and vents the boost line completely. The IP then reverts to "normal" naturally apsirated mode and looses power, but does not shut off. Fuel is regulated at atmospheric pressure, as if there were no turbo.

If unplugging the connector to the switchover valve (where the boost hoses connect) gives you boost, the switch on the manifold is bad or the wire is grounded somewhere. If the switchover valve is open with the wires disconnected, it is bad and needs to be replaced.

Peter

[Maverick]

Peter,

I used a multitester today to try to diagnose the problem with my switchover valve. I’ve come to the conclusion that the switchover valve is fine as it vents with the wire connector on (no boost) but not when it is disconnected (boost). Additionally, the switch on the manifold yields no conductivity to ground. But I still have ground to the switchover valve wire connector. Somewhere the wire is grounded to the connector.

Where could this ground be coming from?

Would it be acceptable to run a new wire from the switch on the manifold to the switchover valve?

Thanks…

Mav

[psfred]

Mav:

Trace the wire (they all have different colors or colored stripes) and see if it has been pinched somewhere that grounds it.

Also, check to make sure someone hasn't gotten it connected to the wrong thing -- there is also a 50 degree C switch for the electronics box that is probably closed cold, open hot. I don't know how close together they are, but there is also probably a switchover valve for the transmission contol vacuum system, and possibly others, too. You may have the wrong wires plugged into the switch.

I don't have a wiring diagram, so can't help too much, but I suspect someone has swapped something around rather than a bad wire.

Peter

[Maverick]

Peter,

The car ran well for the past year after I purchased it from the MB dealer. After I developed a major oil leak through the vacuum shut off switch, I drove the car under 70mph with no quick starts so it wouldn’t leak and therefore didn’t notice the lack of boost. (the 350SDL really drives well without the turbo!) During this time, apparently the crossover switch started causing problems. No one else has been under the hood but me.

Today I cut the wire at the switch on the manifold and ran a new wire directly to the ground side of the connector on the crossover switch. I hope this simple setup works. I mean the car runs great but how do I know it’s protected?

Thanks much…

Greg

[the_good_fellow]

Peter,

Factory setting max. boost is about 9-10 PSI which is controlled by the wastegate. at what PSI then sensor switch will close ?

[psfred]

Boost pressure on a 603 is 0.9 atm or 13.2 psi or so, higher than on the older 617 engine. I suspect the switch closes at 15 psi or so, but I don't know for sure.

The turbo will cheerfully produce 20 psi if the wastegate sticks closed.

Peter