no but thats a great idea.

__________________
1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

No (less than 1.8 in. Hg).

On the 1991 model, there are 3 transducers : one (Y31/1) operates the EGR valve ; second (Y31/2) the vacuum control flap (on the intake, between air filter and EGR port) ; third (Y31/3) the boost pressure control valve. I'm not sure which are the ones "below the airbox" : follow the OUT line and you'll find.

Not sure for the 1991 model, but in 1992 when a control loop fault is recorded by the EDS control unit, the turbo is deactivated until you erase the error code.

Beside the transducers, in 1991 you have 2 other components that can lead to vacuum leak to the transmission : the vacuum reservoir and the vacuum amplifier (both are linked to the vacuum control valve on the side of the IP, see shop manual, section 0500, p.21).

__________________
DR
1992 300D 2.5T 351 000 km

P.S. The intake control flap is apparently there to improve the efficiency of the EGR system (closed when EGR valve and wastegate are open).

__________________
DR
1992 300D 2.5T 351 000 km

thanks

it must be the boost pressure control valve below the airbox. i have noticed that whenever theres are disconnected from the vacuum that the car is much slower, but how is the turbo 'deactivated'. since its powered by exhaust pressure, how could it be turned off?

i appreciate all the help ive gotten on this tread, thanks

__________________
1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

On that turbo, the wastegate is open when no vacuum goes to the boost pressure control valve vacuum unit.

The transducer feeds variable vacuum to that unit so that the intake turbo pressure measured by the pressure sensor (follow the long hose from the ALDA) matchs a stored reference pressure in the EDS. The engine speed, IP control rod travel (engine load), air intake and coolant temperature are also taken into account.

__________________
DR
1992 300D 2.5T 351 000 km

In simpler terms, the 1990+ model turbo wastegate require a vacuum signal to function. No vacuum, and the wastegate stays open, which allows boost to bleed off - no pressure to the intake. That's why "OldSouth" on this forum replaced his turbo with a good old-fashioned mechanical type, that uses a pressure operated wastegate (like the engines up through 1987).

__________________
Dave
Boise, ID
Check out my website photos, documents, and movies!

That's it ! I like those simpler terms

__________________
DR
1992 300D 2.5T 351 000 km

ok, more good news.

tonight, using the mityvac techniques i learned from watching my mechanic, i tracked down the leaks. yes, theres more than one, but surprisingly, none are in any of the vacuum lines, theyre in the transducers.

in total, im losing about 12" hg, thats over half of my vacuum. and here are the places im losing it:

7" hg in the wastegate transducer (the one under the airbox)
2" hg in the EGR transducer
2" hg in the intake control flap transducer
1" hg somewhere in all of the lines in between

are these transducers supposed to consume about 2" vacuum? it seems strange that all 3 would go out and that 2 wouldbe losing the exact same amount.

anyway, my mechanic had disconnected the line to the wastegate when i complained about the transmission, because it was the one losing the most vacuum. this shop mostly works on older diesels than mine, so i guess he didnt realize that i had a vacuum wastegate.

well, when i found out for myself where the vacuum was going, i disconnected the EGR and its sidekick, the intake control flap and re-connected the wastegate (easy decision). now, im still losing vacuum, but instead of 12" hg, its only 7"hg (the mysterious 1" must be somewhere among the other 2 transducers).

and the results speak for themselves. im back into the 11-12sec 0-60 time, and the transmission is shifting very smoothly. i dont think the computer knows that i have the EGR disabled, is it because i disabled the intake control flap as well? dont know, but i thinkk i have a better explanation.

i checked the date on the plate on the drivers door, and even though my car is a '91, it was one of the first '91 models off of the line. the date was August 1990. maybe they hadnt put that in the computer yet?

i really cant imagine this car going any faster than it does now, but if theres a 2.5 owner out there that can do better than 11sec 0-60, i would really like to know.

i am planning to replace the wastegate transducer, but i was just thinking that if there is nothing wrong with the EGR transducer because it is supposed to lose 2" hg, then i could just replace it with that and save myself $90.

thanks for all of the advice

__________________
1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

Vacuum lost is normal for those transducers : when no current is applied to the transducer and vacuum is applied to the VAC side, most of the vacuum is lost to the ATM side and only a maximum of 2" Hg (70 mbar) is going to the OUT side.

When you read vacuum on a transducer, it all depends of the electrical signal sent by the computer to the unit. In general, when the computer energizes the wastegate transducer, it should not energizes the others 2 at the same time.

[ 100 mbar = 2.95 in.Hg ]

Attached Thumbnails

 

__________________
DR
1992 300D 2.5T 351 000 km

yeah, i thought of that, but i wasnt testing the vacuum from the 'out' side, i was testing it by hooking the mityvac straight onto that 4 way splitter coming off of the vacuum pump, then i unhooked the vac supply line from the transducer and plugged it with my thumb. i gained 7" when i took it off of the wastegate transducer, and 2" from each of the others. i did thid with the engine at idle. when i disconnected all three (like my mechanic had it when i first discovered the plugged line) i had a total of 22"hg.

so are you saying that theres nothing wrong with any of the transducers? or that only the wastegate transducer is shot? theres obviously a problem somewhere in the vac system, and im not losing anything in the vac lines. i cant imagine that one transducer is supposed to lose 7"hg, but losing 2"hg would seem reasonable.

thanks

__________________
1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

Grdenko here are the results of my wife's 1993 300D 2.5L Turbodiesel

3 timed runs with a Full tank of diesel, A/C on, Driver weight 200 lbs.

0-60 MPH
10.7 seconds
10.6 seconds
10.1 seconds, power braked

Test done with a G-Tech.

GSXR says it probably isn't accurate.

This thread is how I fixed the same problems you have suffered with. Computer Trouble Codes really saved alot of time and I describe how to do access them in this thread.

93 300D 2.5L Turbodiesel Power Loss

Good Luck!

Sorry, my previous statement :
may not be exact. The main function of the transducer is to deliver a variable quantity of vacuum on the OUT side : I doubt that all of the oversupply of vacuum is evacuated on the ATM side (at near zero F outside, I'm not willing to check this at the moment).

Here's another wrong statement :
Only the control flap transducer and the waistegate transducer work in tandem (control flap is closed when waistegate is opened and waistegate is closed when control flap is opened).

The test procedure in the shop manual states that both the EGR transducer and the waistegate transducer are energized at idle (over 3V for EGR and approx. 4V for waistegate) and the vacuum on the OUT side is near the same (8.8"Hg for EGR and under 8.8 for waistegate).

So I guess that the 2" loss is normal in your testing procedure and the 7" is not. At idle, it could be a bad waistegate transducer or a vacuum leak between the OUT side of the transducer and the waistegate vacuum control or in the vacuun control itself. A mityvac test should give the answer.

__________________
DR
1992 300D 2.5T 351 000 km

thanks alot, ill check the vacuum on the 'out' side of the wastegate transducer. i guess if its alot less than 8.8"Hg, than the transducer is bad. thanks for all of your help.

__________________
1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

last night i switched the EGR transducer with the wastegate transducer to see if that would stop the leak (i have the EGR disconnected) it didnt, in fact now i cant get vacuum anywhere, and it takes a very long time for it to build up even when i have everything over there disconnected. my mechanic replaced the 4 way splitter that comes off of the pump, could he have put in one with a smaller inner diameter?

BTW, if there is anyone thinking about taking out their wastegate vac transducers for any reason, the easiest way to do it is to remove the whole airbox, trust me.

later

__________________
1991 300 D 2.5 Turbo, 220k
also in the family:
1981 240 D 185k
1991 350 SD 185k
2006 S 500
2005 SLK 350

Please buy 2 Vacuum Valve Transducers. Install them.

Replace all of the rubber vacuum hoses that appear cracked or brittle or that fail vacuum tests that are attached to the valves.

Clean the passage between the egr valve and the intake mainifold flap valve piping.

Enable the EGR, please.

Let's hope that the 2 or 3 switchover valves are not malfunctioning/bad.