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  #1  
Old 10-10-2011, 08:41 PM
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1990 300d 2.5 no boost

A friend of mine just bought this car and we are trying to diagnose the no boost issue. I've spent countless hours reading through posts (ALL of them) about the waste gate swap, transducer troubleshooting, eds codes, over boost switch, etc.

First thing I did was check for vacuum to transducers. That all checked out. Then I made sure the lines were all plumbed right and that checked out. I then applied a vacuum to the EGR, the flapper, and the waste gate. Those all work as they should.

So my next step was to run a vac line from the waste gate to the cabin (to pull a vac manually on the waste gate.) Did that and had 14psi boost but the car still wouldn't get out of it's own way. So my next step was to bypass the over boost solenoid. I ran a line directly from the manifold to the ALDA. Yes, I know that I need to be careful doing this. Anyway, drove the car and it ran like a bat out of hell. I couldn't give it more than half throttle in any gear without it boosting past 15psi. My next step was to hook the waste gate back up to the transducer while leaving the over boost bypassed. Got back in, went for a ride and had 0 boost.

I built a nifty little pulse reader this afternoon to see what the EDS had to say. The very first time I checked it 9,3,2 in that order. I realized I left the plug off the MAF last night so we reconnected that then cleared the codes. I checked again and now I get 1 meaning no faults yet, I still have no boost. So in the meantime I'm going to check the over boost switch to make sure its not bad or dirty. Then I'm going to clean the lines going from it to the intake and ALDA. I am stumped as to why I have no boost though with no apparent problem or EDS code. I would really like to try to get this sorted without having to order a pressure actuated waste gate, etc. I'm not saying I don't want to do that mod it's just that we need to get this car drivable/reliable for cheap for the time being and then maybe tackle that project when it gets a bit colder.


Any help would be greatly appreciated.

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Old 10-10-2011, 09:00 PM
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Do the mod that GXSR posted around here and never look back.
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  #3  
Old 10-10-2011, 09:01 PM
92 300D 2.5L OBK #59
 
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Best/Most cost effective option is to swap out the vac operated wastegate with the pressure operated wastegate. It's a hundred dollar fix and eliminates all the stuff in front of the battery. EDS then only controls the idle speed.
This is the thread. http://www.peachparts.com/shopforum/diesel-discussion/212097-wastegate-actuator-swap-om602-962-a.html
It's very long, but well worth it.
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Old 10-10-2011, 09:15 PM
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I appreciate the fast responses, but did you guys not read my post? I'm asking since none of the components are showing a fault. Maybe i'm just missing something.
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Old 10-10-2011, 11:10 PM
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Update: swapped two of the transducers and achieved nothing, still no boost or codes. Also, cleaned the lines to the over boost solenoid, ALDA and intake and tested it by blowing air through both sides freely with 0v going to it. When I apply 12v to it you can hear it click and air will only move one way. So I assumed it was good. I also took the sensor out of the intake and it was almost completely clogged so I cleaned it with brake cleaner and air. Put everything back together, pulled a vac on the waste gate again went down the street got 14psi but still no fuel to back it up. I stopped on the side of the road and unplugged the harness going to the over boost solenoid got back in stepped on the gas and it would spin the tires a bit as soon as it spooled in 1st. So back to square one, only with another problem (over boost circuit). Could it be the sensor in the intake is bad?

Another thing that has me confused is the fact that when I put the mighty vac on the output of the waste gate transducer I get NOTHING, load or no load through various RPMs. Yet when I leave that line coming off the output of the waste gate transducer unplugged to pull a vacuum from in the cabin with the mighty vac, the transmission shift rougher than hell. If I plug it, it shifts perfect. This makes no sense to me.

However, when I run the same test off the output of the EGR transducer at idle there is no vac and it goes up to ~10" or so around 2000-2500rpm. I get nothing off the output of the flapper transducer at various RPMs but I'm assuming this is normal sin there is no boost.

Again, I've read every thread relating to the no boost problem. I'm stuck!
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Old 10-10-2011, 11:43 PM
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The overboost sensor sends a ground signal at ~1.1 bar. It's open below the setpoint. Easy to test.

When you swapped transducers, you might have sent a vacuum signal to the mixing pipe flap and starved the engine of air.

The sentiment is precisely that it's not worth most folks' time to understand and sort the system. There just isn't the knowledge base available to diagnose problems even with error codes. If cash is tight, a trip to a wrecking yard or pick-n-pull might get you a pressure actuator you can modify to fit this car.

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  #7  
Old 10-11-2011, 12:18 AM
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Sixto, are you saying the sensor is somehow adjustable? As far as the test i ran on the transducers goes, everything was hooked up properly and tested properly. I've been through those vacuum lines and connections probably 50 times since last night.
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Old 10-11-2011, 12:22 AM
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Not adjustable. The setpoint is about 1.1 bar. Or maybe it's 0.9 to 1.1 bar. But it's fixed.

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Old 10-11-2011, 12:28 AM
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So are you suggesting that the sensor is in fact bad?
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Old 10-11-2011, 01:47 AM
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I'm suggesting you can check with a VOM or test light whether there's a ground signal from the overboost switch below 0.9-1.1 bar. There is constant +12V to the solenoid when the engine's running.

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  #11  
Old 10-11-2011, 02:04 AM
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Have your tests confirmed that all three transducers (pressure converters) are working properly? That was my problem on my 92 (which only has two). Replaced them both and boost was perfect.

I should add that, when I asked my tech about the possibility of only one being bad, he said that because of the way the two work in tandem it was advisable to replace both even if one was the main source of the problem.
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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
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Last edited by shertex; 10-11-2011 at 06:24 AM.
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  #12  
Old 10-11-2011, 10:40 AM
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Sixto, thanks, i'll try that.

Shertex, how exactly did you test yours? Also, how much did the new ones set you back?
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Old 10-11-2011, 10:58 AM
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Quote:
Originally Posted by ascalise View Post
Shertex, how exactly did you test yours? Also, how much did the new ones set you back?
My mechanic was the one who concluded that I needed to replace my pressure converters...I don't know how he came to that conclusion. When I asked him, "If only one is bad, can I replace just that one," he said, "since they worked in tandem, it's really best to replace both." And perhaps he knew that the SYSTEM of the two of them together was shot, but didn't know which one.

I paid just over $100 a piece from one of the online MB parts distributors.

NIGHT AND DAY difference after I replaced them.
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14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Last edited by shertex; 10-12-2011 at 07:58 AM.
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  #14  
Old 10-11-2011, 09:16 PM
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Ok, i ordered the actuator and fitting for the swap. On to the other problem with the over boost circuit..

I hooked up the positive lead from my meter to the + on the battery and the negative lead to the connection prong on the pressure sensor and its an open circuit. If I run the same test with the wire plugged into the sensor I get 12v to the meter. Does this mean that my sensor is sending ground all the time? Btw, I did this with the car idling.
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Old 10-11-2011, 11:59 PM
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And I thought I was related to Don Quixote...

Far,Far Better Electro/Pressure/Mechanical Engineers than any of us will ever be
have designed THAT system to be virtually Un-Diagnose-able.

(It's a Bloody Wonder any of them "Worked Right" their first 5 years out of
Sindelfingen.)

EVEN IF it's "Fixed" to Factory Specs...It Leaks Turbo Pressure cause the
Vacuum Wastegate setup is never truly completely closed.

You'll ENJOY full PSI until the release setpoint is reached with the Pressure
Operated Wastegate Swap.
(Actually you've already experienced it,by forcing the Wastegate closed.)

Get the Vacuum / Pressure schematic for the Overboost circuit on that YM
124.128 and go back through it.It's something simple that's keeping the ALDA
from "Seeing" the Boost and providing the Fueling necessary to compliment
the Boost.

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Last edited by compress ignite; 10-12-2011 at 12:12 AM.
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