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  #1  
Old 06-10-2003, 04:24 PM
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overboost sensor.....

I just noticed the overboost sensor on top of the manifold is not connected to anything. Is there a tube or wire running from this thing and where is it supposed to go????
Thanks

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  #2  
Old 06-10-2003, 06:28 PM
Old Deis
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There is supposed to be a tube running from a banjo bolt on the rear of the intake mainfold up to the rt side of the onverboost sensor. Then another tube runs from the sensor down to the ALDA on top of the IP. There is an electical plug in that has two wires and goes into the bottom of the sensor.
If that is really your overboost sensor then either there is no plumbing to the ALDA which would give you very poor performance at acceleraction, or someone found the oeverboost sensor to be plugged and just ran the pressure line from the sensor directly to the ALDA.
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Old 06-10-2003, 06:30 PM
Old Deis
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My mistake, I was reading you to have said the crossover valve mounted on the firewall.
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  #4  
Old 06-10-2003, 06:53 PM
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There should be a wire from the overboost sensor to the switchover valve on the firewall. At the moment you have no overboost cut-off of the boost signal to the ALDA. Either you knocked the wire loose or your sensor is stuck on (like the sensor was in my 81 SD) and someone left it off to allow a boost signal to get to the ALDA.

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  #5  
Old 06-10-2003, 09:12 PM
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There has never been a wire on it. Just wondering. The acceleration seems fine....wondering if I am missing something? Can I buy the new sensor and wire on the net anywhere? Nearest MB dealer an hour away.
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  #6  
Old 06-11-2003, 12:45 AM
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There shouldn't be any degradation in performance. What your engine doesn't have is the ability to reduce fuel if the turbo were to blow too hard, say if the wastegate stuck open.

If I remember correctly, the sensor has a bullet plug. If that's gone then you might need a new sensor. There's nothing special about the wire. You can make it out of parts from Radio Shack.

Check the parts forum to see if anyone's parting out a 300D or 300SD of that vintage.

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  #7  
Old 06-11-2003, 10:45 AM
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OKAY.....$70 to replace the "overboost sensor" A few questions on this item. The car seems to run fine without it. What will I gain by replacing this item? I plan on installing a manual boost controller in the future. WHen I install the boost controller will it be in vain if this sensor is working?
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  #8  
Old 06-11-2003, 01:09 PM
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I think you'd want to have a working boost sensor at some point. Wait for a used one to find its way to you.

A manual boost controller won't have any impact on a working or non-working sensor and vice versa.

But you're waiting to hear from someone else, right?

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  #9  
Old 06-11-2003, 05:43 PM
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This sensor is just an extra safety item to protect the engine in case of wastegate failure (stuck closed). Most cars are incapable of producing enough fuel or boost to actually cause damage (the IP's tend to wear, or get weak springs or something, and won't provide enough fuel after ~20 years). I have the overboost circuit disabled on my 1984 300D, and my previous 1983 also had it disabled. I have tested that car with a boost gauge and it won't get over 10psi anyway, well under the ~14-18psi trigger point of the sensor.

Yeah, your can *should* have it working, but don't lose sleep if it's not. But test the boost pressure if you're going to leave it disabled!
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Old 06-11-2003, 07:16 PM
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I have a boost guage installed in anticipation of the boost controller. Max is 7psi. I was hoping to up it to about 10-11 max with the boost controller.
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  #11  
Old 06-11-2003, 07:37 PM
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Spec for that engine is 0.85-0.95 bar (12-14psi). Try disconnecting or clamping the wastegate hose & see what the boost goes up to. If it's around spec, the wastegate is opening too soon - you can open up the can and adjust it, no external controller required. If the reading does NOT increase with the hose pinched or removed, the engine isn't making enough power, and you'll need to try tweaking the ALDA & see if that helps. You should see a power gain from 7 to 10psi, but beyond about 10-11psi, you may not gain anything (depends on the IP's internal full load fuel stop setting).
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  #12  
Old 06-11-2003, 10:06 PM
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I was told previously the stock setting was 7.5 psi. I never doubted that because that was what I was seeing at the guage. Now that I look it up in a Chilton's manual they are telling me 0.7-0.8 bar. I converted that to 10-11.5 psi. If the manual and gsxr are correct something is wrong. I realize eventually I'll have to adjust the ALDA to gain full power at higher boosts. If I pinch off the wastegate hose I won't hurt anything??????? How do you adjust the wastegate without boost controller?????
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'71 Hodaka Ace 100
'72 Triumph T100R
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  #13  
Old 06-11-2003, 10:28 PM
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Whooops! My bad. You are correct. I just checked the factory manual, and the spec for the OM617.95x engine is 0.70-0.80 bar (10-11.5 psi). It's my other two cars with OM603 engines that are 0.85-0.95 bar spec.

Your boost is still a little low at 7psi. The fairy tale is that in an effort to help sneak by stiffening EPA regs in 1983-85, MB shipped the last few OM617 engines a little on the conservative side for boost settings. Some of these engines only put out 6-7psi. Tweaking the wastegate usually got the power back to normal.

Pinching the wastegate hose FOR TEST PURPOSES will tell you if the problem is the wastegate opening too soon, OR if you lack enough fuel to create more boost. Try it and see what happens. On my 1984 (same as your engine), pinching the hose jacks boost up to 15psi, but I get NO more power than I do at the usual 10psi (there's no more fuel to go with the extra air). So in my case, the wastegate it opening per factory spec, but adjusting will get me nothing. You need to test yours & see what happens, every engine is different....
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  #14  
Old 06-12-2003, 01:09 AM
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I have a CA model......that may explain the low boost psi.
BTW, you have a gixxer??????
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Old 06-12-2003, 10:11 AM
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Well, I did have a Gixxer, a 1997 GSX-R750 which I raced (amatuer level) for a while. Couldn't afford to keep that up though. Sold it before I moved (BIG mistake - I miss that bike) and just picked up a mint Mille R, which so far I like quite a bit.. :p

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