We recently replaced the cylinder head on my 1984 300SD and did so successfully I might add , but that's another long story. In the process, however, we discovered that the overboost protection circuit does not exist. The boost pressure sensor (see attached picture of same component, new) is installed on the intake manifold (or boost air pipe per MB), but there is nothing connected to it. As I understand it, (see attached diagram of turbo operation) when boost pressure is monitored to exceed 1.1 bar, this sensor provides an electrical signal to the switchover (aka changeover) valve which interrupts the boost pressure line connected from the banjo fitting at the intake manifold to the switchover valve and then on to the ALDA, thus removing corresponding fuel enrichment needed for additional boost. My real question pertains to the location of the wire that would normally connect to the boost pressure sensor. I'm assuming that it originally connected through the passenger side of the firewall into a harness to which the switchover valve is currently connected, but I find no evidence of such a wire. I've only owned the car for the past three years and haven't had a chance to really work on it until recently. In addition, I have no knowledge of what occurred or didn't occur for "maintenance" prior to me. I would assume that at some point in the past, the sensor malfunctioned so someone simply disconnected it and went ahead and removed the wire. Can someone please tell me how it is supposed to be set up/dressed out? I would eventually like to tweak the boost pressure control valve to achieve turbo boost that is something closer to 1 bar, but I will not mess with it until after optimizing the ALDA (at which point it may not even be necessary to mess with the turbo) and certainly not as long as I have no overboost protection...

Thanks,
-Ken

Boost Pressure Sensor:

http://ken.kdharris.net/kdhbenz/turbo/boost-pressure-sensor.jpg

Turbo Operation

http://ken.kdharris.net/kdhbenz/turbo/turbo-schematic2.jpg

Solution:

Attached Thumbnails

 

So it's just a piece of wire that is entirely visible in the engine compartment and does not run through a harness to get from the switchover valve to the boost pressure sensor?

__________________
-Ken Harris
1984 Mercedes 300SD
2001 VW Passat 1.8T
2004 Harley Davidson FLHRSI
-----------------------------
Der Dieselfanatiker

I believe I followed it and it comes around behind the valve cover (by itself) and then jumps into either the glow plug harness or the one whereever the oil pressure sender goes.

I think I can handle that...thanks Brandon...

__________________
-Ken Harris
1984 Mercedes 300SD
2001 VW Passat 1.8T
2004 Harley Davidson FLHRSI
-----------------------------
Der Dieselfanatiker

Brandon is correct.

The wire suffers pretty badly from the heat all around the top of the valve cover. I recently taped this wire from the harness all the way to the manifold to provide additional protection.

There must be some evidence of a cut wire at the harness right near the #4 glow plug.

Further investigation only served to make me scratch my head again. I've included a couple of pics below which show the plug at the bottom of the changeover valve. As you can see, they are not discrete wire connections, but rather the ignition and boost lines are connected to the changeover valve within a plug. The plug is connected to wires which are inside of a plastic sleeve and appear to go back through the firewall in a harness near the brake booster. My assumption and the basis of my original question is that the wire for the boost pressure sensor originally either came back out through the firewall or ran through the through in front of the firewall closer to where the boost pressure sensor is located; neither of which I can find evidence of...?

Pix (320x320) of Changeover Valve [click either for larger (1360x1360) images]:


Anyway, when I've got more time, I'll try to follow those wires back and see if I can determine their route. When I get tired of that exercise, perhaps I'll just put a meter on the plug to identify the hot wire and then route a new separate wire from the remaining terminal from the changeover valve to the boost pressure sensor line.

If anyone else has the same setup as mine where a plug connects to the changeover valve, perhaps they can tell me exactly where on or along the firewall the wire connected to the boost pressure sensor originates.

In the mean time, thanks to all for the assistance thus far...this forum is truly AWESOME!!!

__________________
-Ken Harris
1984 Mercedes 300SD
2001 VW Passat 1.8T
2004 Harley Davidson FLHRSI
-----------------------------
Der Dieselfanatiker

That wire disappears into the harness. It's not worth the effort to try and track it down from the valve.

As mentioned above, look near the #4 glow plug. The engine harness will have a loose wire there that was probably cut. This is the wire that you are looking for.

Thanks Brian...

__________________
-Ken Harris
1984 Mercedes 300SD
2001 VW Passat 1.8T
2004 Harley Davidson FLHRSI
-----------------------------
Der Dieselfanatiker

Well, I'm almost afraid to say so, but it turns out that my problem with the "missing" overboost protection circuit was of my own making all along (story of my life )...

As mentioned in the opening post to this thread, we recently replaced the cylinder head in this car. In the process of putting everything together, we inadvertently connected the line that should've gone to the boost pressure sensor to an unused temperature sensor in the head instead of the boost pressure sensor. I confirmed continuity between it and one of the lines at the plug connected to the changeover valve, reconnected it to the boost pressure sensor and away we went.

Thanks to Brian Carlton for guiding me to the area around the #4 glowplug. In the process of my investigation of that area I noticed that the line connected to the unused sensor seemed to backtrack a long way from where it exited the plastic sleeve around the harness containing it and the glowplug wires near the #4 glowplug. Then it occurred to me what must have happened and I corrected it. Obviously there was not a ground potential at that sensor, or the turbo would (should) not have worked at all, which it always did...

Muchos Gracias Amigos...

__________________
-Ken Harris
1984 Mercedes 300SD
2001 VW Passat 1.8T
2004 Harley Davidson FLHRSI
-----------------------------
Der Dieselfanatiker