Hey All,

I have questions about the pneumatic governor injection system in my '73 220D W115.

What the heck does the pneumatic governor do? A friend of mine who knows a thing or two about diesel tells me it only comes into play when the throttle is closed, acting as an engine brake.

I'm not so sure. Seems to me the throttle valve would set up some vacuum in the intake at any position (except full open, maybe), and therefore would affect fuel delivery.

I've got the MB Diesel Service Manual, but it does not explain the workings of the system. Can someone explain this system to me, and tell me how it differs from others?

Many Thanks!

It also limits engine speed. Inside that "throttle" is a venturi tube. As the engine speed increases, the airspeed across the venturi pulls the governor to limit RPM.

Look at this thread: 200D, 220D, and early 240D tuning guide.

I'm familiar with the 1960's models, but believe yours is the same. Unlike most diesels, there is a butterfly in the intake. When the butterfly is in the idle position, vacuum is created in a venturi tube in the throttle body. This vacuum is applied though a hose or pipe to a diaphragm at the rear of the injection pump. The vacuum pulls on the diaphragm, and against a spring, to move the fuel rack and limit the fuel to the injectors. As the butterfly is opened, the vacuum lessens as less air goes through the venturi. This causes less vacuum at the diaphragm and allows the spring to push on the diaphragm and the fuel rack, giving the injectors more fuel. The more the butterfly opens, the less vacuum and the more fuel. There a lever on the side of the pump actuated by the throttle, but this does not control the fuel and only comes into play at idle, pressing against the diaphragm, to prevent surging.

Newer diesels, and actually most diesels, do not use a butterfly and directly control the fuel mechanically at the injection pump. The only engine I've seen this vacuum type of throttle control as of yet is on the Mercedes engines up to the mid 1970's.

The butterfly valve on some post-mid80's-present diesels is to increase the pressure differential between the intake manifold exhaust manifold to allow more exhaust gas flow through the EGR valve for reduced emissions.

At last!!! -- I have been searching the net for years to get an explaination of the Mercedes throttle mechanism. I own a 1970 L406DG camper van fitted with a 615 (2197cc) unit. It is fitted with a pnuematic governer as accurately described previously in this thread .
The thing is -- the throttle unit actually comprisses of two butterflys one behind the other with the hindermost's spindle being placed slightly above the centre line of that of the foremost - as one looks into the intake from outside.The vaccum connection to the IP is placed between these two butterflies.
Now-- interestingly on my engine , the hindermost butterfly is free to move - ie there is no "solid" coupling between it and the foremost butterfly ---which is solidly coupled to the throttle linkage. However the hindermost butterfly is openend by a protrusion on the foremost buterfly's spindle as the throttle pedal is depressed. There is no " return" spring or any other device to keep the hindermost butterfly in contact with this protrusion and in fact the " default" position of the hindermost seems to be permanetly open.
Whilst the engine is idling I tried manually moving the hindermost butterfly to the closed position ( so that it did in fact rest on the foremost butterflly's protrusion and was closed along with the formost butterfly ) - result -- very rough idling an increase in engine speed and --- alot of blue smoke!!.
I am guessing that the blue smoke is probably due to worn valve guides and / or stem seals (something that i mean to address in the near future) caused by the high vaccum as a result of my experiment.

What I would like to know is - what is the purpose of this secondary ( ie hindermost ) butterfly - when it appears to never move in operation.( I have watched it as I rev the engine to full throttle) I have wondered whether the throttle assembly on my engine is incomplete in some way - missing linkage or springs etc as the vehicle was in pretty poor mechanoical order when I bought it some 8 years ago. However it has been 100% reliable ,cruizes well ( smokes a bit on over run - but thats probably the valve seals etc) and I am of the school "if it aint broke - dont fix it" !! .
As a control engineer I am just curious to the thinking behind this mechanism.

I will post some pictures of the tthottle mechanism - it is very simular to that of a 220D car but with the IP regulator rod comming of a bell crank on the throttle linkage instead of the throttle assembly itself The throttle linkage all apears to be correct when compared to the owners manual - so i dont think it has been meddled with ion its long life!!
I have not been able to look under the hood of 220D car to see how the hindermost butterfly is operated - perhaps some 220D owner out there who would be kind enough to post a close up picture showing detail of both sides of the throttle assembly ??

Great forum this -Mb owners unite !!

The second butterfly is a crude emergency shutoff. Much like the "STOP" lever on later engines.

The inner butterfly(aka "check valve") has the sole purpose to keep the engine running at low speed in the case it starts running backwards(happens sometimes...) ..lotsa smoke out of the air filter...so that you can shut it off as quick as possible....and yes,it should move freely in the throttle duct...

Thank you both -- A crude shut off device ? -- hmm - interesting it never occurred to me !!¬. Yes I can go for that - thinking about it - that may be why the inner butterfly is slightly offset in the ventri -so that it automatically closes tightly if the air flow through the intake reverses as a result of the engine running backwards -which I agree is perfectly feasible on a diesel as the IP timing would be sufficiently close to TDC to allow that to happen with a hot engine ,albeit running with much reduced engine power. This could happen if one stalled on a slope and the clutch was re-engaged whilst rolling back - nasty!!

While I have you attention - one further question. Do you have the Bosch part number for this type or family of IP's ?? I would like at best, a service manual ( unlikely and difficult to obtain I guess) or at least a cross sectional diagram showing its inards. I am paranoid about bits missing off my engine as I think I may have interveined in some canabalism when it was layed up prior to purchase.
1) I have noticed that the end of IP rack is visible ( ie i can see it moving in and out) inside a threaded boss on the front ( ie drive end) cover. This boss is about the size of a 10mm fuel union. Surely this should have some kind of protective cap over it to prevent the ingress of dirt?
2) These IP's have seperate lubrication which is topped up via a filler cap and the level is set by a level plug on the side. I have noticed that when I add oil through the filler what runs out of the level screw is neat diesel until it is all displaced by the added oil. I guess this is nothing to worry about ( too much !!??) as I have driven 20,000 miles in this condition. I guess that this is "fuel blowby" in the injector rams caused by wear or leaking seals not knowing how it is constructed internally - how fuel much should I expect - and I am somewhat glad that the pump is not lubed by the engine oil - as I would end up with pretty thin oil or at worse a runaway engine!!.
Does anyone have any comment on the amount of diesel found in the IP cam chamber ( ie equivalent to its "crankcase") as described above? I have not noticed that over time the IP pump "over flows" from the cam chamber or filler cap so were does this accumulated blowby fuel all go?