No cable but now that you mention it there is two nipples on the throttle arm as though a cable should be there, did the cable supercede a rod arrangement? my 280 coupe has a rod that goes to the transmission tightened by the throttle action, incidentally, the transmission on the coupe and the one on the TD are identical looking apart from the arm and rod arrangement, what is the purpose of the cable/rod, is it to do with shift timing? I assumed it was to do with the kickdown, could I retro fit a rod or cable to my TD transmission? I have a spare rod and arm that came with a transmission I took from another 280 Coupe as a spare for mine, I ought to be able to fabricate a linkage from my throttle arm to the rod fairly easily if this is the answer, it could be that the original rod has been removed previously for some reason I will have to make a closer inspection. perhaps this is where I have been going wrong all along ie a vital part of the setup not present ?

Felix

It's very difficult to help you because that vehicle appears to be very different from those over here and, furthermore, it doesn't appear to have all its hardware.

The older vehicles had a rod and bellcrank affair. This was subsequently replaced by a cable in later vehicles.

In any case, this mechanism serves to delay the shifts. If the rack is opened a large amount, the transmission will remain in the lower gear and allow the engine to rev reasonably high.

yes and It probably does not help that I have matched an 80/81 tranny to a an 82 engine with bits and pieces and guesswork ie not sure if the bits I have will work together anyway, kinda wish I hadnt started the project, but that just makes me more determined to finish it. thanks again for all your help though much appreciated

Felix

Going back... did you test the VCV...
 

Felix - Have you tested your VCV as I have recommended? Since your VCV is so different from what we are accustomed to seeing, I think this would be appropriate... I would put the hand vacuum pump on the port [where a line is connected]... on the port that goes to the tranny vacuum modulator. Pump the vacuum pump to see if the vacuum holds. For the sake of continued discussion, I'm going to assume it does NOT hold the vacuum. IF this is the case, then put you small finger over the "vent" port connection that normally runs through the firewall and then ends with the line open to the atmosphere. Put you small finger over the vent port while you pump the hand vacuum and see IF it holds vacuum.
Report back on this... for we must find out why you are unable to successfully bleed down the vacuum on the Tranny-Shifter Vacuum control system's vacuum.
Regards,

Some closure at last
 

My "INDY" [ Independent Mercedes Mechanic ] sometimes says...
 

Felix - So vintage diesel Mercedes in GB have 4 different models of VCV(s) and similarly you have matching color coded transmission modulators. I'm a little surprised the German engineers didn't create RED - WHITE & BLUE for us here in the U.S. - My Hungarian "INDY" friend here in the U.S. often laughs and jokes about such things on these cars as being are examples of " Hitlers Revenge " !

In any event you have proven the original title of this THREAD to be accurate... that it is critical to understand the vacuum system on these diesels vintage Engine/Transmission-Shifter Vacuum Control Systems... and to [ one-at-a-time ] systematically isolate any problems with it. And yes, understanding how vacuum is NOT really negative pressure but rather it is pressure below 14.7 psi [ 1 "Bar" ] and that very similar to electrical circuits until air flows in a vacuum control system, you WILL NOT see andy pressure drops... so there is the STATIC state and there is a DYNAMIC state in vacuum control systems. AND YES, these trannys do shift a bit "stiffly" until the fluid has warmed up some !

Thanks for letting us all know how you solved your car's problem there on your side of " the pond " !
Regards,

Most diesel vintage mercs in the UK had manual transmission, The auto set up, notably the colour of valves (which I now know to have different vacuum dumping properties), addition of a rod or bowden cable coupled with triangular throttle linkage etc varies according to year the earliest being the simplest much the same as it does on the US set up, I am guessing here but I think the white plastic VCV was developed specifically for US cars to work in conjunction with the black box and EGR system, Guessing again but the addition of a rod or cable would help to eliminate the rough late changes on cold fluid by adding some persuasion to the change other than just vacuum. anyway the car is going on a 200 mile round trip tomorow before it is passed back to its owner and then the next project looms a FWD audi a6 TDI engine rebuild Oh joy :)

leaky vacuum transducer
 

My '87 300tdt was not shifting right so I checked the vacuum line coming off the vacuum transducer and leading to the transmission modulator. No vacuum. Incoming vacuum line was checked out OK. Where happens in the transducer?

Wes

.......... We need a Vacuum Diag for your Car..........
 

Wes,
Your car is newer than any others we have tried to help with this THREAD and when I look at: http://www.peterschmid.com/vacuum.htm, my favorite resource for vacuum diagrams does NOT cover your model [station wagon?].

So IF you can identify a source for the Tranny/Engine Vacuum Control System in your car, otherwise you might not find much help. Meanwhile, I'll look elsewhere to see what I can come up with!

Q - Is your car a W124 or W126 chassis?

Oh, I see... it is a 300TDT W124 with the 603.960 engine... right?

Regards

Hard shifts -- 1987 300D (W124 w/OM603)
 

My '87 has had "extra-firm" shifts for as long as I've had it. At very low throttle positions the shifting is reasonable -- it usually gets harsher as the footfeed is pressed down. Road speed does not seem to matter. The shifts are not always harsh but can involve any upshift (1-2, 2-3, 3-4). Downshifts are OK, almost always unnoticeable. The trap ox recall has been performed and the entire exhaust system including the turbo replaced. The EGR system has been blocked and removed including all of the vacuum stuff. There do not seem to be any vacuum leaks in the car. Boost has been measured at 11-12 psi at the manifold. I've done the following to try to understand the problem:

Engine off

1. Remove windshield washer reservoir and inspect the Bowden cable where it connects into the throttle linkage and goes down to the transmission. With no footfeed at all, the cable is neither slack nor under tension. As soon as you start to press on the footfeed the Bowden cable sees some pull. Pulling on the cable by hand reveals some spring resistance, suggesting that there is something on the other end, i.e., the cable is at least not dangling under the car somewhere.

2. Disconnect transmission vacuum line from the green dashpot; connect Mityvac to vacuum line, pump down. Read 28 Mityvac inches of mecury, steady with very little decrease over a couple of minutes.

3. Reconnect green dashpot to transmission vacuum line using a tee and connect Mityvac to the tee.

Start engine

4. With engine idling, the vacuum on the transmission line is about 16" Hg. It decreases as the throttle is advanced, to a minimum of about 7" Hg.

Stop engine

5. Mityvac continues to read about 16" Hg in the transmission line. Pushing the throttle to the stop drops the vacuum in the transmission line to 7" Hg. Releasing the throttle sends the vacuum level back to 15-16" Hg.

From what I have been reading here, it seems that the Bowden cable is set correctly, the VCV is working (with possibly a little too much vacuum) and the modulator is not leaking. Yet the shift remains unacceptably harsh.

What should I do next?

Jeremy

Jeremy,
I have zero experience thus far with Bowden Cables... and the same for the turbo(s) on these vintage MBZ(s)... so IF either of these are a factor for you, I'm afraid I would probably NOT recognize it !

The thing I find most intriguing about your POST is in paragraph #5... where you say the vacuum reading keeps coming back after the engine is shut down. IF I can get either of my cheap Harbor Freight vacuum pumps/gauges to work for me I'll have to test my 240D to see IF the vacuum control system performs anything like this.

Q1 - Is your "tranny shifter vacuum control system somehow cross-connected such that it receives vacuum from your car's vacuum reservoir tank?...
and
Q2 - Does your 87 [ what model ? ] diesel even have a vacuum reservoir?

Sam Ross

A1 - I think the vacuum system is 'wired' correctly but I'd be happy to have someone show me that I was wrong.

A2 - It's an '87 300D-T (W124) -- the one in my signature line. It must have a reservoir somewhere because
(a) I have a vacuum gauge permanently installed in the line that feeds the shutoff on the IP. The gauge typically shows 15"-20" Hg after the engine is shut down, and
(b) the shutoff on the IP is forceably kept in the "off" position -- you can feel it -- even after the engine is shut off.

So there's a vacuum reservoir somewhere. One of the vacuum diagrams in my collection shows a line leading over to a reservoir in the right front fender. I have such a line but haven't confirmed that there is a reservoir there yet.

Many of the posts in this thread suggest that the correct vacuum level on the tranny line is 8"-10" going down to near zero at full throttle. However, these posts then go on to say that more vacuum causes slipping. I have the opposite problem, so I'm not sure what to do.

Since I have to do something, I plan to first tighten the Bowden cable to see what happens and then play with the vacuum system, maybe adjust the VCV to see if I can lower the vacuum in the tranny line, see what that does.

Jeremy

Yes, you've properly determined that the VCV and the amplifier are both functional.

Please disconnect the line from the modulator to the amplifier and plug the line (from the amplifer). Connect the Mityvac to the modulator line and pump it up to 15" or so. Run the Mityvac into the cabin so that you can take it for a drive. Drive it and keep the vacuum at 15" (approx.) for the entire test drive.

Report back with result of shift quality.

Thanks Brian for being there with your expertise...
 

Brian,
I was hoping that you were still monitoring our TREAD here for I know you are into these W124 models!
I just wish the vacuum diagrams for these W124(s) were more readily available OnLine. By-the-by, my favorite WebSite for such has changed it's name... from: http://www.peterschmid.com/ to http://peterschmidtransmission.com/ .
But there are no W124 model diagrams there !
Does anyone out there know where we can access the W124 Tranny Shifter Vacuum Control diagrams OnLine?

Brian... I have NOT yet found anything to explain something about your "amplifier"... I presume it is a vacuum amplifier?
Can anyone offer an explanation as to what this component is?
Sam

Brian, I will try your experiment with the Mityvac on the transmission vacuum line this morning and report later today.

Sam, I have attached a vacuum diagram that is the closest fit to my 1987 300D-T (W124, OM603) that I have been able to find in the FSM CD-ROM.

The purpose of the vacuum amplifier (inferring from the name) is to provide a wider range of vacuum changeability than what would otherwise be available from the VCV. How the vacuum amplifier works to do that I have not yet figured out.

You will see a vacuum reservoir in the right fender. I have not confirmed the actual presence in my car of this reservoir but the thick line to it is definitely present. It would be the reason why there is vacuum on the transmission line even after the engine is stopped.

I'm unsure whether there are vacuum orifices in the lines at the tee located directly above switchover valve Y29.

I will be the first to admit that I don't understand how this rat's nest does what it is supposed to do.

Jeremy