Troubleshooting Vaccum connections on my 300D

[1987_300D]

Hello all,

I have been going through all the vacuum connections, repairing and replacing leaks to address my hard shifting2nd gear.....

I found Y30 is missing a connection to the 138 charge air distribution pipe? Where is this air distribution pipe? I cant seem to find where its supposed to connect.

Thanks for any help in advance!!!

[ajnorris]

Thats the overboost protection solenoid. It is likely bypassed straight from the alda to the nipple on the intake manifold.A cranked up alda could give you sort of a hard shift under heavy throttle. If the issue is just first to second most mercedes do that until warm. Also there is a procedure for adjusting the vcv (vacuum contol valve) on the side of the ip. That has a lot to do with shifting harshness. The purpose of that device is to simulate the vacuum a gas motor would have.

[1987_300D]

Where exactly on the manifold is the connection for the overboost solenoid? I have looked and cant seem to find the tap. it is connected to the ALDA just the other side for the manifold is open so I plugged it until I figue out where it goes.....

Regarding the symptoms of the car, I have harsh shifts mostly on second gear. It shifts better when cold and when the engine warms up it shifts harsh especially under 2K rpms.... If I mash the throttle to 2.5-3K rpms it seems to shift a lot nicer!!!

I have checked the vaccum pump, builds good vaccum...fixed all the leaks and replaced almost all the hoses I hold vaccum from the Blue saucer disk to Transmission.....As a last resort I adjusted the VCV down 1/2 turn CCW problem still persists......

[jay_bob]

You have an OM603.960 motor, so 'advanced search ' the forum for "OM603.960 vacuum diagram", there was one in a not too long ago post.

I see you're new here, do you know about the online factory service manuals at startekinfo.com, click on MB Workshop Resources on the left, go through about 3 pages of results to '124 Service Info'.

The boost port is a brass nipple about in line with cylinder #4. It's next to a pressure switch with 1 wire on it. The pressure switch actuates the Y30 overboost solenoid which is on the inner firewall opposite the master cylinder.

Y29 is the boost pressure control valve to the BFS. It is actuated by a 50 degC switch in the head, once the engine warms up it varies the boost level seen by the BFS thus changing the shift characteristics once the engine warms up.

The BFS gets a sample of boost pressure, it's entirely possible that the line from manifold-> Y30 -> ALDA -> Y29 -> BFS has gotten stopped up with manifold goo thanks to the EGR valve.

Another thing to consider is dealing with the EGR and cleaning the manifold, unless your state inspection policy requires intact emissions gear. That may not help your shifting problem directly but it will help in other ways.

[1987_300D]

Thank you for the Information guys!!! Yes I am new to the platform and these diesel engines..... but I always wanted a W124 diesel!!!!

Thank you for the support!!!! I will check out the manual and look for a brass fitting to connect the Y30.....

I live in NH and due to the year of the vehicle no Emission test is required I have not yet removed the manifold so I have no idea what condition that's in.....

[ajnorris]

One other thing I forgot to mention. On the gas models at least there is a switch under the gas pedal that acts like a kick down lever. If that gets stuck closed it can make the car always start in 1st vs 2nd like they are supposed to and it will make it shift from 1-2 hard.. I think the diesels have a different valve body that starts in 1st but it may still have the switch to delay the shift... worth checking at least. Also one thing I have done before is to use a mightyvac in the car to pull vacuum on the trans modulator while driving to see what the different vacuum levels do... sounds like you are getting too little vacuum at light throttle in 1st. If you pull full vacuum with the mightyvac and it smooths up the shift then you know its something with the vacuum system. If it still is harsh you may have to look elswhere.

[jay_bob]

Yes the mechanically controlled transmissions in Diesel applications also have the electrical kick down solenoid.

The switch sits under the pedal, it's a big clunky switch, so you should be able to hear it click when pushing the pedal all the way down. It can get stuck from the carpeting or if something is spilled into it.

If the switch is stuck closed ("on"), then it will keep the transmission from up shifting. You can find the connector on the switch body, try unplugging it to see if that changes anything.

[1987_300D]

I will check the switch but it shifts up and down when its supposed to and when the car is cold it shifts when its supposed to and pretty smooth.....

So there is no doubt in my mind if a vacuum related issue its just a matter of isolating it..... I'm not going to lie I did not think it would of taken me this long.....lol...

I will try the switch and then perhaps invest in a mighty VAC to test further..... Thanks for all the help!!!!!

[1987_300D]

Thank you all for the help I spent some time this weekend and found a broken fitting..... Repaired it and wow what a difference.... Completely different car!!!

The fitting was on a cylinder/ solenoid looking thing behind the passenger headlight..... What are those for does anyone know?

Thanks again for all the help!!!!!

[ngarover]

Search EGR Delete. Spend a couple bucks and buy the EGR block off plate, and remove all the unnecessary EGR related vacuum lines and equipment... It will greatly simplify the setup, and remove any other possible leaks, plus you will gain some performance back. Kind of a win win.



The diagram shows what I first did when I got my 87 300d, Which is still setup this way, but I kept the ALDA on the new engine hooked up. (The red square with 2 vacuum line coming off it in the diagram)