Confounded! Very little power in my '78 300D - Page 2 - PeachParts Mercedes-Benz Forum

ROLLGUY · #16 08-26-2012, 10:28 AM

There may (should?) be a control rod from the accelerator linkage that goes down to the trans. It may need to be adjusted to give proper shift points. The rod gives the trans the throttle position. Otherwise, the trans has no idea when to shift. An automatic of this era needs throttle position and vacuum input to shift smoothly, and at the correct engine RPM.

kerry · #17 08-26-2012, 10:55 AM

There was at least one year in which the transmission was controlled completely by vacuum without a bowden cable or control rod. I don't know if there was a period when the transmission was controlled without vacuum at all. I doubt it. There are a few threads on here concerning the vacuum-only transmission which state which year MB used that method. It was around 78-80 I think.

ROLLGUY · #18 08-26-2012, 11:03 AM

Quote:

Originally Posted by kerry

There was at least one year in which the transmission was controlled completely by vacuum without a bowden cable or control rod. I don't know if there was a period when the transmission was controlled without vacuum at all. I doubt it. There are a few threads on here concerning the vacuum-only transmission which state which year MB used that method. It was around 78-80 I think.

I believe it is 1981 only. 1980 and earlier had the control rod, and 1982 and newer had a cable. My brother and I both have 1981 cars, and both have no control rod or cable. They DO have a VCV, and are very persnickety to get shifting correctly.

Shortsguy1 · #19 08-26-2012, 11:04 AM

Not particularly relevant to the OP's question, but the my w115's transmission has only a mechanical linkage to it and no vacuum control.

Graplr · #20 08-26-2012, 11:13 AM

Quote:

Originally Posted by ROLLGUY

Is this a manual trans car? It will not have a VCV if it is manual, but it should if it is an automatic. However, I can't see a VCV in either photo.

I was under the assumption that these early 617s have no VCV. I don't have on on my early 79 and don't believe there ever was one.

Phil, to adjust the shift points you need to adjust the linkages on the right side of the engine inbetween the air filter and the engine. I forget if lengthening or shortening causes earlier or later shifting but for some reason I remember thinking to myself it was backwards of what it intuitively seemed.

ROLLGUY · #21 08-26-2012, 11:13 AM

Quote:

Originally Posted by Shortsguy1

Not particularly relevant to the OP's question, but the my w115's transmission has only a mechanical linkage to it and no vacuum control.

How is it not relevant? The OP has shifting problems, and the car in question is a 123. Your 115 and it's equipment is NOT relevant to the OP's question.

Phil_F_NM · #22 08-26-2012, 11:58 AM

I DO have the control rod on the right side of the engine and all the linkage is there. I'm going to pop off each linkage joint today (one at a time) and lube them up with some white lithium. After that, I can start messing with the linkage to transmission.

On this car, there is no evidence of a VCV nor a bracket on which it would sit, so here's my question regarding shifting points:

Does the linkage provide for a linear adjustment of shift points in a non-VCV trans? I can see how a VCV would or could easily help the trans to shift in a log curve or at a different rate according to vacuum. Without the VCV, there is only hydraulic pressure and the linkage so my thought is that shifting would be more linear with accelleration unless there is a cam lobe somewhere down below providing for a variable/logarithmic set of shift points.

Maybe I'm getting ahead of myself but I see this in camera repair as well (German cameras, Leicas to be specific.) When adjusting focus in a rangefinder, the manufacturer provided for close-up, infinity and focus rate. All are precisely adjustable according to mechanical cam adjustment. I see no reason why Mercedes wouldn't have done the same thing with their transmissions. Just getting it figured out will be the fun part.

I digress.

Thanks for all the help so far. She's getting her motor cleaned today (gently, not with a pressure hose) and I'm going to do some more interior work.

I'll post any further issues I have but so far it's looking good. You've all been a great help. I wouldn't have caught that aneroid valve vent since the port is exactly the same size as the vacuum ports in the vicinity. After this, I'm going to have to find the right vacuum connectors with the proper orifice size to get the proper amount of vacuum for all the systems. For now, it's working quite well.

Phil Forrest

MB-Owner-in-ind · #23 08-26-2012, 12:22 PM

To adjust your shifting linkage you will see under the exhaust manifold there is an adjustment point. You have someone press the accelerator down to the down-shift switch and you loosen then tighten this adjustment. It will move to the proper spot bu itself if it is free.

If you are looking for more power check the start of delivery on the IP pump. I advanced mine and was able to climb a hill @ 35 mph that required down shifting before.

Also this car takes off in 2nd gear unless you floor it on take off or shift it into "L" and back to drive while standing still. 1st gear was there for towing.

Diesel911 · #24 08-26-2012, 12:33 PM

If you have an Automatic Transmission; if you have a Metal Rod that goes from the linkages back to the Transmission you will not have a Vacuum Control Valve.

As previously said the Aneroid; the Disc and Square shaped item on top of the Transmission is not connected to Vacuum. It is exposed to the Atmosphere.
It is there to change the amount of Fuel the Fuel Injection Pump Injects when you change altitudes. If not exposed to the Atmosphere it is not going to work right.

MB123Diesel · #25 08-26-2012, 01:32 PM

Just thought I'd chime in here since i recently adjusted the shift linkages in my '79 300D. A few others mentioned it, and they are correct; this trans has NO vacuum control. Shift points are controlled by the rod that runs along the passenger side of the engine block below the intake/exhaust manifolds. Lengthening the adjustment points will cause the trans to shift sooner while shortening them will cause it to shift later. Additionally, there is a bushing at the end of the control rod where it connects to the lever on the side of the trans below and slightly behind the firewall. I found that mine was completely missing causing weird shift behavior. Hope this helps.

-Chris

kerry · #26 08-26-2012, 06:10 PM

Quote:

Originally Posted by Diesel911

If you have an Automatic Transmission; if you have a Metal Rod that goes from the linkages back to the Transmission you will not have a Vacuum Control Valve.

As previously said the Aneroid; the Disc and Square shaped item on top of the Transmission is not connected to Vacuum. It is exposed to the Atmosphere.
It is there to change the amount of Fuel the Fuel Injection Pump Injects when you change altitudes. If not exposed to the Atmosphere it is not going to work right.

I had a metal rod and a VCV on my 79 116 300SD.

Peterac · #27 08-26-2012, 10:18 PM

On my 79' 300D without VCV, ie mechanical rod shifting. There is a mechanical modulator control on the transmission box. On the RHS at the back of the trans you will find a nut (can't remember the size maybe 15mm). Remove this nut it is just a cover for a flat head screw inside. The screw is the adjuster. Turn this (can't remember direction) quarter of a turn at a time either direction and note if shifts are later or sooner. Adjust accordingly. As the shifts are made later they also become softer, so there is a bit of fiddling to get it where you like it. I believe it is clockwise for later/softer shifts.

Peterac · #28 08-26-2012, 10:37 PM

http://www.peachparts.com/shopforum/diesel-discussion/304479-79-300d-upshifting-problem-2.html

Try this link. The mechanical modulator does affect the shift points as well as the soft/harsh change though!

Cheers.

Diesel911 · #29 08-26-2012, 10:41 PM

Quote:

Originally Posted by kerry

I had a metal rod and a VCV on my 79 116 300SD.

I certainly do not know everything about all the years and Models.

On my 84 300D a Vacuum Control Valve and a Vacuum Controlled Modulator.

My understanding is that the Rod was replaced by the Bowden Cable on the Later Models.

ROLLGUY · #30 08-26-2012, 11:20 PM

Quote:

Originally Posted by kerry

I had a metal rod and a VCV on my 79 116 300SD.

I would add that all the turbo models have a VCV from '78 -'85 (116, 123,126).

I stand corrected about the VCV on the '79 300D (there is not one). It seems that pre-1981 the non turbo cars had control rod only, no vacuum to the trans.
We all learn something new every day!