Strangest shifting problem?? - PeachParts Mercedes-Benz Forum

Knight Rider521 · #1 11-09-2004, 10:03 AM

The transmission in my 300sd has been acting very strangely the last few years. Here is the situation. When it is summer, spring, or early fall, and thev ventilation comes through the center console vents, the car shifts beautifully. But when the system changes over as if it were heating the car, and the air vents to the front windshield and floor, I get alot of hard shifting throughout the gears and this strange "clunk" for lack of a better word, when I get down to about 11 mph. This strange "cluck" feels like a harsh downshift, but the transmission is already in 1?? the strange thing though, is that if I turn on the compressor, with the heat on, only with the double arrows (one up and on down) it shifts great again, as if it were still sending air out of the center vents. Has anybody else had a similar problem?? I figure that the problem is vacuum related, but do not know which vacuum elements or hoses should be replaced or checked. Thanks in advance,

Knightridder 521

#2 11-09-2004, 12:20 PM

You might try to go around the vacuum lines that lead into the heater, just to correctly diagnose the problem. The vac line to the trans is easy to trace, keep it plugged in and remove the line leading into the heater. Drive the car and see if the harsh shifts stop.
The vac to the trans softens shifts. Takes a fair amount of tuning to get it right, but from what you are saying I am guessing you have a large vac leak to the heater control in there. That leak is most likely under the dash, at least behind, or in, the ACC. There are some very short vac lines in there that are prone to leaking, and there are some vac driven servos that can leak. If you are losing enough vacuum to cause the trans to shift that way, I would guess you have an open connection. Maybe a line is completely off from a connector. A mighty vac might be a useful tool here.

H-townbenzoboy · #3 11-09-2004, 12:20 PM

The 300SD we have does that also, although the clunk will sometimes stop without the acc or sometimes, it'll clunk with the acc on. The mechanic said it's something about vacuum, and that the tranny may need to be rebuilt. How many miles do you have on yours?
-Joe

whunter · #4 11-09-2004, 12:44 PM

Quote:

Originally Posted by Knight Rider521

The transmission in my 300sd has been acting very strangely the last few years. Here is the situation. When it is summer, spring, or early fall, and the ventilation comes through the center console vents, the car shifts beautifully. But when the system changes over as if it were heating the car, and the air vents to the front windshield and floor, I get a lot of hard shifting throughout the gears and this strange "clunk" for lack of a better word, when I get down to about 11 mph. This strange "cluck" feels like a harsh downshift, but the transmission is already in 1?? the strange thing though, is that if I turn on the compressor, with the heat on, only with the double arrows (one up and on down) it shifts great again, as if it were still sending air out of the center vents. Has anybody else had a similar problem?? I figure that the problem is vacuum related, but do not know which vacuum elements or hoses should be replaced or checked. Thanks in advance,

Knightridder 521

There is nothing wrong with your transmission.

What you describe is a pure vacuum problem.

Get out the MityVac and vacuum routing diagrams.

Best guess:

Your climate control system has a VERY BAD VACUUM LEAK, or the transmission vacuum modulator tube is connected to the wrong port, it is possible that the vacuum modulator is bad, or it can be a combination of these issues.

Please don't waste your time and money thinking rebuild/replace transmission.

Knight Rider521 · #5 11-09-2004, 04:15 PM

I will get my hands on a mighty vac, hopefully for cheap, and take the vacuum measurements this weekend. Where can I get a copy of the vacuum routing diagrams and their proper vacuum readings. In case this helps, determine the problem, The air vents from the correct locations at the correct times. I knew that the problem wasn't a bad transmission though, because it shifts great when I change the air venting locations.

tangofox007 · #6 11-09-2004, 05:27 PM

Given the symptoms that you describe, you most likely have a leak in one of the climate control elements that actuate the doors that route air flow in various CC configurations. The elements have one or two vacuum connections. You should be able to isolate the problem by plugging the vacuum lines that are used in the configuration(s) that cause your transmission problems. (The problem could be in the element or associated plumbing.) There are repair kits available for the vacuum elements.

Knight Rider521 · #7 11-09-2004, 07:19 PM

Tangofox007, the vacuum elements which you refer to are the contraptions with a reddish top and black body with a metal element sticking out one end correct. If I recall correctly, these units are behind the dash and are somewhat difficult to get to. Is there anyway to isolate these units, or temporarily override them, let's say, make it so that the air travel pattern does not change from center vents to floor and window, or so that it flow to all the entire time and fix the problem??

whunter · #8 11-09-2004, 07:28 PM

Quote:

Originally Posted by Knight Rider521

I will get my hands on a mighty vac, hopefully for cheap, and take the vacuum measurements this weekend. Where can I get a copy of the vacuum routing diagrams and their proper vacuum readings. In case this helps, determine the problem, The air vents from the correct locations at the correct times. I knew that the problem wasn't a bad transmission though, because it shifts great when I change the air venting locations.

Vacuum Diagrams W123 and W126 models
Vacuum Diagrams

Diesel Giant · #9 11-09-2004, 07:49 PM

My 84 300D use to do the clunk thing. The vacuum signal is way too low to the modulator valve. My prob was the green vac line that goes to some vac switch under the dash. I believe it controls the fresh air flappers. Just disconnect the green line in the engine compartment and plug it up. The fixed my prob. When I get some spare time I will find out which vac dash pot is leaking.

tangofox007 · #10 11-09-2004, 08:07 PM

Quote:

Originally Posted by Knight Rider521

Tangofox007, the vacuum elements which you refer to are the contraptions with a reddish top and black body with a metal element sticking out one end correct. If I recall correctly, these units are behind the dash and are somewhat difficult to get to. Is there anyway to isolate these units, or temporarily override them, let's say, make it so that the air travel pattern does not change from center vents to floor and window, or so that it flow to all the entire time and fix the problem??

Some are harder to get to than others. You might try changing to the climate control set-up that causes the transmission problems while watching to see which elements are actuated when you make the change. Then plug the vacuum line at the element that is applying suction at that time. If that solves the shifting problem, you have identified the offending component. Remember that the problem could be the lines and/or hoses that supply vacuum to the actuators. If the problem occurs when going from cooling to heat, the flap actuator that cuts off the center vents is suspect.

Knight Rider521 · #11 11-12-2004, 10:59 AM

What is the easiet way to access the vacuum lines which I need to be testing. It appears to me that the majority of the lines I should be concerned with are under the fuse box. What do I need to take a loose to remove or move the fuse box to access the lines. Also what is a "good vacuum" which I shoul be looking for when I connect to the lines??