Would somebody tell me how does transmission up shifts. Please don't refer me bowden cable. Ok lets work out this way with an example;

Assume a car is standing still on top of a mountain, and you start moving it downhill without throttle but engaging the transmission (in D). You are not touching the throttle but car is accelerating (because of gravity) and upshifting. Now what is causing it to upshift???

How Stuff Works

didn't help much. don't want theroy. want to know how it works in a w124.

It has to do with the engine speed and the speed at which the vehicle is moving. Back in times before computers it was controlled by vaccuum, the engine produces a lot of vaccuum at idle but less as you increase RPM, because you are opening the trhottle and increasing air flow through the intake which lessens vaccum to the accessories, that is why you have vaccuum resevoirs and why the cars with vaccuum control heating and AC would do wierd things when you accellerated quickly. The modulator would get the vacuum signal from the engine and upshift according to drop in pressure. Diesels are a whole different story though because their vaccuum systems are so different. Now computers do it all. But like you said if you left your engine idling, started down a hill and didn't press on the gas, the speed is what upshifts, the tranny has to keep up also as you move faster the engine will start to increase in rpm thus decreasing vacuum and signaling a shift allowing the rpm to go back down. This is done through the clutches in the transmission. With the new computer controlled transmissions, the engine speed and vehicle speed are all monitored as well as the throttle position. When you floor it, it knows to downshift. The computers also learn driving characteristics of each driver and adapt. Electromagnetic silenoids now take the place of vaccuum modulators. I hope that helps in the explanation. I am not an expert, but that is what I have learned myself as to an explantion of how it works. Thanks and have a great day!
Regards,
Brian

__________________
Brian W. Heitman

www.MBCA.org/northtexas

Join MBCA!!!

04-17-2004, 08:15 PM
C32AMG
Registered User Join Date: Mar 2004
Location: new york
Posts: 183

The primary pump, supplies pressurized fluid to the hydraulic circuit. This pressure applies bands, clutches and lubrication for all moving parts. The most hydraulic circuits are found in the valve body. As the fluid moves into the valve body it has the potential to actuate every single valve. When the valves move they redirect the pumps pressure to control the application of bands and clutches thus providing up and down shifts and different ratios through the planetary system. The pumps working pressure is always the highest pressure in the hydraulic system.

All other pressures are derived from this maximum pressure and is reduced and controlled by regulating valves. The primary control valve is the pressure regulator valve. Mercedes calls this valve the control valve working pressure. It controls and regulates the amount of working pressure in the transmission. This pressure varies depending on which way the valve moves. If the valve moves towards the spring, which sometimes breaks, we have low working pressure. If it does not move, or moves a little, we have high working pressure. There are other pressures that balance and regulate the pressure regulator but when we have 210 pounds of working pressure in a vehicle in drive, the problem is usually the pressure regulator.

The governor is the road speed input from the transmission. It has no regulating pressure from a stop and does not control a first gear start in any valve body. In a Mercedes first gear start valve body, the 1-2 command valve is stroked in the first gear start position; by design once the car is started, commanding the B2 band on and the F, one way clutch locked. If the valve body were a second gear start, then the one two command valve would be stroked in the 2nd gear start position commanding the B1 and B2 bands on.

The description of operation in that PDF is for a 1996 C22, which Is a 722.4 transmission that starts in second gear.

What they are referring to by the engagement dampening circuit, is the accumulator cutting in oil circuit wish is a dampening system designed to soften shift engagements from a dead stop (not at road speed):

The dampening circuit just cushions the selected gear engagement.

N to D = B1 and B2 = 2nd Gear
N to 3 = B1 and B2 = 2nd gear
N to 2 = K1 and b2 and f is locked = 1st gear
N to R = K2 and B3 = Reverse




In the neutral position there is no power flow between the engine and transmission, no bands or clutches are applied. This is also the towing and push start feature of the car.

By moving the transmission lever from neutral to drive at road speeds, you are going from a pressure less system (neutral) to a high pressure system (drive). At that high pressure you are probably freeing up whatever is stuck in the valve body.

I think the problem is in the valve body. If that car was at my shop I would change it.


In any 722.3-4-5 MB or Porsche 928 if you move the selector lever manually to second, the transmission will start in first gear. This is engine breaking. Try that as a test.




This is a foot note to my above post.


How a transmission shifts is based/ derived from a balance /unbalance of pressures


The main plays are

Governor, road speed input oil.
TV Bowden cable / rod, load input
Modulator pressure load and speed dependent.
Working pressure, pump

Shifts happen when governor pressure moves the shift / command valves in the valve body (control center) in opposition of TV, Bowden cable/ rod, control pressure, making the connection between main pressure and the appropriate band and clutch drums

The valve body is an analog computer.

__________________
Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.

wbain5280 The valve body is an analog computer.






All the shifts in a W124 are hydraulically controlled, no ECU (computer)

so is there any way i can change the behavoiur of my tranny not to shift to 4th until reaching 60Km/h(37mph), because right now it upshifts to 4th at 40Km/h when i let off the throttle to apply brakes.

bowden cable sets the shift points while you are opening the throttle, what thing should manages to prevent upshift when you let off the gas pedal??? this is all i want for a long time.

Is this behaviour (upshift when letting off throttle) because of Vacuum Leak or Bad Modulator, becuase i adjusted my modulator and nothing happened, i still have hard shifts.

My point exactly. No electronics or digital programs are involved, only hydraulics and valves. We are talking about the same thing.

__________________
Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.

Based on the excellent post below, transmissions are under fluid pressure.

If I have a significant leak, say in the pan gasket, should I assume pressure is reduced and my shifting performance will be not be optimal?

I ask because I had a transmission service yesterday, which involved a new filter (the old one was filthy, but no debris or metal) and pan gasket to replace the faulty one, and I am noticing a difference. I had a pretty bad leak -- literally hemorrhaging tranny fluid when the car was parked, which I assume was a pan gasket leak.

__________________
2008 E350 4matic / Black/Anthracite

------------------------------------
Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

The Bowden cable delays delays the upshift when you have the pedal depressed, the more depressed the later the shift. When you let off the pedal to apply the brakes, all the transmission 'knows' is that you have let off the pedal and it is now time to upshift.

The easiest way to prevent upshift is by manually selecting '2' '3' and 'D' at the appropriate times. Leave it in 2 or 3 and it won't upshift when you let off the pedal!

As explained, upshifting on pedal lift is normal behavior!

As you can see in the following link, the vacuum proportioning valve on the IP, the vacuum modulator on the transmission, and Bowden cable adjustments should be made in a specific order to achieve the desired results.

http://www.continentalimports.com/ser_ic20242.html

Best Regards,
Jim

I don't think the fluid in the pan is pressurized, any more than the oil in your engine sump is pressurized. The oil in the transmission, and the engine, are pressurized for internal use, and drain back to the pan to be recirculated and repressurized.

A leak will lower the fluid level, and transmissions are a lot more sensitive to fluid level, to the point where drivability is affected. The engine oil level only affects the cooling capacity of the oil, until it is so low that air is entrained and it foams.

Best Regards,
Jim