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Mercedes-Benz Transmission terminology for dummies (722.XXX series transmissions)
I find that a lot of people immediately go to the internet and ask "why is my transmission not working" only to receive a number of seemingly cryptic answers filled with letters, numbers, and foreign terminology. Here is a quick crash-course on the most common terms and references when talking about these transmissions.
B1 - controls 2nd gear. A bad B1 band or piston will make the car shift from first to third.
B2 - most common failure in Mercedes transmissions for years, controls first gear, "shifting into drive"
B3 - reverse gear brake band. Some models of MB transmissions can be adjusted externally to reduce slipping and flaring when engaging reverse, others do not have this feature. If you have an older Mercedes with no reverse, the B3 is the most likely cause. I see this especially in the gas w124s and w201s for some reason, many many cars on the market with no reverse. This requires a trans rebuild to fix.
K1 spring - this is a spring in the valve body that controls the 2nd-to-3rd gear shift. Over time, both the K1 and K2 springs wear out and cause the transmission to flare a bit between shifts. There are "repair kits" available with new springs and seals for the DIYers, not a difficult fix. These springs actually control how the corresponding clutches (called K1 and K2 clutch packs) operate.
K2 spring - similar to K1 controls the 3rd-to-4th shift, along with reverse and overdrive.
"Flaring" - this refers to the RPMs spiking in between shifts under normal driving. This is usually caused by either overly-soft shifts (worn out springs and/or vacuum issues).
"slipping" - This means the car goes in and out of gear, typically creating a jerking motion under acceleration, sometimes also a bad jutter.
Vacuum tuning - The 722.xxx series transmission shifts are controlled by vacuum. A line comes off the engine and plugs into a part called the "vacuum shift modulator" on the passenger side, that will denote how firmly or how softly the car will shift between gears. If you completely disconnect the vacuum line, shifts will be extremely firm (neck-snapping in my diesel!). You can adjust the modulator by turning a plastic tee-connector on the outside of the transmission.
Bowden Cable - This adjusts the shift points. The Bowden cable is attached to the throttle linkage, and basically tells the transmission how far down you're pressing the accelerator...floor it and the shift points will be predictably higher. This typically cable stretches over the years and needs tightening, which can be done on top of the valve cover at a white cable-screw followed by a rubber accordion cover.
Symptoms of a bad B2 piston - If your piston is scratched or scored, it will "catch" on the outer bore of the transmission housing, bushing or seal. This causes intermittent engaging of the forward drive gears. Basically, you can shift into drive, but the second you put any load on the transmission it slips. You won't be going anywhere fast. Many times a sticking B2 will turn into a broken B2 piston since the hydraulic pressure is extremely high, and scoring is only going to get worse (i.e. don't wait to fix it!).
Symptoms of a bad B2 band - this is what I'm dealing with at the moment. The piston itself is fine, no catching, marring, or scoring. However, there is no spring tension on the B2. I am able to push the piston in and it will not push out against my pressure - it should have quite a lot of pressure on it, so much that it is a difficult task to replace the outside cover without some sort of improvised "tool" jammed in between it and the transmission housing (I used a pipe the first time). There is NO drive, no forward motion whatsoever. To expand even more, you will not see a change in RPMs when shifting into drive, and not hear any hydraulic actuation under the car when you shift. Strangely, when mine went, there was no debris in the pan indicating it was a clean break or the debris is stuck somewhere (very possibly the valve body).
Sent from AutoGuide.com App