Mercedes Benz Transmissions Repairs
Early transmissions such as the 722.3 usually have recommended service intervals of every 60k. The fluid used should carry the correct specification i.e. MB 236.7 which is Dexron II D. You can call us if you want an estimate on a transmission service or need information on the service interval. We will look up and use the correct fluid unless you request otherwise. It is common to use Dexron III, but be aware fluids with the correct Mercedes Benz specification should only be used. In most cases Dexron, unless it is marked on the bottle with the MB specification, being met may not be approved. If possible we always drain the torque converter, because omission of this step leaves 4 liters of transmission fluid in the vehicle. With the onset of lifetime fill electronic transmissions in the late 1990s the torque converter drains were omitted. These transmission are controlled by vacuum and modulator pressure, which can be adjusted and have some weak areas where vacuum leaks are common.
The 722.6 has been a very reliable electronic transmission and is much simpler to repair than its predecessors. One problem that was ongoing with this transmission is leaks at the plug adapter for harness connector. There are various versions of this adapter and o-rings. Some have tabs, and the adapter is best replaced complete with o-rings. If the adapter o-rings should be black, red, or white o-rings then they are prone to leak. Connectors with tabs will also leak. The electronic transmissions are lifetime fill in most cases and have a lock on the dipstick. If you replace the adapter you need to adjust the fluid level with a special transmission dipstick tool. The fluid used is not Dexron III but a variant of it, so don’t use ordinary A/T fluid in an electronic transmission with lifetime fill. The correct fluid is 001 989 21 03, MB 236.10. This fluid is backward compatible with all earlier transmissions. The dipstick is a special tool and the dipstick cap has an anti-tamper plastic lock. The fluid after MY 2006 is the same in a 722.6 as a 722.9. Don’t use the 722.6 fluid in a 2006. The newer fluid is identifiable by its lightly purple color. The newest fluid is backward compatible with all Mercedes transmissions. Some late model 722.6 transmissions have a scheduled transmission service, oil, and filter at 39k. Transmission service completed with the correct fluid could only be beneficial, even if it isn't called for.
Early 722.6 transmissions had a poor valve body conductor plate design. This part has the starter lock out switch and a transmission VSS sensor built into it that powers the shift solenoids. If it fails you may have an intermittent no-crank no-start condition and the transmission may shift irregularly because the speed sensor signals needed to calculate smooth shift adaptations are faulty. This part can be replaced with the transmission in the vehicle and the valve body removed, which is good to do if considering a transmission service on an older vehicle. If you are experiencing shifting concerns or start delay, please contact us for details as we can scan for codes that would indicate you need this repair or other transmission repairs.
One note on torque converters and transmissions is there considerable variation in these parts and the best repair is installing a Mercedes-Benz remanufactured unit as it will be an exact match. Used parts will in most cases not match and may be problematic even if they are good, Control units can be very specific and may not work correctly. Dealer remanufactured is the highest cost option but is the best and fastest turn around. Alternately, we can rebuild your transmission, and have your existing torque converter rebuilt depending on the extent of the damage. Torque converters that need to be rebuilt are sent out for this repair. They are punch marked, cut in half by a dedicated machine and cleaned of any debris. The lockup clutches are replaced if needed and welded back together on a dedicated machine. We don't recommend replacing one with an aftermarket remanufactured one, as you are best to have the one from the vehicle rebuilt. There are too many variations of these to expect you will get the correct one, although it will fit, internally it will have a different design and stall characteristics. A new unit from Mercedes is the best repair but it may be over 3x the price of having yours rebuilt. In many cases the torque converter is replaced simply because it may be contaminated with material from the failed transmission and if it is reused the new transmission may be damaged. Having it opened inspected and rebuilt eliminates this concern.
Another issue with the 722.6 involves transmission fluid wicking up the harness into the control module and causing the module to malfunction. Usually, the harness from the transmission to the ETC module is replaced and the module is replaced provided the leak is repaired. In some cases this may be more than is actually needed but it usually fixes the concern. Transmission fluid does not actually damage the module and can be flushed away with board cleaning solvent or electrical contact cleaner as a first attempt. Always repair the leak before replacing a module.
Another issue with the 722.6 was in Valeo radiators leaking coolant into the transmission fluid, which causes harsh engagement followed by a droning vibration. There is a glycol test to determine if this is the case. If strongly positive the radiator and torque converter are replaced and the transmission and lines are flushed. The dealer used to replace the transmission also but no longer does this commonly. If the contamination is slight the radiator only is replaced and the transmission and lines flushed. Consider having the transmission clutches replaced if the contamination is high, i.e. visible contamination. The clutches are very sensitive and it may not be fully repaired unless the radiator, torque converter, and transmission are replaced or rebuilt. If you have this complaint and the glycol contamination is not present you may need a new transmission control unit. The newest software can correct an adaptation issue with the torque converter than can also be the cause. This should be diagnosed by a specialist. The transmission control unit is replaced and adapted to repair this because the transmission software is not flash updatable as it is on the predecessor.
The next transmission in the series is the 7-speed 722.9. It has no dipstick tube and can only be filled or checked from below. If your vehicle has a 7-speed you may have noticed that 722.9 transmissions have no hill holding capacity, and they roll backwards with your foot off the brake. Some have a hill holding feature where they use the brakes to hold briefly. The reason is this, when the 722.9 was designed it had to fit into the same envelope as a 722.6, as it was going into vehicles already designed and in production. If you change or modify the transmission tunnel you have to repeat all the crash testing as it is a crumple zone. Since the 722.6 has five gears and the 722.9 has 7 gears, the extra Ravigneaux gear set that makes this possible that has to fit into the same package, requiring something non-critical had to be omitted. From the 722.6 design two sprag clutches were omitted. These are essentially freewheel elements that spin in one direction but lock up in the other. Without them a transmission will not lock up when it rolls backward in 1st or 2nd gear. If you have ever wondered why a 7-speed automatic doesn't hold a hill like the 5-speed automatic does that’s the reason, it's not broken.
There are numerous issues with the 722.9 particularly the early ones. Contact us if you are having problems with one. These as the 722.6 can rebuilt and repaired readily.