Reviving a High Mileage Automatic Transmission

How to improve performance and prolong the life of a tired gas or diesel automatic transmission

I do not normally recommend the use of additives in good low mileage transmissions. On the other hand, if a transmission has over a hundred thousand miles and is exhibiting poor shifting performance, I don’t hesitate, just as long as I have tested and adjusted all external transmission controls first. For more information on D.I.Y. shift diagnosis, adjustment and fine tuning see related products below.

Problem & Solution

Common among these chassis:

W108 W109
W110 W111 W112 Sedan
W111 W112 Coupe

Now don’t get me wrong. I am not saying that additives should be used in all cases of poor shifting. Many shifting problems (especially on diesel automatics) are caused by loss of vacuum or by improper adjustment of the shifting controls. One must first determine that the vacuum modulator is working properly, the proportional shift valve is delivering the correct vacuum, and the bowden cable or pressure control rod is adjusted properly. If these conditions are not dealt with first, an additive won’t solve your problem.

Transmission fluid additives are designed to do one of three things: 1. Soften and expand external rubber seals to stop or diminish fluid leaks. 2. Soften internal o-rings and clean passages to improve shifts controlled by the valve body. 3. Increase friction between worn clutches to reduce slipping during shifts. Understanding these properties will help you decide when it is time to try an additive.

In my experience, certain shift problems are more likely to be helped by using an appropriate additive:

  • Very abrupt shifts:  By abrupt, I mean the kind of shift that “jars your teeth.” (Note: This condition can also be caused by a bad modulator or no vacuum getting to it. This MUST be checked first). Abrupt shifts can be caused by valves inside the valve body sticking or “hanging up” in the passages. An additive can remove the gum or tarnish in these passages thus freeing the valves for smooth travel. This is especially true if the car is not driven often or has just come out of long storage.
  • Transmission reluctant to shift after long periods of storage (sticking valves)
  • Transmission not shifting into 3rd or 4th gear: This is often a valve body problem and may be helped by using an additive. Keep in mind that 1980 and 1981 non-turbo diesel automatics may not shift into 4th gear when vacuum is lost to the modulator.
  • Transmission slips excessively during shifts or takes over 2 seconds to engage from park to reverse or drive: This condition can be caused by worn clutch packs and can only be permanently fixed by removing and overhauling. But it is possible to extend worn clutch life by using an appropriate additive. Burnt fluid smells are a sure sign your clutches are slipping and overheating. (Note: Keep in mind slipping during shifts can also be caused by improper adjustment of the modulator or too much vacuum to it. - be sure to tune your transmission BEFORE resorting to a viscosity additive!)

I use two different brands of additive. TransX is a super thin additive that I use exclusively for leaks or problems associated with a sticking valve body. Lucas transmission additive is more viscous and I use this additive for slipping problems associated with worn clutch packs. One quart added to your transmission is recommended. Do not over fill. Remove one quart of fluid before adding or in the case of burnt fluid do a complete fluid change before adding the Lucas additive.

Adding Lucas additive (round bottle) to a worn transmission. Note cutting hole in bottle to vent. This will increase fill speed... :-)

Conclusion:  I often use an additive in an attempt to "wake up" older transmissions or  get them to "behave" a little better. Most of my success has come when doing this on vehicles that have been in storage for long periods of time or are not driven often. Some transmissions will indicate typical symptoms of sticking valve in the valve body which can result in abrupt shifting, late shifting and sometimes not even shifting at all. Before using an additive, first go through the proper procedures to check to see if it is a vacuum problem. Many shifting problems are related to either a lack of or too much vacuum getting to the vacuum modulator.

I would like to share with you a another success story that happened on August 16, 2009.  This occurred when bringing my daughter, Kaia's, 1968 280SE out of storage to get it ready for some summer cruising. When taking it on a test drive, I found the automatic transmission was racing up to a high speed and shifting with an abrupt clunk.  I had to grit my teeth every time it changed gear. I took it back to the shop and inspected the vacuum line going to the modulator and replaced the rubber hose connector. I took it back out on the road and it did the same thing.  I decided it was time to add some TransX additive. You cannot believe what happened. It was like a miracle!  For the first 8 to 10 shifts it continued to jerk but it suddenly made an unfamiliar shuddering noise during one shift and cured itself.  Within one day of driving it is now shifting better than it ever has.  Not only does is shift smoother, but it now shifts at the proper RPM and even down shifts better. How is that for a success story!  Kent Bergsma

Kaia's sweet W108 280SE ride. She calls her Mary Jane...

Final Recommendations: How do you know you need a replacement? If no adjustment or additive helps then proceed to look inside the transmission. If your transmission fluid smells burnt and/or you take the pan off the transmission and find excessive metal particles in the bottom then it is time to quickly start looking for a replacement. Do not rule out installing a good used transmission as long as you purchase it from a reputable wrecking yard with a warranty. I do not recommend purchasing from a private party unless you can road test the car before the transmission is removed. I do not recommend you have a general transmission shop overhaul your transmission unless they can show you they have rebuilt a large number of Mercedes automatic transmissions. 

Kent has written a manual on how to remove and install an automatic transmission. We also have some special tools and supplies that will help you do this job yourself. Also, anytime you remove the pan for inspection you must change the filter and pan seal.

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