Automatic Transmission Erratic Shifts

Proper diagnosis is priority one - then repair! DIY testing and repair kits available.

I have a number of kits and manuals that will help you solve your diesel automatic transmission shifting woes. As these old automatics continue to age the problems become more complex and the solutions more challenging. 

Problem & Solution

Common among these chassis:


You will find these kits and manuals helpful in sorting out your own transmission problems. My 61x Transmission Tuning Manual is the longest manual I have written to date, but the customer feedback has made it all worth the effort. It is nice to know we are saving "one transmission at a time."  Kent Bergsma

When "de-bugging" an automatic transmission shift problem you need to go about it in a systematic manner. Just throwing money and parts at transmission shift problems will usually lead to much frustration and loss of money.

Before assuming the worst I recommend you do the following:

  • Don't spend any money on expensive parts until doing a proper diagnosis
  • The manuals and test kits I offer will help you isolate the problem
  • If you are having a shop work on your transmission be extra cautious
  • Get more than one opinion if seeking help from a transmission shop
  • Look outside the transmission first. Many problems are external

Don't even consider a rebuilt replacement until you read this transmission tuning manual! It will walk you through 8 phases of diagnostics and repair to help you get your transmission shifting properly. You may be surprised to learn just how many problems can be "adjusted out."  One customer found his transmission shifting with a hard jerk and sometimes it would not even go into fourth gear. His transmission shop told me he would need a rebuilt transmission ($2200). He brought it to our shop for Kent to take a look at it. Kent was convinced something was wrong with the modulator. Upon looking under the car he found the vacuum line fitting at the modulator had fallen off. Cutting a new piece of rubber short hose the line was back on in less than 10 minutes. Cost of repair $0.00 (freebie). 

Don't let something like this happen to you. Do your homework and proper diagnosis BEFORE you let anyone tell you you need a new tranny!


125 page transmission tuning manual and associated repair kits will help you "fix it yourself" and possibly save hundreds of dollars.

Kent's tuning manual covers in detail the critical adjustment procedures to the automatic transmission modulator and the VCV (proportional vacuum shift valve where applicable) On later 616 240D and 617 model turbo diesel automatics these two valves work in conjunction with each other and must be working and adjusted properly if you ever hope to get your transmission shifting as it should!  And as I have always said, you will know your transmission is on its game when you drive your Benz and don't notice it every time it shifts!

The shift adjustment process must first start with the vacuum modulator. This vacuum control unit mounted on the side of the transmission must be tested and adjusted to specs BEFORE you move on to other adjustments. Unfortunately this can not be done by guess or by feel. There is a pressure port on the side of the transmission that you must tap into and connect to a pressure gauge. The engine must then be started and the modulator adjusted to a set pressure for that specific transmission. It is not a difficult job with the proper test tools and information, and can be done in less than an hour. Too much pressure and abrupt shifts will occur. Too little pressure and you may experience slipping or flaring during shifts. If you are experiencing consistent abrupt shifts it could be that your modulator has failed completely. 

But you can't stop by just installing and adjusting a new modulator. The engine's vacuum system along with the VCV and 3/2 valves that deliver the right volume of vacuum to the modulator at the right time must also be tested and adjusted. Too little vacuum can lead to abrupt or jerking shifts. Too much vacuum to the transmission modulator can lead to soft or flaring shifts.  Sounds complex but it is not rocket science. Kent will walk you through the process step-by-step with his illustrated and easy to follow written instructions. 


Sometimes it may only be an inexpensive part that solves the problem. See all our related products below. Read the full descriptions of each as they will educate and enlighten you on the "secrets" of automatic transmissions.

Sometimes adjusting and tuning will not help. In that case CLICK HERE to read Kent's recommendations for last ditch efforts. There is always the possibility that in the end you will need to install a replacement transmission.... At least you will know will confidence that is your only and best option.