Does Your Automatic Transmission Really Need Replacing?

Be careful to not let someone sell you a rebuilt transmission!

The following are key signs your transmission may be in trouble. 

Problem & Solution

Common among these chassis:

W108 W109
W110 W111 W112 Sedan
W111 W112 Coupe
  1. Burnt smelling transmission fluid (dark in color)
  2. Fine metallic particles in the bottom of the oil pan
  3. Delay in shifting out of park. More than 1 second to engage in reverse or drive
  4. Won't shift into higher gears or does so reluctantly
  5. Transmission jerks or slips between shifts

But before you let anyone try to convince you to replace it, DO proper diagnose first!  Here are some diagnostic tips and some things you can do to improve performance and extend its useful life:

No. 1 and 2 mean trouble. There is not much more you can do than change the fluid and filter and pray. The smell of burnt fluid indicates high levels of clutch wear and friction leading to high heat buildup. Evidence of metallic dust in the bottom of the pan usually means excess gear or bearing wear. These indicators do not necessarily mean your transmission will fail next week or even next month. By keeping the fluid clean and using a transmission additive you can push more life out of a dying transmission. You will need an additive with a viscosity enhancer. I like the Lucas brand. This can reduce some of the clutch slippage and friction thus reducing heat. WARNING: this additive should only be used in high mileage transmissions showing true signs of slipping.You should also plan on being a little more gentle with it.  Avoid rapid acceleration if possible, shift manually when you can, and when starting out let the transmission fully engage before hitting the throttle.

No. 3 is a good indicator of total mileage and wear. When at a standstill with the engine running, move the shifter quickly from park to reverse or drive. Note how long it takes to engage. You will feel the slight clunk. If it engages immediately that is a good sign you have a strong transmission. If it takes close to 1 second to engage you have normal wear for a higher mileage transmission (over 150,000 miles). Don't panic. There is still some life left. If it takes 2 seconds or longer to engage - well, it's wearing out. This is an indicator only. Fixing the problem would require overhaul. 

No. 4 and 5 can mean many things and should not be of concern until YOU do some detective work. I can not begin to imagine how many Mercedes Transmissions have been replaced over the years that really did not need replacing. I have been witness to quite a number of examples of this myself, and I only live in one very small corner of this country. I remember one 300SDL a customer brought to me to find out how much I would charge to replace his transmission. He had just come from a shop that told him his transmission was shot and he looked like he wanted to cry. As I have learned many times in dealing with these type problems - I told the customer NOT to panic until we checked it out (remember - proper diagnosis comes first). I started by pulling the dipstick - looked at and smelled the fluid. That looked ok. When I test drove the car the transmission was shifting with a violent jerk. So hard it would almost snap your neck. Back at the shop it took me less than 5 minutes to find the problem. The part cost about 10 cents maybe. Test drove the car again and the transmission shifted perfectly. Now you can image how happy this customer was. I told him there would be no charge, but he slipped me a $20 bill anyway.  How does that compare to the $2000 the shop had quoted him for a rebuilt transmission! Read on to find out what I did....


A. Tune your engine first: This is way too often overlooked by owners trying to solve transmission shifting problems! Since throttle position regulates the vacuum to the modulator, it stands to reason the engine must be producing the correct power for the throttle position chosen. It also must be a smooth power curve. If your diesel has weak acceleration and power fix this first! Fuel injectors play a very KEY role here. On a recent 300D turbo I was working on, you can not believe the difference in shifting performance after installing newly tuned and balanced Monark Fuel Injector Nozzles.

B. Think Simple: When dealing with transmission shifting problems, always start with the simple things first. Many diesel automatics use vacuum to modulate and control shifting. That being the case, it stands to reason you should always begin by checking over your vacuum system. Begin by looking for loose vacuum connections in and around your engine compartment. You can't believe how may times I have found loose or incorrectly connected vacuum lines. Check all rubber hose connections for tightness. Refer to your vehicle's specific vacuum diagram for correct hose routing. 

C. Test for vacuum output and leaks: You will also need a vacuum gauge to test the output of your vacuum, as well as isolating vacuum leaks. We carry a number of different products on our website to help you with troubleshooting and repairing your vacuum system. 

NOTE: So, do you want to try to guess what the problem was with the 300SDL? That's right, it was a loose vacuum connection at the transmission modulator. Oil had soaked and swollen the rubber connector to the point where it just fell off. We now supply oil resistant and reinforced vacuum hose that you can install to prevent these kind of problems. 

D. Inspect and repair the 3/2 switchover valve assemblies:  1980 to 1984 diesel models have small vacuum switching valves located on the valve cover. These valves control the functions of the EGR as well as the vacuum control shifting valve (VCV).  I don't think I have ever seen a 3/2 valve assembly that did not need some type of repair. Most models have a black cover that protects this assembly. You will need to remove the cover tune to inspect for damage and leaks. Most valves have worn off levers, leaking seals at the input shaft, and loose or cracked hose connections. This assembly should be rebuilt before proceeding with any further transmission shift debugging. Complete overhaul kits are available here.

E. Replace Vacuum Modulator at transmission and set modulation pressure:  To read more about the importance of your vacuum modulator and why it should be replaced click the link that follows. Proper modulation pressure is critical to timely and smooth shifting performance! 

F. Tune your transmission for Peak Shifting Performance:  After all the above are taken care of, only then you will be able to fine tune your transmissions shift patterns for optimum performance. While most of the above repairs are science, I have found that tuning the transmission is more art than science. See the Manual below for Auto Transmission Tuning for Peak Performance.

If none of the above help and you still find the transmission will not shift or slips excessively when shifting then start looking for a replacement.  Don't be afraid of used if you can purchase from a reputable recycler.