First lets start off and explain
what a differential is and what it does. A differential converts the
turning force of the drive shaft into the turning force of the wheels,
the differential also allows the rear wheels to turn at different
speeds (such as going around a turn). The differential splits the
engine torque 90 degrees by using a ring and pinion gear, the drive
shaft turns the pinion which turns the ring gear, the ring gear is
attached to the axles thus splitting the power to the
wheels. To calculate the gear ratio of a rear end one divides the
number of
teeth on the ring by the number of teeth on the pinion. The higher the
number
the lower the car is geared, and vise versa for lower/taller. So if one
replaces
gears for a lower set (higher number) one will have better
acceleration, but
a reduced top end, and decreased gas mileage. So now that we have a
basic knowledge of how a differential works lets get on to the swap.
Ring and Pinion Gears
From the factory all U.S. bound E420/400E's came with a very high rear end ratio of 2.24 (Euro models had a 2.65). This high ratio is great for highway cruising and for fuel economy. However a car with this much power deserves to have gearing that makes better use of the power. Since Mercedes did not install a 2.82 differential from the factory (like they should have), I took the matters into my own hands. The first decision was to choose the appropriate gear ratio, the choices I found are 2.65, 2.82, and 3.07. The 2.65 was not enough of a change and it would not have made enough of a difference for my personal taste, the 3.07 was too low of a ratio, and would greatly reduced gas mileage, and highway drivability. My choice was the 2.82 it was the perfect compromise between acceleration, gas mileage, highway cruising, and top speed. However if one never drives on the highway then the 3.07 would be a good differential for around town.
Now for the hardest part of this mod the... finding the correct gears. There are two ways to change the gear ratio on a car, one is to just replace the ring and pinion gears (ring and pinion gears must always be replaced as a set) and keep the old differential housing, the other choice is to replace the whole assembly (commonly called a pumpkin). I did a lot of research and found out that, not only would it be cheaper to buy a used pumpkin then replace the gear assembly but also much easier.
The search began to try and find out what Mercedes car received a 2.82 differential using the same pumpkin as the E420/400E. After looking at the pumpkin on my own car I found a 129 part number (SL), I then found out through more research that the E420 shares the same differential housing, and most of the same drive line components as a R129 SL. Knowing that the SL uses the same parts I then searched for the gear ratios for the SL and none were 2.82, but rather 2.65 (SL500/SL600) and 3.46 (SL320). I then found out that the W140 S-class shares the same differential as the R129 SL. After weeks of searching on the net for a list of the gear ratios for the 140 I found a site in Japanese that stated the 400SEL had a 2.82 rear end, after looking into the matter more I found out that all 140 S-classes with the 4.2 liter 119 V8 had the 2.82 rear, these cars include; 92 400SE, 93 400SEL, and the 94+ S420. Finding the correct car to pull the differential out of is only part of the story.
An option on all E420/400E's was ASR (a type of traction control that limits rear wheel slippage). Why am I talking about ASR? Well the differentials on an ASR car and a non ASR car are different. The non ASR car (such as my car) has a single sensor hole for the ABS system sensor, the single sensor is located on the pinion gear shaft, it senses the shaft speed for the computer. While the cars equipped with ASR have two sensor holes, one on each axle output shaft (these are used for both the ABS and ASR systems). The only years that the 140 S-class came without ASR was 92-94 (ASR was an option on all early 140 S-classes), from 95 on the S-class came standard with ASR, thus any differential from 95 on will not work on a non ASR car. My particular differential came out of a 94 S420 without ASR.
Installation of the differential is quite easy, just pop the old one out, take the back cover off the original differential put it on the new one and bolt back up, the back cover on the S-class is different due to a different mounting design. Here are some pics however there is not that much to look at.
Ring and Pinion Gears
From the factory all U.S. bound E420/400E's came with a very high rear end ratio of 2.24 (Euro models had a 2.65). This high ratio is great for highway cruising and for fuel economy. However a car with this much power deserves to have gearing that makes better use of the power. Since Mercedes did not install a 2.82 differential from the factory (like they should have), I took the matters into my own hands. The first decision was to choose the appropriate gear ratio, the choices I found are 2.65, 2.82, and 3.07. The 2.65 was not enough of a change and it would not have made enough of a difference for my personal taste, the 3.07 was too low of a ratio, and would greatly reduced gas mileage, and highway drivability. My choice was the 2.82 it was the perfect compromise between acceleration, gas mileage, highway cruising, and top speed. However if one never drives on the highway then the 3.07 would be a good differential for around town.
Now for the hardest part of this mod the... finding the correct gears. There are two ways to change the gear ratio on a car, one is to just replace the ring and pinion gears (ring and pinion gears must always be replaced as a set) and keep the old differential housing, the other choice is to replace the whole assembly (commonly called a pumpkin). I did a lot of research and found out that, not only would it be cheaper to buy a used pumpkin then replace the gear assembly but also much easier.
The search began to try and find out what Mercedes car received a 2.82 differential using the same pumpkin as the E420/400E. After looking at the pumpkin on my own car I found a 129 part number (SL), I then found out through more research that the E420 shares the same differential housing, and most of the same drive line components as a R129 SL. Knowing that the SL uses the same parts I then searched for the gear ratios for the SL and none were 2.82, but rather 2.65 (SL500/SL600) and 3.46 (SL320). I then found out that the W140 S-class shares the same differential as the R129 SL. After weeks of searching on the net for a list of the gear ratios for the 140 I found a site in Japanese that stated the 400SEL had a 2.82 rear end, after looking into the matter more I found out that all 140 S-classes with the 4.2 liter 119 V8 had the 2.82 rear, these cars include; 92 400SE, 93 400SEL, and the 94+ S420. Finding the correct car to pull the differential out of is only part of the story.
An option on all E420/400E's was ASR (a type of traction control that limits rear wheel slippage). Why am I talking about ASR? Well the differentials on an ASR car and a non ASR car are different. The non ASR car (such as my car) has a single sensor hole for the ABS system sensor, the single sensor is located on the pinion gear shaft, it senses the shaft speed for the computer. While the cars equipped with ASR have two sensor holes, one on each axle output shaft (these are used for both the ABS and ASR systems). The only years that the 140 S-class came without ASR was 92-94 (ASR was an option on all early 140 S-classes), from 95 on the S-class came standard with ASR, thus any differential from 95 on will not work on a non ASR car. My particular differential came out of a 94 S420 without ASR.
Installation of the differential is quite easy, just pop the old one out, take the back cover off the original differential put it on the new one and bolt back up, the back cover on the S-class is different due to a different mounting design. Here are some pics however there is not that much to look at.
