While the S65 engine of the E9x M3 has been the subject of fairly extensive study by many tuners and BMW enthusiasts since the car’s development, we feel that the knowledge base for the car’s incredible DCT transmission has remained quite limited, especially when compared to the wealth of information available about the car’s many drivetrain, suspension, and control systems.

With this in mind, we began to take a closer look at the DCT to understand what limitations the transmission had, and where those limitations would stop us and our fellow enthusiasts in enjoying our M3s. Over time it became abundantly clear that the transmission hardware was overall incredibly robust. Even after installing multitudes of ESS VT2 supercharger kits on our client’s cars and seeing those same cars log hundreds of track miles for several years, we’ve seen no hardware failures to date.

Clearly, then, the physical hardware seems to be robust enough for the most demanding M3 driver in even very stressful situations. From what we’ve seen, if the M-DCT transmission has any one weak point, it is temperature control. Damage to the clutches and other internal components can occur if the transmission temperatures climb above a safe level. Fortunately for all of us BMW M has protected us by forcing the M3 to enter a safe mode when the transmission temperatures rise above acceptable levels.

It is possible to achieve high enough operating temperatures to engage limp mode if operating in very high ambient temperatures while simultaneously making very high power levels. Although we’ve heard of very few instances where this has occurred, two of our clients with very high output engine setups operating in extremely harsh have encountered BMW’s DCT safe mode. Although both have only seen limp mode a couple of times, this prompted us to investigate further. Oddly, even basic information regarding the transmission’s operating temperature ranges was largely unknown by the BMW enthusiast community. After repeated queries, even our best contacts at BMW were not able to say exactly where the M3 chooses to enter safe mode to protect the DCT transmission, or what criteria are used to calculate this critical temperature!

I wanted to share the information we do know with the M3Post community, in hopes of expanding the community’s knowledge of the M-DCT transmission.

The DCT transmission uses two coolers to control temperature: a coolant to oil heat exchanger and a oil to air heat exchanger.



The coolant to oil heat exchanger facilitates a faster transmission warm up by circulating transmission oil through a bypass valve in the transmission thermostat (the thermostat is part number 9 in the above diagram). The transmission oil exits the thermostat and travels to the top of the transmission via a soft line (part number 12 in the above diagram) where it enters a heat exchanger. This heat exchanger uses warm coolant from the engine to help the transmission oil get to operating temperature more quickly during a cold start. The oil then returns to the thermostat block through pipe number 13 in our diagram, and re-enters the transmission. The coolant is circulated to the heat exchanger via an auxiliary coolant pump which is electronically controlled via the M-DCT computer and can be turned on or off when needed. This system will first bring the transmission oil up to operating temperature more quickly for better cold starts, and then use the engine cooling to maintain a stable transmission oil temperature once operating temperature has been reached.



At 203 degrees Fahrenheit, the thermostat opens oil flow to the front mounted oil to air cooler, positioned in the passenger side bumper opening. This oil to air cooler will work to maintain a stable oil temperature from that point forward. Part number 1 in the diagram above is the supply line to the oil cooler, and part number 2 returns the cooled oil to the transmission.

This covers the transmission cooling system during normal operation, but what about when the oil temperature exceeds BMW’s acceptable parameters? This is what we set out to explore. To help our clients better understand how their DCT transmission is operating, we created an adapter that can interface with a temperature sending unit to give an accurate reading of the DCT oil temperature.



We are still waiting for our friends at BMW to give us the maximum allowed temperature value, but in the mean time we will communicate with our clients who have already installed our DCT oil temp adapter and gauge set to monitor their experiences as they use their cars both on the street and the track.

I apologize for the long post, but I do hope that we can begin an open discussion about the DCT trans and educate the BMW community!

 

SSP does upgraded clutch packs for the M3 DCT box and also its special DCT gearbox oil that is known to lower operating temps and smooth out shifts. They are also working on a trans cooler for this car.