Transmission plug adapter pins CRD

[nudibranches]

interesting...so reading this we think the ABS fault is triggering the P0702? Or am I reading it wrong? (Wouldn't be the first time

[JeanLuke]

P0702 is a general fault the ECU throws that just says, "Something is wrong with the driveline." The details are from the TCU. The TCU relies on the ABS wheel speed sensors for some of the control functions, so it will throw errors if sensors are faulty, and the ECU throws a generic P0702, which is what actually ignites the MIL (I believe).

[Adrian D]

100% correct
The transmission doesn't have an output shaft speed sensor (why, Mercedes? WHY?), so it relies on the wheel speed sensors and t-case position sensor to determine the output shaft speed.

[JeanLuke]

Originally Posted by Adrian D

100% correct
The transmission doesn't have an output shaft speed sensor (why, Mercedes? WHY?), so it relies on the wheel speed sensors and t-case position sensor to determine the output shaft speed.

It's even more sinister than that. It also looks at the relationship between the four wheels and throws an error if it sees something implausible. The bugger in this is that if the maths does not work out between engine RPM, gear, transfer case ratio, diff ratio and the arithmetic mean (aka average) of the two rear wheel speeds, it throws an error.

Not a problem with a normal diff because the maths will always work. Big problem with a lunchbox locker or a Detroit locker because the tailshaft does not equal the arithmetic mean of the two rear wheels, but rather the slower of them. The TCU sees this as transmission slip.

Similarly, a 242 transfer case conversion can sometimes cause issues when you have rear wheel slip in 2WD. The TCU can throw a hissy-fit.

[Clarky]

Originally Posted by Adrian D

100% correct
The transmission doesn't have an output shaft speed sensor (why, Mercedes? WHY?), so it relies on the wheel speed sensors and t-case position sensor to determine the output shaft speed.

Yes it does, its part of the conductor plate.
I have the same transmission in my WH and when I did a ratio change it went straight into limp mode as it detected an out of range wheel rotation compared to the transmission speed sensor.

[Adrian D]

Originally Posted by Clarky

Yes it does, its part of the conductor plate.
I have the same transmission in my WH and when I did a ratio change it went straight into limp mode as it detected an out of range wheel rotation compared to the transmission speed sensor.

Unfortunately, the speed sensors in the transmission are internal only, the actual output shaft speed is calculated

One of the configuration parameters in the TCU is the axle ratio. The gearbox calculated the expected output shaft speed based on its internal ratios and calculates an expected wheel speed based on diff and t-case ratios (source: the company I work for develops the software used for testing automatic gearbox and ecu firmware at Daimler)

[nudibranches]

Originally Posted by JeanLuke

It's even more sinister than that. It also looks at the relationship between the four wheels and throws an error if it sees something implausible. The bugger in this is that if the maths does not work out between engine RPM, gear, transfer case ratio, diff ratio and the arithmetic mean (aka average) of the two rear wheel speeds, it throws an error.

Not a problem with a normal diff because the maths will always work. Big problem with a lunchbox locker or a Detroit locker because the tailshaft does not equal the arithmetic mean of the two rear wheels, but rather the slower of them. The TCU sees this as transmission slip.

Similarly, a 242 transfer case conversion can sometimes cause issues when you have rear wheel slip in 2WD. The TCU can throw a hissy-fit.

Yes I've heard that, but why won't it throw a code when you lock the diff(s) with an air locker when off road? Both wheels are then locked and the diff is no longer open? Is that because the wheels are able to slip more easily across gravel, loose surface as opposed to a road?