Measuring Transmission Slip

[Sparky8370]

there are three speed sensors on the allison.

input speed
turbine speed
output speed

the difference between input and turbine speed determines how much the CONVERTER (not the trans itself) is slipping. Obviously if its unlocked you are going to see a lot of "slip"...because thats what a torque converter does. Once its locked you should see zero difference, or something small, like +/-5 rpm. If the converter is slipping, you'll see a number thats ~15rpm or higher when its locked up.

The turbine speed sensor basically measures the input speed of the trans itself...like the RPM of the "other end" of the converter (called the turbine)...the output speed sensor measures the speed of the output shaft. The transmission has a fixed set of ratios. The TCM is constantly calculating the "actual" ratio of the turbine vs. output speed based on which gear the trans is in to determine if the trans (clutchpacks) are slipping or if there is some other internal problem in the trans that is preventing it from transmitting the proper gear ratio equivalent for a given gear/speed. This has NOTHING to do with the converter slip.

The ratios of the allison are as follows.

1st gear - 3.10:1
2nd gear - 1.81:1
3rd gear - 1.41:1
4th gear - 1.00:1
5th gear - .71:1
6th gear - .61:1

So, if the trans is working properly and not slipping, the ratio/mathematical equation between the turbine speed sensor and output speed sensor should correlate with what gear the trans is in. For example if its in 3rd gear (regardless of wether the converter is locked or unlocked, remember the allison is a clutch to clutch automatic trans, it is ALWAYS locked positively in one gear...there is NO freewheeling or overrunning so the ratios are always constant/fixed), which is 1.41:1, the turbine speed should be 1.41 / the output speed.

Say the turbine speed is 2500rpm. 2500rpm / 1.41 = 1773rpm. SO. the output speed sensor SHOULD read 1773 rpm if the trans is not slipping and working properly. The TCM constantly calculates this ratio, and does it for all of the other gears too. If the ratio gets too far out of range, it interpenetrates this as the transmission slipping and goes into limp mode and sets an "incorrect ratio" DTC.

make sense???

ben

Ben, what were you thinking about when you were getting ready to say "limp mode"? Because earlier in that sentence you said "interpenetrates" instead of interprets.

[Boost]

bad boy

This is a very useful and informative thread. Keep up the great work!