Powershift, FORScan & GISS

[AlCan]

Hi Guys,

As you'll know, I've been learning about the first two items, now up to the third...

This has been (still is) a real puzzle to me, and I'm hoping someone can solve it for me - Mondaveo, with all your Speed Sensor / FORScan experience, can you shed any light?

The 6DCT450 has (as we all know) two clutches and two input shafts.

As someone who like to understand things, it makes complete sense that there should be a Speed Sensor for each input shaft. There's a third Speed Sensor for the Engine RPM which it reads off the outer clutch drum. That's the one that connects internally to the TCM by a plug and wires near the main Circular Connector on the front of the box. (Diagram pg 42)

I have a copy of the ATSG Powershift training manual(?) from 2012 which explains a lot of things about the transmission, but sadly, not all of it. And it has a few mistakes and inconsistencies.

One inconsistency surrounds the Input Shaft Speed Sensors. On pg 41, it clearly mentions these three speed sensors: Engine and Odd & Even Input Shafts.

The input shaft speed sensors must also operate by Hall Effect, and read gear teeth on the relevant shafts.

The lower half of pg 42 shows all the sensors stalks that protrude from the TCM into the transmission, with arrowed descriptions. One says: "Reads Input Shaft 2 Speed". All Good.

Another says: "Reads Input Shaft 1 Speed via Output Shaft 2". That one really had me guessing - how can it do that?

The diagram on pg 30 is even more confusing: "Output Shaft 2 Speed Sensor ... Hole". What use is that? In effect, since both output shafts are geared to the differential ring gear, it's really a vehicle speed sensor.

Today, finally, I figured it out. It's not quite clear, but what is must mean is: "Input Shaft 1 Speed Sensor Hole" and "Reads Input Shaft 1 Speed via Gear 5 (idler) on Output Shaft 2".

To me, that makes perfect sense (ratio around 1:1 - clever) as there has to be a Speed Sensor for each input shaft, since it's key to clutch control.

[FWIW, the Mitsubishi 470 redesigned box info is much clearer, and there are two clearly identified input shaft speed sensors shown.]

Which brings me to my real question:

Is there a second PID for the other speed sensor? Or are both speeds somehow kludged together into the one GISS? To me, that's really no use at all but maybe that's it, or maybe the FORScan guys have missed it.

Any thoughts and comments greatly appreciated.

[AlCan]

Update:

Done some more Googling, but no one seems to have (publicly) explored the GISS PID in relation to these gearboxes more than Mondaveo and me.

Had another think...

Also had another look at the data I have already captured from my transmission and have concluded that GISS is a kludge of some sort. As might be expected, it's a "drifty" datastream. It reacts to gear changes (both odd and even) but doesn't really seem to relate to anything definitively. It seems to drift about.

This, I imagine is because in the P/S transmission, most of the time (while driving), only one clutch and one set of gears is engaged, and if there's no clutch slip, then that input shaft speed will be the same as engine speed.

However, the other input shaft will be connected to a disengaged clutch, and except for when a gearshift is pending, will itself be disengaged - no gear will be engaged in this half of the gearbox, so this input shaft will be free to do whatever takes its fancy.

Most likely, because it's a wet clutch and the clutch plates are a large surface area, there will be appreciable drag despite being disengaged, and the "free" input shaft will generally 'follow' engine RPM. In other words, during acceleration, it will probably lag slightly behind, during rapid deceleration it will likely run a bit ahead, and for steady state, it will probably equal engine RPM.

That's a lot of probablies which I think accounts for it being what I call a "drifty" variable - and why, as far as I can see, it's very little use.

I'm assuming for now, in the absence of anything more authoritative, that GISS is simply the average speed of the two shafts, but since most of the time one shaft is floating about and its speed is seemingly quite indeterminate, the net result is of very little use [for diagnosing live shift-related problems.]

Here, now, just imagined, is my best guess at what the designers intended: MAYBE, during the Clutch Learn and/or Shift Fork Learn procedures when the vehicle is stationary and options are limited, GISS is used to calibrate the individual clutches or gearshift forks.

To explain, if one side (say Odd gears) of the gearbox is engaged (with Odd clutch fully disengaged, of course), then the Odd input shaft (and the Odd clutch output-side plates) will be stationary. In this scenario, with the Even gears remaining in Neutral, GISS will therefore be the exact speed of the Even input shaft (divided by two).

This can perhaps be used to determine when the Even gears clutch begins to engage - and who knows - maybe they even use synchro cones to load up the free shaft as part of that process.

And vice versa.

It may also be used during Shift Fork Learn to establish when a synchro is engaging. (If the relevant clutch is lightly engaging, that input shaft will start to slow down).

If these guesses are correct, then it explains why GISS exists, but unfortunately it doesn't much help me solve my problem - what is my transmission doing before it skips 2nd gear?

Anyway, food for thought...