The new 2010-11 F/awd RX350, like the early FE, has a switch so the driver can manually put the F/awd system in "lock" mode. Above 25MPH the system reverts to automatic mode. Also like the early FE it has a temperature sensor that is used to give the driver a "heads-up" that the F/awd system beginning to overheat. If the driver ignores the heads-up and the F/awd system's temperature continues to rise the F/awd system is simply disabled with an indicator to notify the driver of same.

Sounds VERY familier.

Hmmm...

Makes one wonder why Lexus thinks this method can work when it clearly did not work out very well when Ford tried to make use of the technique.

One additional bit of information is that I was told the "new" Ford Explorer, rising from the grave of the FreeStar and Taurus X, uses water cooling for the F/awd system's PTO/PTU. Maybe there's hope for the new restyled FE/FEH after all.

 

I've posted quotes before from the '09 FE/FEH manual where it mentions what happens when the rearend ATC unit overheats and what the driver should do but it doesn't say how the driver is notified of the condition.

 

Oops, it appears that I mis-spoke.

The only temperatrue sensor I could locate regarding the RX350 F/awd system was the ambient OAT, Outside Air Temperature. Then I found....

"The 4WD system will be disabled, DTC C1299, when one of the following is met:

-Condition that controls coupling calorific value is predefined value or more.
-The estimated transfer oil temperature exceeds the specified value.
-Vehicle is driven and energy load tire radius difference beween front tires and rear tires is more than 3 percent."

Apparently the temperature of the transfer lubricating oil is a computed valve. But does "transfer" mean PTO or the actual electromagnetic clutch..? Or both..??

Aisin Patent: http://www.docstoc.com/docs/51118057...Patent-5954618

Sounds to me like an afterthough, during testing the "transfer" was found to be overheating and the quickest fix was firmware revision.

 

After a few minutes Googling the consensus seems to be that the 4WD indicator on the instrument panel begins flashing/blinking if the "transfer" temperature is rising close to the point of overheating and then comes on solid if the temperature actually rises to the overheating point.

 

If so, there's nothing in the owner's manual about this. In the Workshop manual it states to let the vehicle idle for ten minutes for cooling.

The manual states that the ATC goes into 100% locked mode till it cools but there's no temperature sensor in the rearend, so how does it detect this? Further more with a full 100% locked mode would cause steering control problems.

 

The way I read this is that if the rear clutch/lubricant "threatens" to overheat it will go into FULL LOCK mode to reduce the clutch heating, to HELL with the other components now under the FULL stress of driveline windup, tire scrubbing, etc. If the heat continues to rise then the system is completely disabled and if you're actually "off-road" the advice is to stop for long enough for things to cool off.

The F/awd 2010-11 RX350 seems to be computing the temperature of the "transfer" oil in order to exert this override control.

 

There's only one cable going to the rearend with two wires to operate the clutch and that's it.

 

Yes, with the Ford FE/FEH I am of the understanding that once the manual F/awd rear clutch engagement switch was removed (2007..?) the driver had no way to inadvertently ABUSE the system so the sensor was no longer needed. I am also of the understanding that at some (later..?) point the control firmware was revised in order to reduce the functionality of the system and thereby reduce the failure rate due to heating of the components.

It seems really odd to me that this system partially locks the clutch when turning as that procedure will clearly result in a higher level of driveline windup and tire scrubbing. If anything a R/awd system would be doing the REVERSE of this, removing engine torque from the front during turns.

Strange.

 

The over temperature condition, lockup, etc. is still mentioned in the '09 FE/FEH Workshop manual. This what I've been stating, it's in that manual but nothing in the owner's manual.

 

There are enough sensor inputs that a computer, with enough inexpensive processing power, could easily calculate, in real time, the temperature rise of the lubricating oil within the rear clutch and even the PTO/PTU. Apparently that's the method that the new 2010-11 RX350 F/awd system uses since it too gives an indication of this same situation but without an actual sensor.

The patent I linked also gives a clear indication of computing capability being used to determine the heating level of the ATF in an automatic transmission resulting from a relative new use of the lock-up torque converter clucth. Heating as a result of the use of a torque converter lock-up clutch to automatically allow "slip" just as one might do with a "stick" clutch at initial start-off movement.

And that patented lock-up torque converter clutch "slip" control seems to fit nicely with the way these new F/awd electromagnetic rear clutch systems are being used, limiting, controlling the amount of "slip".