You are not logged in or registered. Please login or register to use the full functionality of this board...

Allison HT-741 Shifting
#1
This is recorded for posterity.

Last year I changed out the transmission oil and put in TES-295 qualified oil (Transynd equivalent). Since that time there has been a gradual alteration in the shifting pattern. Whereas originally it would shift from 1-2nd with a soft shift that dropped the engine RPM from 2100 to around 1700, then when the RPM got to 2100 again the converter would lock up and drop the RPM to about 1800, then when it got to 2100 again the 2-3 shift would drop the RPM to 1450 pretty sharply, and when up to 2100 again the 3-4 shift would behave like the 2-3 shift. Now, sometimes at the 2-3 and 3-4 shifts the RPM would only drop to around 1700 unless I immediately released the throttle and then the RPM would drop to the usual 1450. When one of these "partial" shifts would happen I would release the throttle for fear that the clutches were slipping. No codes were showing up (actually not a surprise since that light had been disconnected - it works now). Two Allison shops could find nothing wrong with the tranny.

Today I went for a test drive to see what would happen if I didn't back off the throttle when the "partial" shifts happen. They behave the same as the 1-2 shift: hit 2100, drop to 1700, hit 2100 and feel a shift or converter lock up that drops RPM back to around 1800. I also noticed that that second shift or lock up could occur at less than 2100 and that I could make it happen any time by releasing the throttle.

It turns out that there are two standard shift patterns:
1-2C-2L-3L-4L
or
1-2C-2L-3C-3L-4C-4L

For some as yet unknown reason my transmission is migrating toward the second of these two. I will be doing research as to why and if I need to do anything in the meantime. The good news is that there really is nothing wrong with the transmission. It is just the ECU changing the shift pattern, which harms nothing. The better news is that this new shift pattern keeps the engine better wound up. I may want to keep this new pattern since at 1450 the engine is a bit sluggish in acceleration. Fuel mileage may suffer a fraction, probably not enough to notice.

The other thing I should not is that when an analyzer is plugged in that senses the transmission ECU the dash indicator for the transmission goes blank.
Jon Kabbe
1993 coach 337 with Civic towed
Reply
#2
Just out of curiosity, has your obvious extensive research indicated that the Allison "learns" the driving habits and shifts accordingly?

Do you think that with a couple of years of familarity that you are more prone to hammering the throttle now?

The best tale I've heard about how to drive a screamin jimmy was to slam the door on your hand on the way in.
Richard and Rhonda Entrekin
99 Newell, 512
Subaru Outback Toad
Rhonda's FB (Rhonda Duggan Entrekin)
Reply
#3
None of the allison manuals have mentioned "learning" capabilities.

I've wondered if my driving had changed. If it has it's really subtle and subtly increasing.

The only thing I can think of at the moment is that messing around behind the dash changed a connection to the ECU that triggered this change. Again, the manual mentions nothing about this possibility.

All in all, this has prompted to learn a lot about how this system functions. It is a well conceived design, but far from simple and obvious.
Jon Kabbe
1993 coach 337 with Civic towed
Reply
#4
I find that I am often manually going through gears on some grades as I hate to lug the 8V92. For some reason it bothers me to see it much under 1600 rpm. On flat ground the times between 1450 & 1600 are pretty short and don't bother me.
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)