OK, here's the question: In fifth gear (OD), is the torque converter supposed to be locked all the time or can it be unlocked?

Here's my understanding of what it means to have a locked torque converter:
Engine RPM does not increase for a given speed, such as when traveling up or down a hill. If going down-hill with more energy than the engine can brake it (foot off of the throttle & without using the brakes), the engine speed will increase.

Unlocked performs as follows:
Engine RPM cycles between several hundred revolutions to provide more power when trying to maintain a speed, such as when traveling up a hill (increase in RPM). But this also means that the RPMs reduce when going down the same hill.

First of all, are my understandings of the above locked behaviours correct?

It seems that after a year of reading other's posts about our transmission, many people's transmissions can unlock the torque converter in fifth gear. Mine, as long as I have owned the truck - since new, does not. In gears 2, 3, & 4 the transmission seems to behave "unlocked", and I think it locks when in "no OD" mode (I'm not sure about this).

However, I've never seen it unlocked in 5th gear. Is this normal?

Thanks for the lesson.

Assuming you are talking about NON-tow/haul mode. My understanding of tow/haul mode, is that the tranny is ALWAYS locked in gears 2-5 with tow/haul. Out of tow/haul, there is less lock in 2-5. I'm NOT sure about OD OUT of tow/haul and being locked (think it can lock, but it doesn't HAVE to be)...
[edit]I'm not sure that my original statement about 1st always being locked is correct, actually.

Also, there is this (http://www.allisontransmission.com/service/faq/index.jsp?CategoryID=11#91) which the last part seems to indicate that all gears can lock and slip in both modes, but that T/H puts you in more severe lock conditions:




<TABLE width="100%" border=0><TBODY><TR><TD colSpan=2>Q. Tow/Haul vs. Normal Mode - What's the difference between Tow Haul and Normal Mode? </TD></TR><TR><TD width="5%"></TD><TD width="95%">A. Tow/Haul (T/H) shift mode significantly changes the transmission's shift pattern to reduce shift cycling and to deliver better performance, control, and cooling when towing/hauling heavy loads. For instance:


Upshift points are raised at light to mid throttle position to use more of the engine's available power for accelerating.Downshift points are raised to enhance engine braking to help slow the vehicle.
During deceleration, the torque converter clutch (TCC) remains applied at closed throttle to much lower vehicle speeds to significantly improve the effect of engine braking.
During acceleration, the TCC is applied in 2nd range and remains applied in 3rd, 4th, and 5th. This improves the drivetrain efficiency and significantly lowers transmission sump temperature when towing heavy loads. In Normal mode, TCC generally applies only in higher ranges and is dependent on throttle position.

</TD></TR></TBODY></TABLE>



-JM

My questions are for the behaviour in non Tow/Haul mode.

Yeah, I think I remember reading somewhere that 1st is always locked. But fifth is the gear I'm interested in.

Also, which gives better mileage Locked or unlocked T/C?

As long as there is no "slippage" both have the same mileage. The problem with unlocked, is that it can "burn" the ATF if you are slipping under heavy loads/high torque. Locked has more jerky/abrubt throttle responses (like a manual trans), so the A/T normally unlocks as you get into the lower/higher torque gears (non-tow/haul) and higher rpms when decellerating.

The trans is never locked in 1st.All models

The 2004.5 locks only in 5th & seems to stay all the time
But will come in later uder hard throttle

The 2001 to 2004 units can lock in 4th or 5th.
This is out of T/H.

In T/H the torque locks after 2nd & stays in till in down shifts to 1st.

That's it, That's all

Tom

In the O/D position at wide open throttle only, the converter will lock at the top of second gear stay locked till 3-4 shift when it unlocks and relocks and stays locked through 5th.
mike

I always tap my t/h button to get it to lock when traveling around 38-42mph. Just watch the mileage go up

So its normal to feel the tranny lock up about halfway through second when pulling a load??? Mine feel slike it slips, then locks up around 2500 RPM. Its almost like I hit another gear in second.

The trans is never locked in 1st.All models

The 2004.5 locks only in 5th & seems to stay all the time
But will come in later uder hard throttle

The 2001 to 2004 units can lock in 4th or 5th.
This is out of T/H.

In T/H the torque locks after 2nd & stays in till in down shifts to 1st.

That's it, That's all

Tom

Not sure where you hear the 2004.5 only locks in 5th gear. I own one and in Normal Mode it locks in 4th and 5th under light load as well as 2nd and above when at WOT. In Tow/Haul mode it locks in 2nd.

So, is this normal???