Re: [A/S] transmission temperatures

[Jim Dunmyer <jdunmyer@xxxxxxxxxxx>]

Jon,
  You're should be safe following the owner's manual recommendations as 
far as OD vs. 3rd for towing. Some transmissions, specifically the 700R4 
used in 1/2-ton GM vehicles, will have a very short life if left in OD 
while towing. Others, such as those used in GM's 3/4-ton vehicles, are 
just fine towing in OD.

Most of the heat in an automatic transmission doesn't come from the 
transmission itself, but from the Torque Converter. During most 
operations on the road, the torque converter clutch is locked up, making 
it much like a manual clutch, so NO heat is generated by the TC. When 
you lock out OD, the transmission will lock the TCC once the vehicle is 
up to speed. If OD is selected, the TCC will remain unlocked until the 
transmission has shifted to OD. At least that's the way it works in the 
Dodge. For that reason, I'll lock out OD if I'm cruising at 45 MPH; that 
forces it to stay in 3rd and allows the TCC to lock instead of its 
shifting to 4th and leaving the TCC unlocked. In my truck, the 3-4 shift 
will happen at 40 MPH, but the TCC will not lock until it reaches 50 MPH.

If you have a tach, you can pay close attention to what's going on and 
figure out the exact sequence of operation, if the TCC is locked or not, 
etc. You determine if the TCC is locked by letting the speed stabilize, 
then add just a bit of throttle kinda quickly. ("jab" the throttle) If 
the tach doesn't move to amount to anything, just follows the speed, the 
TCC is locked. If the tach jumps with the throttle, but the speed lags, 
the TCC is UNlocked. When you remove your foot from the throttle, notice 
how the tach immediately drops way off; that's because the TCC unlocks.

The quickest way to heat up your transmission is manuevering to park 
your trailer, especially if you need to back uphill or if the ground is 
a bit soft, requiring more throttle to get 'er to move. The other bad 
condition is a hard, long pull in 2nd, such as a very steep hill at slow 
speeds.

FWIW: it won't hurt your engine nor your mileage to let the engine run 
at 2500 RPM or more. My old Jeep CJ-5 cranked 3000 RPM at 70 because of 
the 3.73 axles.

                                  <<Jim>>