RE: [A/S] New Suburban

["Perry Joe Scudder" <perryjoe@xxxxxxxxxxxxx>]

Hi All,
I put 65,000 miles on a 2000 GMC 2500 with the 6 L Vortec engine with a 4:10
rear ratio while pulling a 10,000 pound trailer all over the country.
The first thing I learned about the Tow / Haul (T/H) switch it the fact it
has a "learning" feature that controls the shift into overdrive points. The
theory ( according to my dealer ) is the transmit ion keeps up with your
driving habits for a while, and locks in on shift points to match your
habits. I could move the shift points up or down by driving at a new speed
while in overdrive and tow / haul. You have to get your vehicle to the speed
you want to drive before switching to T/H, then drive at that speed in T/H
for a while. I found it was very easy to move the shift point "UP" but very
difficult and time consuming to move it "DOWN"
I found this out from the dealer when I took the truck in because it would
not shift into Overdrive at 55 mph. I had run in OD at 55 mph on the next to
last trip I had taken, but could not do so on the last trip. The dealer
explained how it worked, and reset the "Memory" back to "zero", and it would
shift into OD at 45 mph I believe it was. I then ran on the Interstate at 65
in T/H for a couple of hours on the next trip, and it was reset to the 65
mph again. As I said at the beginning, I learned to work with the T/H
switch, and could get it to do what I wanted. I liked to run in T/H because
it would take on a bigger hill (or more head wind) without shifting down out
of OD when it was in T/H.
I don't know enough to go into detail on the "free wheeling" but it has to
do with the torque converter locking and unlocking. There is nothing wrong
if you freewheel when you let off the gas when you are in T/H.
I traded my truck for a 2004 with the Duramax / Allison power train, and the
T/H switch works TOTALLY different in this truck. I have not worked with it
enough yet to figure it out, but it now makes the truck act like it has an
exhaust brake (engine holds the truck back) when I hit the brake going down
hill. It does not do this at all unless you are in T/H. It seems to act the
same way as before as far as changing the shift points going up grades
though. If anyone has got it all figured out yet, how about posting the
answer for the Duramax / Allison set up.

Joe Scudder
2004 GMC 2500 HD Sierra SLT Crew Cab, Short Box, 4 WD, Duramax / Allison
pulling a 34, Airstream Travel Trailer