I started doing this years ago on an old '85 Caprice, and got into the
habit. I still drive in D, except on the highway on my 97 Taurus. Haven't
had any problems (knock on wood), and the original tranny has 125,000miles
on it (AX4N I believe? It's the stronger of the 2 trans offered with the 3.0
If you have the AX4N(19 bolt pan), you are lucky to have that
combination! The Vulcan will pull into OD Lock-up around 36-37 mph, and
has good torque in that range, so you are sacrificing MPG by not driving
in OD and letting the computer do the thinking. The AX4N is very
adaptive to conditions in OD - you will likely never have a problem.
Leave the bands & clutches engaged, idle in "Drive". Much less wear
than dis-engaging & re-engaging all the clutch packs, bands & servos.
The only slipping/stalled component at idle in (D) is the fluid
connection at the converter... leave it that way. With electric fans,
the cooling system of the vehicle is more than adequate.
The admonition to put the vehicle out of gear when idling was to allow
the engine to run faster to move cooling air through the rad (and
tranny cooler) to avoid overheating. Idling fast in gear produces a
fair amount of heat in the converter.
With today's computer controlled engines and transmissions, and
electric cooling fans, this is no longer an issue.
Put it in drive and leave it there.
As for OD, when running heavily loaded, towing a trailer, or running
in hilly terrain, it is recommended to lock out OD (run in D insted of
OD, or press the OD lockout button, depending on the car). This WILL
increase fuel usage marginally, but the transmission does less
"hunting" - therefore fewer application and release cycles, causing
less wear on the trans components.
Under these conditions you SHOULD be running at reduced speed anyway,
so the fuel consumption hit IS marginal.
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