Join the Car Talk Community!

I have a Mitsubishi Montero with an automatic transmission It...

RSS
Dear Tom and Ray:



I have a 1990 Mitsubishi Montero with an automatic
transmission. It is incredibly reliable and I hope to have
it for many years. I recently got a new job and my daily
commute now takes me down a long, winding mountain road --
and, naturally, back up at the end of the day.

With Overdrive "on," I pick up speed too quickly going
downhill and need to brake a lot. Uphill, the vehicle seems
sluggish and shifts back and forth a lot between third gear
and Overdrive. The manual recommends switching Overdrive
off for long grades like this, but I worry that doing so
will place undue strain on the engine and transmission. Any
thoughts? -- Curtis

TOM: The owner's manual is absolutely right, Curtis. We
tell most people to just leave Overdrive on and forget
about it, but your commute just happens to contain the two
specific exceptions to that rule.

RAY: When you're driving down a long, steep grade, you MUST
downshift to keep the car's speed under control -- and
turning off Overdrive IS downshifting from fourth gear
(Overdrive) to third gear.

TOM: Why not just use the brakes? Because under those
conditions, the brakes can overheat and fail. And that
could result in your taking an unexpectedly dangerous
shortcut to work (straight down the side of this mountain).

RAY: In fact, on a really steep hill, you may need to
downshift to second or even first gear to keep the car
under control.

TOM: The other time you want to turn off Overdrive is when
the vehicle is "hunting," which is something you want to
avoid. Now, before we get all those nasty telegrams from
the zealots at the National Rifle Association, let me
explain that in automotive terminology, "hunting" is when
the transmission can't figure out which gear to be in, and
it keeps switching back and forth between two gears.

RAY: When you're climbing a moderate hill, or traveling on
rolling hills at 35-45 mph, you may find the transmission
shifting a lot between third and fourth gears. While this
is not nearly as harmful as losing your brakes, it can be
pretty annoying. And in cases like these, you can solve the
problem by turning off Overdrive and forcing the
transmission to stay in third gear.

TOM: In neither one of these cases will turning off
Overdrive have any negative effect on your engine or
transmission. Now, if you forget to turn Overdrive back on,
and get on the highway and drive 600 miles to East
Treetrunk, you'd put a little extra wear on the engine and
get poorer gas mileage, so if you turn it off, remember to
turn it back on.

RAY: But poorer gas mileage still pales in comparison to
taking the "instant" route to work, doesn't it, Curtis?

You must be logged in to leave a comment. Login / Signup
Support for Car Talk is provided by:

Donate Your Car,
Support Your NPR Station

...and get a tax break!

Get Started

Find a Mechanic


Go



Submit