1999 Lexus LS rolling backwards in Drive

rockland

I have just purchased a 1999 Lexus LS with 49K on the odometer. Had a safety check from the selling dealer AND had it looked at by the local Lexus dealer before purchasing it. I parked at my residence with the car's nose pointed uphill on a modest incline. On going to use the vehicle, I started the car with my foot on the brake and then shifted into drive. I took my foot off the brake. The car, in D, proceeded to roll backwards down the incline and almost into my garage doors! Only by applying the the brakes and then quickly moving to the gas peddle was I able to get it up the incline. Advice on the problem before I go to the Lexus dealer. Thanks.

zueslewis

but where on Earth do people get the notion that an automatic transmission (in a forward gear) is supposed to hold a hill? It's not.

A manual transmission, in the same instance, would cause the car to cough, sputter and die.

There's nothing wrong with the car, it's just an incorrect perception of the operation of the transmission.

rockland

This is interesting since four other vehicles, a Lincoln, a Maxima, a Pontiac and a Volvo, all automatics, did not display the same propensity to free wheel. Is there a reason why these other vehicles did not behave in a similar manner? Thanks for the input.

zueslewis

some roll, some don't. Now of course, if the thing offers no resistance at all, that's another story.

Transmissions aren't a device to keep you planted on a hill - that's up to the braking system.

rockland

The car moved backward seemingly with minimal resistance. What could the cause(s) be? Thanks for the insights.

zueslewis

and resistance - but don't expect a solid hold on a hill.

alcan

Whether a vehicle will roll backwards on an incline is determined by torque converter design, engine idle speed, vehicle weight, and the percentage of grade. Transmission operating pressure is not a factor. Some vehicles will tend to roll more easily than others, due primarily to the characteristics of the torque converter. If there are no other driveability concerns, the vehicle is probably operating within design parameters.

zueslewis

my point is that auto transmissions do not hold cars on hills (with any form of guarantee) and are not designed to.

bolivar

until a car with an automatic transmission can overcome the law of gravity, it will roll backwards in gear down a hill.

Cars are different.

Their rolling resistance, idle speed, power transmitted through the torque converter and transmission at idle speed, brake drag, etc, etc, etc are all variables here.

But, by golly, don't believe all us posters, take it to the Lexus dealer. The service people need to get their daily snicker, and you will be it.

paul29

can hold on a hill. I had a MB 1976 300d and a 1983 300dt which would stay put on almost any hill in drive without any assist from the brake or fuel pedals. These cars could also be tow started as they had a reverse pump in trans. The Lexus service guys may snicker, but they don't know it all yet

swschrad

that say don't use the engine and transmission to hold your place on a hill, use the brakes.

if you have such a line in your Lexus manual, you know why now. you have to two-foot off a hill from a stop, I guess, but at least it's not two feet and three pedals, like you have in manual trannies.

oh, yes, that's manuals WITHOUT a hill-holder feature, which is most of them.

wwest

I don't know this for sure, but is it possible that Lexus is now, when the vehicle is setting idling, using the lockup mode of the torque converter (removing the torque "multiplier") to increase overall MPG?

If they are, wouldn't that make it a lot easier for the car to roll backwards while in gear?

zueslewis

in overdrive, not the primary gears, so 'no' is the answer there.

wwest

But do you know of any reason not to engage it at idle and "unload" the engine to increase the overall MPG rating.

For myself, I can't think of any reason not to use the lockup mechanism in this manner.

This might also account for the throttle tip-in hesitation some Lexus owners are complaining about. In the past I have advised that this hesitation might be the result of the engine ecu delaying WOT activity until the "WOT" VVT-I and the intake plenum settings can be "finished". But it certainly could be that time is needed to release the lockup solenoid, or maybe all of the above.

paul29

lock the torque converter at idle while stopped the engine will stall or car will jump ahead or back depending on the direction selected.The same effect as popping the clutch on a standard.

wwest

Sorry, never thought that one through very well, did I?

Hmmm....

So, why doesn't an automatic transmission, especially today's version, with an ECU that "talks" to the engine ECU, automatically shift into neutral if the E-throttle is at the idle position?

paul29

have to because the torque converter allows slippage when in gear at idle.( the engine driven vane is spinning and the trans vane is stationary ie held by the car brakes , slight incline etc ).The engine is the power source for a/c, p/s, p/b, heat,electrical, etc so is kept running. You would have to be idling in gear for long periods to increase the consumption to any degree and the usual situation is light traffic or traffic lights which at most can be measured in seconds . In major tieups you would probably put your vehicle in park or neutral to give your brake foot a rest and keep vehicle from moving.

q45man

Lockup TC is only used in the top two-[sometimes three] gears to improve mileage. In a 5 speed AT 1,2,3 are never locked....the small clutch could never take the stress.

Its all about fuel economy now!

At TC stall [the output turbine stopped] the entire excess power of the engine [a few HP at idle]is heating the ATF to hold a 4300 pound car on an incline [even a 10% one]

http://home.a-city.de/walter.fendt/physengl/inclplane.htm

would be between 15 and 20% or 860 lb/ft.

Not many engines put out this much extra [x 1st gear and diff say divided by 12 or 70 lb/ft] at idle when coupled thru a low efficiency TC.

If you raised the in gear idle speed to 1200 rpm it will hold before the TC explodes. But on a level road idle would make you travel at 13 mph with your foot off the gas!