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Throttle lag / Downshift delay, hesitation

Posted by wwest (My Page) on
Fri, Feb 2, 07 at 21:29

If you browse about your will find owner complaints of throttle lag or engine delay/hesitation for Ford, VW, Toyota, Lexus and others, all concerning vehicles with automatic transmissions and mostly FWD or front biased AWD vehicles.

It is my firm belief that sometime in the mid to late ninties someone, or some group, with TONS of clout over the automtive industry issued an edict that the safety of FWD and front torque biased AWD had to be brought into line with their RWD and rear torque biased AWD brotheren.

My vote goes to the automotive insurance industry. Accident statistics most readily available, certainly with the CLOUT and enough synergy with the industry to want to keep this on the QT.

The safety issue involved the potential for loss of directional control due to engine braking, especially FRONT engine braking, in wintertime adverse roadbed conditions. There is also the issue of the potential for engine braking to interfere with ABS, again especially detrimental for FWD vehicles.

So, late in the last century the shift pattern/schedule was revised across the industry to address the safety issue. The new shift pattern dictated that anytime there was a FULL lift-throttle action by the driver the transaxle would be quickly upshifted so as to prevent any significant level of engine braking.

The problem that quickly arose from this change was that if the driver quickly returned to acceleration "mode" the engine was now at idle and the just previously commended upshift would deplete the ATF pressure/flow reserve. With little or no ATF pressure/flow available the subsequent downshift due to the driver's re-application of pressure to the gas pedal could not be quickly completed.

As evidence of Lexus has now replaced a LOT of early RX300 transaxles.

By 2001 Lexus had figured out the problem and increased the displacement of the ATF oil pump, gear type oil pump, to provide more pressure/flow at engine idle.

So, the 2001 RX300's, even with all equipped with the extra ATF cooling via the tow package, OVERHEATS the ATF to the point that the recommended traansaxle ATF service interval has declined from infinity (the life of the vehicle actually) to every 15,000 miles.

What to do, what to do...??

Oh, I know, let's use DBW, e-throttle, to delay the onset of engine torque until the subsequent downshift can be completed, the clutches firmly seated.

So the RX330 used the old standard ATF gear pump displacement but was equipped with DBW "to protect the drive train".

Regretably some one else in engineering had already decided that the VC, Viscous Clutch, in the AWD version was contributing to the overheating of the ATF and so it was removed, not to return until the advent of the RX350.

Is Lexus listening, do you suppose?

The final FIX...

SNOW mode...Assuming the new shift pattern upshift technique is to help alleviate accidents due to loss of directional control arising for engine braking, why not just have a SNOW mode that can be activated by the driver, by a rain sensor, or if the OAT hovers around or below freezing?

Upon a full lift-throttle event in SNOW mode the transaxle would remain in the same gear ratio (ready to SURGE forward on command) but the engine RPM, via DBW, would not be allowed to fall enough to provide a significant level of engine braking to the driven wheels, FRONT, rear, or ALL.

Absent being in SNOW mode the shift pattern could be the same as it was pre-2000, NO upshifting on full lift-throttle events.

No HIGH potential for engine braking, FRONT especially, to put your life at risk or interfere with ABS if the roadbed traction is satisfactory.

I guess on second thought ABS interference via engine braking might still be an issue. But that could addressed by keeping the upshift pattern but delaying it until the brakes are applied.


Follow-Up Postings:

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RE: Throttle lag / Downshift delay, hesitation

Firstly, there's not much engine braking on automatics.

Secondly, you are far more likely to lose control on a RWD car with braking on a slippery surface beacuse the back axle swings around (try braking with only the back brake on a motorcycle on a wet road).

An upshift to prevent engine braking would be to meet emission standards because it reduces the vacuum pressure in the inlet manifold.

An automatic never runs in idle unless you put it there. And downshifting when you floor the throttle has always been standard on all automatics.

I think the overheating would be because of the AWD problems, problems with synchronising the rear and front drivetrains ?
Landrovers with viscous diffs used to chew up tyres.


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