Re: [A/S] Light Truck Diesel Engine Exhaust Brakes

["Harvey Barlow" <harveybarlow@xxxxxxxxxxxxxxxx>]

Jerry,

Yes, you are correct.  I think downhill engine-braking in overdrive fourth
gear or direct drive third gear of a gas-engined truck applies considerably
less deceleration than does a turbodiesel-powered truck with its exhaust
brake activated.  My guess is the manufacturer expects that the automatic
will live behind a gas engine but knows that it will not behind a diesel
with exhaust brake.  We need to remember, the factory warranty provided by
the manufacturer is only 36 months and 36k miles.  The automatic
transmission in my '94 Ford behind a gas 460 big block failed at 103k.  The
torque converter began chattering on lockup.  It had pulled a 9k pound 34'
A/S about 40k - 50k miles when it failed.

The torque converter lockup feature in modern automatics is provided by
hydraulic pressure known as line pressure in the transmission.
Engine-transmission computers control the function.  High line pressure is
only applied under heavy throttle.  So when the driver's foot is off the
throttle, line pressure is very low, and the torque converter is either
free-wheeling or is lightly applied.  I am not an expert and my information
may not be precisely correct, but is a fair representation of the facts.

There are other factors to consider when adding an aftermarket box or chip
that will provide deceleration torque converter lockup.  The transmission is
designed to handle certain specified torque loads when accelerating, but the
torque load applied when decelerating (using engine braking) is applied to
different components.

Harvey