GMC Lock-up Torque Converter Clarification

[scott widmer]

mypentium wrote in reference to the THM425 transmission:

> ...it suffers from the lack of
> a lock up converter; a design innovation that began in the early 80's
> when downsizing of power trains began. With an overdrive lockup
> convertor and overall 3.66 gearing, there would be plenty of power in
> all three gears, and with the .75 transmission gearing O.D. would offer
> as opposed to the 1.00 of third gear...

In reference to Bill's post above, I wanted to address some
mis-information regarding lock-up torque converters. Contrary to the
beliefs of some, the terms "lock-up torque converter" and "overdrive"
are NOT synonymous. They are two very different and independent
features of transmissions so equipped (and the THM425 is not one such
transmission).

The function of a lock-up torque converter is simply to provide a direct
mechanical coupling from the engine to the input shaft of the
transmission (bypassing the traditional fluid coupling). This is
desirable because it increases the efficiency of power delivery to the
trans by eliminating slippage. The lock-up portion of the converter is
nothing more than the addition of a clutch (similar to that of a manual
transmission) in series with the torque converter and a means to control
it's engagement (computer or vacuum modulation).

Most transmissions are "programmed" to engage the torque converter
clutch (TCC) during light to moderate, relatively steady state driving
conditions above a predetermined speed (

 

[patrick flowers]

Thanks Scott for the info in your post of Wed, Nov 11. I won't quote
your article to save space. FWIW here are some of my thoughts about
overdrives that might be useful.

Overdrive originated with standard transmissions and the conventional
front engine, rear drive axle arrangement. In this configuration, high
gear provided direct drive from the engine so that the driveshaft turned
at the same speed as the engine. The rear axle ratio was selected to
provide a compromise between power and economy for high gear operation.
Attempts to improve economy (ei: less engine wear and better fuel
mileage) by lowering the rear axle gear ratio caused lack of power and
reduced acceleration particularly in high gear.

To solve this problem, a separate gearbox was fitted between the
transmission and the driveshaft. This unit provided either direct drive
when "disengaged" or a step up gear ratio when "engaged". It was
manually engaged when cruising at highway speed and was essentially
another gear above high gear. This configuration was usually available
as an option and became known as overdrive.

The concept was improved in manual transmissions with the 5 speed
transmission. This design included a ratio equivalent to overdrive as
5th gear, thus making separate overdrive gearboxes obsolete, and has
been referred to as a 5 speed overdrive transmission. Front drive
import cars commonly used a 5 speed manual transaxle. In this
configuration, the final drive was an integral part of the
transmission and 5th gear was an overdrive ratio. .

Automatic transmissions are essentially a servo-controlled gearbox
connected to the engine with a torque converter. The torque converter
has two great characteristics. It can provide enough slip to substitute
for a clutch so that the engine can idle in gear, and it can multiply
engine torque at low rpm's to augment gearing. When you get past lower
speeds it reverts to being a direct fluid coupling from engine to
transmission. This coupling has slippage inherent in its design that
results in about 25% loss in rpm before reaching the mechanical gears of
the transmission. That percentage of horsepower (and fuel) is turned
directly into heating your transmission oil.

During the oil crisis, auto makers hit two birds with one stone by
introducing the lock-up torque converter. It eliminated the slippage and
also provided an overdrive ratio. Consider the GM TH350C (other makes
and models may be similar). High gear is 1.00 to 1 transmission gear
ratio. When you are driving along the at a steady highway speed, the
torque convertor is slipping at about 25% below engine speed. When you
select overdrive, the clutch inside the converter engages, locking the
two halves of the converter together mechanically. The engine is now
turning slower by 25% and creating a transmission gear ratio of .75 to 1
which is an overdrive ratio.

Automatics have evolved into front drive automatic transaxles with the
final drive integral to the transmission, and most of them incorporate
overdrive ratio gearing for economy. It is often difficult to isolate
overdrive gearing as being something separate. I think the determining
characteristic of overdrive is the existence of a suitable overdrive
ratio. Lock-up converters are frequently referred to synonimously with
overdrive as in the following (and informative) site: Olds FAQ --
Transmissions. Also included are links to a color T.C. illustration:
Drivetrain Tech and technical article: Torque Convertors - what they
are....what they do!

I don't know what problems might exist with mechanical reliability
beyond my own experience. I remember GM overdrive trannys in the early
80's had problems that seemed to be caused largely by driving in town in
OD, and on the highway in heavy pulling situations where it would "hunt"
in and out of overdrive. The remedy was to only shift into OD during
highway cruising.
- --Bill

 

[patrick flowers]

>
> Thanks Scott for the info in your post of Wed, Nov 11. I won't
> quote your article to save space.

> --Bill

Whoops! No I haven't changed my name to Bill. This was a "multipart"
post that I cleaned up and sent on to the list. Obviously I didn't do
sumptin right.

Patrick
- --
Patrick Flowers
Mailtoatri63

The GMC Motorhome Page
http://www.gmcmotorhome.com

 

[mypentium]

I did not realize that the mailinglist is unable to process links.

The links I quoted in my post of Thurs, Nov 4 are listed below in
brackets:
- -- highlight address--hit edit/copy--edit/paste to blank browser
location bar-- hit enter.
- --Bill

Lock-up converters are frequently referred to synonimously with
overdrive as in the following site:
Olds FAQ --Transmissions. (
http://www.en.com/users/thall/olds/oftrn.htm )

Also included are links to a color torque convertor illustration:
Drivetrain Tech ( http://www.ntcnet.com/~dragon64/drivetrain.html )

and technical article: Torque Convertors - what they are
....what they do! ( http://www.fbody.com/torque.htm )

 

[clark searle]

Automatic transmissions are essentially a servo-controlled gearbox
connected to the engine with a torque converter. The torque converter
has two great characteristics. It can provide enough slip to substitute
for a clutch so that the engine can idle in gear, and it can multiply
engine torque at low rpm's to augment gearing. When you get past lower
speeds it reverts to being a direct fluid coupling from engine to
transmission. This coupling has slippage inherent in its design that
results in about 25% loss in rpm before reaching the mechanical gears of
the transmission. That percentage of horsepower (and fuel) is turned
directly into heating your transmission oil.

I was interested in your comment about the slip ratio of the torque converter.
You said 25%. That means, for example, on the highway with the engine at 2400
RPM, there would be 600 RPM slip in the converter and a transmission speed of
1800 RPM. I'm surprised that there would be that much slippage going down the
road. I certainly don't know much about torque converters, but I was estimating
that the slippage under typical road load would be about 200~300 RPM.

Any substantiation available from anyone?
Clark
78 Kingsley
Mid-Michigan