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Mechanical override: Lockup torque converters

Chrysler launched its new lockup torque converters on its 1978 cars. While not a completely new idea — Packard and Studebaker had done it — the lockup converter had not been tried for quite some time.

The torque converter creates a fluid linkage, so cars can sit still while in Drive; they also tend to reduce vibration and make the car feel less “jerky.” The lockup replaces the normal fluid interface in the transmission with a mechanical one, but only when needed. That saves a small amount of gasoline and increases usable power a little; at higher speeds, the engine doesn’t have to work as hard, and the transmission fluid doesn’t get as hot.

The lockup itself was done through a special clutch.

lockup torque converter

Below those speeds, the lockup could also engaged when the driveshaft 850 rpm on V8s, or 1100 rpm on the sixes (not far above idle, for V8s). At higher road speeds (27 mph for V8s, 31 for sixes), the lock-up engaged at the same time as the upshift to third gear.

The torque converter “unlocked” when downshifting, or when the driveshaft dropped below 850 rpm (V8s) or 1100 rpm (sixes).

Most of the 1978 cars had the lock-up torque converter; it wasn’t used with the 440 V-8 engine, cars ordered for areas above 4,000 feet altitude, California slant sixes, the Super Six, or any car with the Heavy-Duty Package.

According to long-time mechanic and tuner Hemi Andersen, “The original lockup speeds were too low, which caused the engine to labor and bog down. Chrysler sent out a modification kit, with a new lockup valve and heavier spring, as a free warranty repair. To install it, one had to take the valve body from the transmission. It locked up the torque converter at around 42 mph, rather than at a low of 27 mph.

“There were also frequent failures of the 904 TorqueFlite transmission itself, requiring major components to be replaced. It took several years for these problems to be resolved.”

hemi andersen with 727

A lockup A-727 transmission has about 5/8” of the end of the smaller input shaft machined smooth. If the splines go out to the end of the shaft (except for about a 1/8" bevel), it’s not a lockup transmission. (Thanks, Joe Reiss for sending this, and Rick Allison of A & A Transmission for noting it.)

When the Torqueflite was adapted/re-engineered into a front wheel drive version, the lockup was activated electronically. Eventually, the rear drive transmissions followed.


by Tannon Weber

Not all 727 transmissions made after 1978 were lockup; towing-package 727 transmissions were commonly non-lockup, and would have the 24 spline input shaft. The A-518 overdrive also has 23 splines. The output shaft has 29 splines.

Non-lockup A-904 transmissions from 1968 on have 27 splines; the 1978 and newer lockup A-904 transmissions have just 26. A-500 overdrive transmissions also have 26 splines, with 25 on the output shaft.

Every modern transmission uses a lockup torque converter; refinement of the system allows it to be used under far wider conditions than the original.

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