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TPS: Throttle position sensor

Mechanical override: Lockup torque converters

The 1978 Chrysler, Plymouth, and Dodge cars went “back to the future,” to find an efficiency measure that would soon become standard on every car in the industry — the lockup torque converter.

1988 lockup diagram

Pioneered by Packard and Studebaker, the lockup torque converter had not been used for quite some time when Chrysler engineers put it back into action. Torque converters are used on automatic transmissions (and, in some vintage cars, semi-automatic transmissions) to smooth out the powertrain and, more importantly, to let the car sit at stop signs and traffic lights, without having to shift into neutral. They do that by using moving oil, rather than a direct mechanical link, to connect the engine to the drivetrain.

lockup torque converters

Mechanically locking up the torque converter at times saves some gasoline and increases usable power; at higher speeds, the engine doesn’t have to work as hard, and the transmission fluid doesn’t get as hot, so the transmission (or at least the fluid) lasts longer.

lockup clutch

The way Chrysler implemented it, there was a mechanical clutch as well as the fluid link. As Tom Hand wrote, “ The torque converter had a piston and friction disc inside it that locked the turbine solid to the impeller front cover. This eliminated slippage in the converter and helped fuel efficiency.”

More specifically, Chrysler added a piston to the inside of the torque converter, and put a material with high friction onto the inside of the impeller housing. When oil pressure reached a certain point, the piston was pushed against the high-friction part of the impeller housing, locking it up and providing a 1:1 mechanical connection. Since there was no damping of engine vibration through the fluid link, they added ten damper springs between the piston and turbine.

diagram 2

To move the piston with hydraulic pressure, engineers added new oil passages through the transmission input shaft.

The oil pressure in the three-speed automatics was controlled and powered by new passages in the valve body, creating two new valves, one for starting lockup and the other for disengaging it. The circuit for starting the lockup process began in second gear, with pressure pushing onto a switch valve, increasing as the transmission input shaft rotated faster; however, a fail-safe valve prevented the pressure from reaching the lockup clutch until pressure shifted with the move to third gear (or direct drive).

lockup torque converter

Seeking any edge in economy, Chrysler used the lockup converter on most of its 1978 cars, except for the edge cases — cars with 440 engines, high altitude setups, Super Sixes, heavy duty packages, and slant sixes sold in California. Originally, it locked up as the transmission shifted to third gear, when the driver reached 27 mph on V8 cars, and 31 mph on slant sixes. It could also engage at any speed, in third gear only, when the driveshaft dropped to 850 rpm on V8 cars and 1,100 rpm on six-cylinders. It unlocked when downshifting, or when the driveshaft slowed below those speeds.

Long-time mechanic and tuner Hemi Andersen wrote that “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 27 mph. (In 1988, a Chrysler tech manual specified lockup and disengagement speeds as being around 24-50 mph, depending on the engine and axle; so, when accelerating lightly, the transmission could shift to third, stay there for a time, then engage the lockup clutch.)

“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.”


TorqueFlite expert Tom Hand agreed:

A few of the early complaints with the lock-up units were sluggish vehicle performance, pinging, bad cold weather performance, dying in reverse and most of all, vibration and surging after lock up. Diagnosis showed some units were locking too early and dragging the engine down (restricted cooler lines or worn oil pump drive gears sometimes caused the converter clutch to lock in reverse, which killed the engine).

The fix was to install a different spring in the valve body to raise the lock-up speed. The pump clearances were also tightened to make sure fluid was available to keep the converter operating properly and prevent lock-up in the wrong gears.

In later years, the lockup speed was adjusted from time to time as engine control systems changed; fuel injection, for example, allowed lockup to be used more often.

Chrysler continued to add lockup torque converters to its lineup; the 1979 cars saw it used on just about all other cars, except “efficiency-size models and small specialty cars,” California sixes, and 360 four-barrels with towing packages. There were no lockup torque converters in the company’s early front wheel drive automatic transaxles; those had to wait for the 1987 car-line.

Starting in 1986, Chrysler recommended that all transmissions with lockup torque converters — going on back to 1978 — use the new Type 7176 (ATF+3) transmission fluid. The company suggested that this could resolve “lock-up shudder.” Numerous Allpar contributors have confirmed this.

The transmission locking up can feel like another shift, and can show up on tachometers, since the engine can revolve more slowly while the car goes the same speed (since there is no loss through the transmission fluid). According to a Chrysler mechanic’s guide, the lockup converter should allow no more than a momentary increase of up to 250 rpm at engine speeds over 50 mph, when the accelerator is pushed to “detent position just short of kickdown.”

Electronic activation

The company developed an electronic control for the lockup converter, which debuted on the 1986 trucks and vans. The 1986 system kept the same torque converter, switch valve, reaction shaft support and pump assembly, and input shaft. Now, though, a computer made the decisions, fed by the coolant temperature sensor, distance sensor, a carburetor switch, and a vacuum transducer (which converted vacuum readings into electrical readings).

electronic lockup control

The unlock solenoid on the valve body built up or released pressure on the piston to lock or unlock the torque converter. The carburetor switch told the computer to unlock the converter when the throttle was closed; the vacuum transducer told the computer about engine load.

With this system, there was no lockup when the car was cool (antifreeze below 150°F) or if the throttle was closed; there was also a delay after the throttle opening. If these conditions were passed, the speed and vacuum were taken into consideration.

With the new “part throttle unlock” design, used starting with the 1986 A-999 and A-904T truck transmissions (for the 318 and 225 engines), the computer unlocked the torque converter when heavy throttle was applied at speeds under 50 mph. There was no lockup below 40 mph; then, up to 55 mph, if the vacuum was between 5 and 18 psi, the converter would lock. It would unlock if the speed dropped below 40, or pressure went over 20 psi or under 3 psi. Once vehicle speed went over 55 mph, the system would lock... unless the driver pushed the pedal to the metal. Under wide open throttle, the torque converter would unlock just before the transmission kicked down to second gear.

Chrysler allowed shadetree mechanics to troubleshoot the system; a failure would set fault codes in the computer (for relay error, vacuum sensor issues, coolant sensor issues, or speed switch issues).

Today, lockup is far more aggressive, faster, and more tightly controlled — and not limited to one gear.


Even after 1978, A-727 transmissions on cars with the towing package generally did not have lockup converters.

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 and Rick Allison of A & A Transmission.) Torque converters from 1978 to at least 1988 should have a “LOCK-UP” sticker if they are lockup designs.

input shafts

Non-lockup A-904 transmissions from 1968 on have 27 splines; the 1978 and newer lockup A-904 transmissions have just 26.

Even after the TorqueFlite name was dropped and transmissions went to four speeds, heavy duty automatics might not have the lockup feature. Later transmissions that evolved from the Torqueflite (e.g. 4xRH) kept the lockup wiring separate, even into the 1990s. On some truck transmissions, for example, the overdrive solenoid would have a two-prong connector if there was no lockup converter, with a three-prong connector for the lockup (a second three-wire connector on either would be for the neutral safety/reverse lamps).

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