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Four-Speed Chrysler Automatic Transmissions

Chrysler used two basic types of four-speed automatics.

The first, used primarily in cars and minivans, was originally called the Ultradrive or A-604, and eventually came to use names such as 41TE and 42LE; these were a completely new design when launched in 1989, and had computer controls from the start.

The second, used primarily in trucks, was originally designated A500 and A518, and includes the 44RE and 46RE/RH; these are essentially the old three-speed Torqueflite with an overdrive unit added.

A604 automatic

Torque converter lockup (Richard Gathmann)

Both Ultradrive-style and truck-style automatics use lockup torque converters — a mechanical clutch engages to lock the engine physically to the transmission, rather than conducting power through the transmission fluid. This eliminates losses from the torque converter, improving gas mileage and power, and reduces fluid temperature for longer life. If you're maintaining a pretty constant speed/throttle position, it will stay locked up until you let off the throttle or need more power (climbing hill, passing, etc.). Letting off the throttle will quickly unlock the converter so that the engine can freewheel and save gas.

The Ultradrive (A-604) and later automatics were designed to have lockup so all transmission control modules are compatible. In the A604 and 41TE, lockup can occur in both third and fourth gears (and second under special conditions) between 27 and 50 mph, depending on engine load and throttle position. Three-speed automatics lock the converter in third and, if fluid temparatures are high, in second, to help cooling. Lockup torque converters first appeared in Chrysler vehicles on the TorqueFlite three-speed automatic.

Variable line pressure

Some four-speed transmissions gained variable line pressure (VLP) for 2006; VLP was phased in throughout the line so that all model year 2007 transmissions had it. For 2006, transmissions connected to the 3.7 liter V6, as well as the 2.7 engine in the LX series, got VLP.

The VLP transmission frequently adjusts its internal fluid pressure to reduce parasitic losses from the pump and hydraulic system, increasing gas mileage and lengthening the transmission lifespan; that change allowed for numerous other improvements, increasing the transmission’s efficiency.

Transmissions for cars and minivans: A-604 / Ultradrive / 4xLE / 4xTE automatics

Brian Meyer wrote:

A604 control diagramInitially called the A-604 or Ultradrive, this transaxle was the first, and for a long time the only, fully electronic transaxle. It has come a long way from 1989, when every day a new service bulletin came out revising this part or that procedure...

Now called the 41TE (4 forward ratios, load range 1, transverse mount, electronic), this transaxle still sees duty in minivans and other Chrysler front-drive transverse applications, like the Pacifica, Stratus/Sebring sedan, etc. (finally replacing the tried and true 31TH, née A470, in the Neon).

Pulse-width-modulated solenoids (remember the ratcheting noise?) act directly on different clutch sets to attain the different ratios; because of this, the valve body assembly is simpler, compared to those where solenoids act to divert fluid to one shift valve or another.

The solenoids are controlled by a computer (TCM) that monitors and adjusts transmission operations, using input from sensors such as speed sensors on the input and output shafts, as well as engine data.

There are no bands or mechanical holding devices (sprags, roller clutches, etc). All ratios are supplied by five different clutch packs (Low/Rev, Underdrive, Overdrive, 2/4 and Reverse). Compared to other domestic front drive transaxles of the time, like Ford’s AX4N or GM’s 4L40E, the 41TE was lighter, smaller and less mechanically complex, with better performance.

The electronic controls allow the computer to monitor performance and adjust solenoid actuation and timing accordingly, maintaining shift quality throughout the life of the vehicle and detecting problems when they arise.

The 41TE was adapted for use in front-drive cars with longitudinal-mount engines (LH bodies) as the 42LE (originally A-606). Principles of 41TE design were incorporated in Chrysler's electronic 45RFE rear-drive transmission (another excellent Chrysler design that can provide up to 8 different ratios depending on programming!).

Through diagnosing and servicing Chryslers, I developed a great appreciation for their ingenuity and elegance of design.

Chrysler engineer Chris Theodore wrote:

You know, it was a great idea. It was clutch-to-clutch shifting. No under and over clutches, fewer parts, electronic control; but the prior VP of engineering had decided to bet the farm, and they put it into all the Chrysler models at once. Usually when you start out with new technology you want to get a little field experience. Make sure all the bugs are out and everything else is worked out.

A-604 four speed automaticThe A604 was a revolutionary development, but it suffered from Lee Iaccoca's desire to rush it into production. It was the first electronically shifted hydraulic automatic transmission that used fuzzy logic to learn to adapt its shifting pattern to match the driver's habits and tastes, as well as to compensate for internal conditions. No other automaker had attempted to replace the many valves and servos in a transmission with simple solenoids controlled by a computer. What's more, Chrysler was often not followed by others in their use of a "limp" mode, to take the driver home even in cases of control failure. The limp mode deliberately restricted the driver to second gear so problems would be addressed.

Aside from the electronic control mechanisms, the four-speed automatic is a conventional automatic with hydraulically applied clutches that shift a planetary gear train.

The problems were worked out over the course of years (Chris Theodore’s story of the A-604 / Ultradrive fixes.) “There was no smoking gun. We had a good team of engineers, but there were just literally hundreds and hundreds of bugs. We just started banging away and banging away cutting them down and knocking them down one at a time. There were supplier issues, manufacturing issues and engineering issues, there were… At first Iacocca was looking at me kind of cross-eyed not necessarily believing what I was telling him. I said look, this is what’s going to happen next. I’d show him here’s what warranty claims are going to be next month and here’s what they’re going to be the month after... It was a long haul, but slowly we worked our way out of it.”

These transmissions take a few days of getting used to a driver, during which they may act confused, but after that, they vary the harshness and speed of shifts to meet the driver's tastes. Problems can arise if you have two different drivers, each with different tastes and habits. (Using two profiles linked to the memory seats was never done.)

Chrysler reportedly put over a million miles of testing on the A604 before its first use in 1989, which is when they discovered that Dexron fluid was not good enough. However, the company wrote that Dexron was good enough if their own fluid, ATF+3, was not available. They also failed to adequately inform oil change places and corner mechanics; even some dealers apparently told customers they could use Dexron. The result was a terrible reputation for quality - we have been told by one transmission rebuilding establishment that the horrific return/repair rate on their own transmissions fell to normal levels when they switched to ATF+3, and that was around ten years after the A-604 was first introduced!

The 42LE was based on the 41TE, with a stronger final drive unit and necessary changes for the longitudinal engine placement. Center-section components were refined to cope with the added torque, and upgraded clutch packs and barreled axle shafts were used. The solenoid pack was integrated into the transaxle for quieter operation. The torque converter was the same size, but had a unique cover and longer input shaft. In the final drive unit, a hypoid ring and pinion gear set delivered power to the differential and from there to the drive axles.

The truck transmissions: A500, A518, 44RE, 46RE/RH, etc

These transmissions continued the bulletproof line of the A727, A904, and A999. In essence, they were an A999 transmission (which mixed 904 and 727 specifications) with a lockup torque converter and overdrive planetary gear set.

A-518 automatic

The gear ratios of these transmissions were often not well-matched to the engines, hurting gas mileage and perceptions of power though it saved money. Generally, vehicles should be just above their peak torque while at cruising speed, which is generally around 65-75 mph in North America, to provide a good mix of mileage, hill-climbing, and passing acceleration. However, money was not available to do this. The effect was mitigated by the wide torque bands of the engines.

Trade journal surveys indicate that the warranty costs were roughly the same as the three-speed A904 and A999 transmissions, and only a little more than the legendary A-727.

The 45RFE transmission

This was an electronically-controlled transmission with five forward ratios including an alternate second gear ratio for improved performance for passing and better fuel economy. It was designed for Jeeps but was used later in Dodge trucks; its first use was in the 1999 Grand Cherokee V8.

The transmission had a tall, 3.00:1 first gear for initial acceleration. Three planetary gear sets combined the widest range of gear ratios available in any transmission in its class when launched. The transmission was built at the Indiana Transmission Plant in Kokomo, Indiana, in a new 1.2-million square-foot facility. [45RFE and 545RFE transmission page]

Other transmissions - Engines

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