CC Tech: 1961 Mercedes-Benz K4A Automatic Transmission – “Mercedes Builds A Brain Box”

 

Back in 1961, Mercedes-Benz belatedly introduced its first in-house automatic transmission, known by the prosaic designation K4A 025 and offered in the company’s new W112 Mercedes 300SE flagship sedan. In the December 1961 Car and Driver, Karl Ludvigsen offered a detailed rundown of the new transmission, which we offer here with some explanatory notes and kibitzing about the Mercedes transmission’s resemblance to the original GM Hydra-Matic.

The caption between the photos at the left of the above page reads: “Transmission designer [Hans-Joachim] Förster poses with Rudolf Uhlenhaut, the chief development engineer, for Jesse Alexander’s camera during a demonstration run in France. Below, Uhlenhaut takes the 300SE into a sharp curve on the Solitude circuit near Stuttgart.”

 

Ludvigsen began:

It’s not called Daimlerdrive or Mercedomatic; it’s merely the Daimler-Benz automatic transmission. Whatever it’s called, it’s news that one of the world’s most engineering-minded car builders has created a self-shifter for its own use.

This first full-automatic box from Stuttgart has no fundamentally radical features, but rather a beautifully refined and combined amalgam of known methods to produce a transmission specially matched to Mercedes engines, Mercedes cars, and Mercedes drivers.

The above statement might give the mistaken impression that Daimler-Benz had never offered automatic before, which wasn’t true: The big Mercedes-Benz 300c and 300d — the so-called “Adenauer Mercedes” — had offered a Borg-Warner DG three-speed torque converter automatic from September 1955, although those cars weren’t produced in large numbers, and the automatic wasn’t offered on smaller Mercedes models.

 

He continued:

Native European automatic transmission developments have been rare and not notably successful. Among the all-mechanical units, only the Hobbs has had some commercial success. The electrical Smiths Easidrive is used by Rootes. Several have combined torque converters with some kind of automatic shift: Rover’s Roverdrive, Borgward’s Hansamatic, Renault’s Transfluide—all defunct. Others have used fluid couplings: Fiat’s 1900B, Daimler’s Fluid Flywheel, and Mercedes’s own Hydrak. But oddly none have followed the very successful and efficient Hydra-Matic line of marrying a fluid coupling to an automatic planetary transmission. This is what Mercedes has finally done.

Hydrak, which Mercedes eventually preferred to refer to as the “hydraulically operated automatic clutch,” was developed by Daimler-Benz in collaboration with Fichtel & Sachs, which also offered its simpler Saxomat automatic clutches to a variety of other German manufacturers in the late ’50s and early ’60s.

 

Hydrak combined a vacuum-servo-controlled plate clutch, made by Fichtel & Sachs, with a fluid coupling (not a torque converter), made by Daimler-Benz. Like the old Chrysler Fluid Drive, the fluid coupling eliminated the need to release the clutch at idle, while the plate clutch would automatically disengage when the driver touched the shift lever and automatically reengage when the shifter was released. It was somewhat fussy to use, and both its vacuum servo and its electrical controls could be troublesome, so it was not very popular, and some cars that originally had it were later retrofitted with conventional clutches.

 

While “native European” automatic transmissions were not numerous, a point that was probably of some concern for Daimler-Benz was that Opel had recently began offering a true automatic transmission on its six-cylinder Kapitän L: The unlovable three-speed Model 5 Hydra-Matic (the same light-duty Roto Hydra-Matic also found on the U.S. Oldsmobile F-85) was optional from December 1960. The Mercedes automatic was clearly well along in development by then, but Opel offering a fully automatic transmission probably forced the issue on the Mercedes automatic’s commercial introduction.

 

The following statement had some steam coming from my ears:

Is it just coincidence that Mercedes began work on this type of transmission in 1956, just 17 years after Hydra-Matic was first announced (in 1939 for the 1940 model year)? That, of course, is the life of a U.S. patent.

This was a completely fatuous insinuation, and it wasn’t even accurate. Different sources make different claims about when work began on this transmission; Road & Track said 1953, and Daimler-Benz had some German patent filings in this area going back to about 1951. Also, if you actually look at the patent plate on a first-generation Hydra-Matic transmission, you’ll notice that there are whole multitude of different patents listed, covering a span of almost 15 years — it’s not like there was one unitary Hydra-Matic patent filed in 1939. I don’t think there’s any question that Daimler-Benz had studied the original Hydra-Matic, but Ludvigsen’s obvious implication that they had copied it as soon as the patents expired shouldn’t be taken seriously.

 

Ludvigsen continued:

Chief engineer Nallinger and transmission designer Hans-Joachim Förster had explored other paths, including overdrive and the Hydrak clutch control, introduced in 1957, but finally concentrated on this combination of a hydraulic coupling with a four-speed automatic box. They were designing around a maximum torque capacity of about 190 pound-feet, since that’s about what the three-liter six delivers, but they also wanted the unit to be usable on the 220 series and even on the small fours, down to the diesel, with only minor internal modification. This has been accomplished.

The early four-speed automatic was officially called K4A 025: “K” stood for “Kupplung,” which is German for “Coupling”; “4” was the number of forward speeds; the “A” indicated the version, and “025” was the nominal torque capacity in kg-m (or mkg, as it’s often abbreviated in German sources): 25 kg-m is about 181 lb-ft. Daimler-Benz also developed a heavy-duty version of this transmission, the K4B 050, for the M100 V-8 engine in the Mercedes-Benz 600, later also used in the Mercedes 300SEL 6.3.

 

Ludvigsen continued:

The hydraulic coupling itself generally follows Hydrak practice but is refined in one important way. Most couplings have a turbulence-producing ring around their inner diameter to destroy efficiency at lowest speeds and thus reduce “creep”; Mercedes has thrown this out in favor of a new angular contour at the periphery of the coupling (visible in the cutaway) that sharply accelerates the fluid as it makes its rotary rounds. This acceleration gives the coupling a rapid increase in effectiveness (decrease in slip) as the speed of the input turbine increases from idle. By this means the coupling can supply ample torque capacity without having too-slow takeup from rest. An external heat exchanger keeps the coupling fluid cool.

The cutaway below is not terribly revealing of what Ludvigsen was talking about, but in the March 1962 Road & Track, John R. Bond explained the concept this way:

It was discovered that a 12-in. coupling could literally be sawed off around its periphery, giving a diameter of 11 in., and still have less slip than a conventionally contoured coupling designed to fit in the same space. At 2000 rpm, for example, the conventional coupling slips about 8% at wide open throttle. The Mercedes unit slips only half as much under the same conditions (4%).

The caption of the above cutaway reads:

In order to prevent creep, the coupling periphery is shaped with angular contour to accelerate rotary motion of the fluid. Coupling fluid is cooled both by radial fins and by an external heat exchanger. Parking ratchet is at rear, to right.

The first part of the caption is completely wrong: The contour of the coupling periphery was shaped to reduce slip by increasing rotary acceleration. This came at the cost of increasing the transmission’s tendency to “creep” at idle, not decreasing it. Nonetheless, it did make for a very tight coupling. The factory service manual for the K4A 025 transmission indicates that coupling slippage at road-load is only about 2 percent, which was extremely good for a non-lockup transmission with no torque splitting.

To get four forward speeds and reverse, Hydra-Matic needs three separate planetary gear sets; the new three-speed H-M uses two. Mercedes is proud of the fact that it has squeezed four speeds and reverse out of only two planetary sets by clever planning. In this way the overall length has been kept just three inches longer than the manual box, and the weight has been held to 130 pounds wet—30 pounds more than the manual unit.

A dry weight of 130 lb made this transmission at least 70 lb lighter than a single-coupling four-speed Hydra-Matic, but roughly the same as the bigger Roto Hydra-Matic found in full-size Oldsmobiles and B-body Pontiacs in this era, which had over twice the Mercedes transmission’s torque capacity.

 

Ludvigsen continued:

As usual in such units, gear selection is by wrapping brakes and multi-disc clutches. The clutch discs, by the way, are supplied by Borg-Warner’s spring division, from the U.S.! For first gear the brake bands of both planetaries are applied, giving a total reduction that’s the product of the two sets: 2.52 X 1.58 = 3.98 to one. For second gear a clutch locks up the rear planetary so only the 2.52 ratio of the front set is operative. For third the two gearsets “change hands,” the rear one being braked and the front one locked for a ratio of 1.58. For direct, both brakes are released and both clutches are applied for a solid lockup—excepting the coupling, which is always in the circuit.

These ratios are similar to later four-speed Hydra-Matic transmissions, although Hydra-Matic got its highest numerical ratio from the rear gearset, where the Mercedes box was the other way around.

This ratio spread was far from ideal: First was a little too low (high numerically), so with the gear selector in “3” or “4,” the transmission would normally start in second gear, which was a little too high (low numerically). To get a first-gear start, you had to mash the accelerator far enough to trip the kickdown switch or else move the shifter to “2,” which then gave you a very firm 1–2 shift accompanied by a big drop in engine speed.

 

Ludvigsen continued:

Thus far the operation is just like the original four-speed Hydra-Matic, though the latter does actually place the coupling between the two main planetary sets in operational sequence if not in physical fact. Hydra-Matic adds another planetary to get reverse; Mercedes adds another clutch and brake at the front planetary to get reverse rotation and multiplies the result by the rear set’s ratio to come up with 4.13 to one going backwards. Thus the two sets are used together only in low and reverse, the least-used ratios.

In a four-speed Hydra-Matic, engine power flowed through the front planetary gearset and then through an intermediate shaft that drove the fluid coupling. I think Ludvigsen’s description really understates the significance of this arrangement, which was quite unusual, and a defining feature of the early Hydra-Matic transmission family: Hydra-Matic essentially used the planetary gearing to give the fluid coupling different slip characteristics in each gear, increasing slip in first and reverse to reduce idle creep while greatly reducing slip in third and fourth. Although Daimler-Benz had some similar design priorities, their K4 transmission didn’t do anything like that, and it’s really the biggest and most dramatic difference between the Mercedes automatic and the four-speed Hydra-Matic.

That unusual arrangement was also why the four-speed Hydra-Matic needed an additional gearset for reverse. The way you normally get reverse in a simple planetary gearset is to hold the planet carrier stationary, but in a four-speed Hydra-Matic, the carrier of the front gearset drives the intermediate shaft, so holding it stationary would stop the fluid coupling from rotating and stall the engine; the carrier of the rear gearset is affixed to the output shaft, so holding it stationary would keep the car from moving. Daimler-Benz was able to eliminate the third gearset by adding an extra clutch and brake, but that meant they couldn’t use the kind of torque-splitting power flow that Hydra-Matic did to increase efficiency.

Early four-speed Hydra-Matic transmissions had used band brakes, which had some advantages, but needed periodic adjustment and contributed to the transmission’s tendency to shift with a jerk. Later Hydra-Matic generations switched to sprag clutches, which were smoother and didn’t need adjustment, but involved some additional complexity for engine braking and neutral. Daimler-Benz decided to stick with bands:

Free-wheels can be used to get quick, minimum-wear brake band engagement on full-throttle downshifts, as the Hydra-Matic shows, but Mercedes preferred to do this by hydraulic design instead of mechanical manufacture. The anchors of the brake bands are spring-mounted so that they move slightly as the band’s servo action takes effect after the drum rotation changes direction, as it does after that planetary’s clutch is released. Motion of the anchor opens a valve that applies full pressure to the brake band just at that instant. By applying the brake fully just at the point where the drum stops turning, so little wear is experienced that the brake bands will never need to be adjusted.

This two-stage band engagement arrangement, which was very clever, used oil pressure on the shift valve to modulate the flow of oil to the band servo, as illustrated in these diagrams from the shop manual:

To keep clutch application smooth, the pressure in the attendant hydraulic system is modulated according to the engine’s intake manifold vacuum, which Mercedes feels runs sufficiently parallel to engine torque. This torque-vacuum relationship is also used to initiate the automatic upshifts, which take place when the same performance can be obtained in the next higher gear with lower engine revs, hence with lower fuel consumption. Automatic downshifts are signaled by a governor, which acts imperceptibly and only at extremely low speeds. Throttle position, short of full “kickdown” depression, plays no part here, so Mercedes has dispensed with the Hydra-Matic’s worst failing: that sudden downshift at part-throttle that really wasn’t required. Avoiding this is specially important on wet or icy surfaces, where a downshift and sudden torque increase can flick the tail out suddenly and dangerously.

The basic control principles of the Mercedes transmission were broadly similar to Hydra-Matic, or any number of other pre-computer automatic transmissions: Each shift valve was spring-loaded to open or close at specific pressures; the transmission output shaft operated a centrifugal governor to generate shift pressure based on road speed, with the shift points raised or lowered based on torque demand. The fluid coupling Hydra-Matic family used a two-stage governor and signaled torque demand based on throttle position, but the Mercedes transmission governor had four centrifugally controlled shift valves, and the transmission varied shift points based on manifold vacuum, which also modulated line pressure to the bands and clutches.

Unlike the later Turbo Hydra-Matic and other automatics that used intake vacuum in this way, the Mercedes control system didn’t include an aneroid bellows to compensate for differences in outside air pressure. This wasn’t entirely satisfactory, and the later K4C 025 transmission used an accelerator linkage rod to control shift points, retaining the vacuum modulator to vary line pressure.

One similarity between the Mercedes transmission and the four-speed Hydra-Matic that Ludvigsen didn’t mention and that wasn’t obvious in the cutaway was that the K4A transmission had both front and rear oil pumps, with the latter set up to take over most of the work of maintaining line pressure at higher road speeds to reduce power consumption. The rear pump also allowed push- or tow-starting of a stalled car, something that had been a big preoccupation of early automatic transmission designers, but that had been falling by the wayside in the U.S. — newer American automatics had mostly dropped rear pumps to reduce size and weight.

That’s Ludvigsen in the photo above, presumably at the wheel of the 300SE on the Solitudering track. He continued:

“Kickdown” to the next lower gear can always be obtained if needed; the governor determines which two gears are available to the driver at a given moment. More important, the driver has complete control over the box through a steering column lever and a small, rectangular indicator on the dash. In positions “3” and “2” he can hold the transmission in third and second gears respectively, to whatever revs he chooses. There’s no automatic upshift to overrule the driver’s desires. In “2” position, specially laid out for hauling trailers up alpine passes, the box will shift back and forth between low and second at part-throttle according to load. In positions “4” and “3” the car normally starts in second gear, to minimize creep, but first can be engaged by stomping down all the way.

The four-speed Hydra-Matic had offered much of the same functionality since 1952: With the “Dual Range” controls, you could hold the lower gears for longer, giving better performance (and, on Controlled Coupling transmissions, better engine braking) and a fair amount of manual control. The difference was that Hydra-Matic would generally still upshift automatically to prevent over-revving, while the Mercedes transmission wouldn’t override the driver’s selection even if it meant a blown engine.

 

This was more a philosophical point than a technical one, and to be frank, I think it risked putting more faith in the driver than was necessarily merited. The question of whether or not automatic transmissions should override manual gear selection has persisted well into more recent decades; ironically some later German automatics, like the Porsche Tiptronic automatics of the ’90s, have been far more prone to second-guessing the driver’s intentions in perplexing ways than the Hydra-Matic arrangement Ludvigsen disdained. He went on:

Mercedes philosophy of design for the driver shows in this unlimited hold in each gear; it also shows in the arrangement of the brake pedal. Some years ago Uhlenhaut said that he felt it was unwise to encourage drivers to learn to brake with their left foot in an automatic car, as it would lead them astray when they sat in a stick- shift car again. Correct or not, the 300SE has a normal brake pedal that’s definitely placed next to the accelerator.

Seeking to discourage left-foot braking was a retrenchment rather than a statement of principle for Mercedes: Early Hydrak-equipped cars had used extra-wide brake pedals for exactly that purpose! Daimler-Benz later reversed course on this, so later cars with the automatic clutch (starting sometime in 1958) switched to the same narrower brake pedal as cars with conventional clutches, a practice continued with the early automatic transmission cars.

 

Ludvigsen added:

On the road the Daimler-Benz automatic transmission is hard to fault. There is fairly strong creep at idle with the indicator at “4”, but not much more than in current U.S. cars. Actual movement of the shift lever was more awkward than we expected, being a bit stiff and clumsy at first acquaintance. Experienced factory drivers coped superbly. The automatic upshifts are certainly noticeable—more so at higher speeds—but not at all objectionable. At full throttle the upshifts take more than a second or two to go through, reflecting the solid, direct drive supplied by the hydraulic coupling. Uhlenhaut told us that this transmission costs only 8 bhp at peak engine revs.

According to John R. Bond in Road & Track, Mercedes admitted that top speed did drop by about 3 mph with the automatic.

 

As for the 300SE in which the new automatic was installed, Ludvigsen observed:

The new [M189] engine is responsive and flexible with more than enough power for this modern-day “Grosser Mercedes.” Fuel consumption is not startling with 16.2 mpg at a steady 68 mph dropping to 14 mpg as the right foot gets nearer the floor. However, this is a luxury car and tycoons needn’t worry about mpg.

This was referring to the all-aluminum 2,996 cc M189 six in the W112 Mercedes 300SE, which had just gone into production in April 1961. One reason its fuel consumption was so heavy was its gearing: With automatic, the axle ratio was 4.10, so the engine was turning 3,800 rpm at 70 mph!

 

Ludvigsen concluded:

The new box is standard equipment on the new 300SE, which qualifies hands-down as the world’s most luxurious compact with its servo disc brakes, air suspension, power steering and the new aluminum-block three-liter six that we discussed under “New Engines” last month. This car probably won’t be in full production for a few months—and not many will be made anyway—but the automatic transmission should be available now on the 220 series. Price? $350, which many M-B buyers will plunk down willingly for this most-controllable brain box.

Automatic was standard on the the 300SE and 300SEL in the U.S., but in Europe, it was still an extra-cost option on the W112, just as it was on lesser Mercedes models. The automatic was predictably popular in the U.S., but I don’t think European buyers were quite so enthusiastic: There was a noticeable penalty in performance and fuel economy, especially with smaller engines, and the transmission was expensive (about 1,400 marks in West Germany, almost £200 in the UK).

 

The K4A 025 transmission’s shifts were always firm — Daimler-Benz had been preoccupied with minimizing the performance penalties of automatic, not with maximum refinement — and even though it didn’t need band adjustments, it was a fairly finicky device, so there were various minor maladies that could make its operation even jerkier. For 1961, it wasn’t too bad compared to contemporary rivals, especially for buff book testers who appreciated its tight feel, but compared to a later Turbo Hydra-Matic or TorqueFlite, it felt like a relic of the ’50s, which in some ways it was. By 1966, Car and Driver would complain, “The automatic transmission seems to take forever to shift—like an old Hydra-Matic—and when it does, the shifts are hard and a little jerky.”

 

This early K4A 025 transmission was offered in a wide range of Mercedes-Benz models through May 1969, when it was superseded by the somewhat revised K4C 025 (and the heavier-duty K4A 040 for cars with the 3.5-liter M116 V-8). It’s now fairly obscure, and increasingly a specialist item in terms of service and repair. Even more than the early Hydra-Matic (to which I think it’s less similar than Ludvigsen implied here), I think the early Mercedes automatic is clearly an engineer’s transmission, from an era when Daimler-Benz was still an engineering-dominated company — it wasn’t long on user-friendliness or ultimate smoothness, and was somewhat eccentric, but it was versatile, no-nonsense, and outstandingly efficient.

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