Giving Slip the Slip: Lockup Torque Converters and Split Torque Automatic Transmissions


During this period, Ford also revived the split torque concept, first for the AOD, Ford’s first four-speed overdrive automatic, and then for the ATX, the company’s first automatic transaxle for front-wheel-drive applications.


The Automatic Overdrive (AOD) transmission, introduced for some full-size FoMoCo cars in the 1980 model year, was derived from Ford’s older three-speed FMX automatic. It used a Ravigneaux compound gearset with three long and three short planet pinions on a common planet carrier, two sun gears (one with 36 teeth, the other with 30), and a single annulus (with 72 teeth) affixed to the output shaft.

Color diagram of a 1980–1991 Ford AOD transmission © 2016–2017 Aaron Severson

Ford’s four-speed AOD transmission has both a turbine-driven primary input shaft (medium blue) and a narrow central direct drive shaft geared to the torus cover (red). In the first three forward gears, the main input shaft drives the rear sun gear (orange) through the forward clutch while the annulus drives the output shaft (dark green). The reaction member is the planet carrier (purple) in first and the front sun gear (light green) in second. In third and fourth, the direct clutch links the direct drive shaft to the planet carrier. The fuchsia rectangles represent the transmission’s three one-way clutches; the upward-pointing black triangles indicate that the clutches allow rotation only in the direction of engine rotation. (author diagram)

As in the FMX, the torque converter turbine drove the smaller rear sun gear in first, second, and third gears. Two one-way clutches (supplemented in Low by a brake band) could alternately hold the carrier or the larger front sun gear, providing two forward reduction ratios. Where the AOD parted ways with the FMX was in the direct drive third gear. Instead of locking the front sun gear to the input shaft in high, as the FMX did, the AOD used an additional direct clutch to lock the planet carrier to a central shaft that passed through the main input shaft and was driven directly by the engine.

In third, engine torque was split between the rear sun gear, which rotated at turbine speed, and the planet carrier, which rotated at engine speed. Because the carrier always rotated faster than the rear sun gear, the long planets overdrove the larger front sun gear. This forced the short planet pinions to resolve the speed difference and drive the annulus forward at a speed slower than the carrier, but faster than the rear sun gear, demultiplying hydraulic slippage by 58.3%. (We’ll spare you the math, which with Ravigneaux gearsets is very cumbersome.) For example, if the engine was turning 2,000 rpm and converter slippage was 100 rpm, the annulus would rotate at 1,958.3 rpm, reducing net hydraulic slip from 5% at the turbine to 2.1% at the output shaft.

1980–1991 Ford AOD transmission showing power flow in 3rd gear © 2016–2017 Aaron Severson

In the Ford AOD’s split torque third gear, both the forward and direct clutches are engaged, allowing both the rear sun gear and the planet carrier to drive the annulus forward. As the yellow power flow arrows in this diagram illustrate, input torque is then split between the rear sun gear and the carrier and recombined by the planet pinions. Both bands are released in third while the transmission’s two one-way clutches (both fuchsia) are ineffective, although the stator’s one-way clutch can lock if turbine speed falls significantly below engine speed. (author diagram)

The AOD’s new overdrive fourth gear was created by releasing the forward clutch — thereby disconnecting the rear sun gear from the main input shaft and the turbine — while engaging a brake band to hold the front sun gear stationary. With the direct clutch still engaged, the carrier, rotating at engine speed, overdrove the rear sun gear, forcing the annulus to rotate at 1.5 times engine speed — an ratio of 0.67:1. The torque converter turbine continued to rotate in fourth, but was no longer connected to the planetary gears, so there was no hydraulic slippage. This lockup wasn’t available in any of the lower gears.

1980–1991 Ford AOD transmission showing power flow in 4th gear © 2016–2017 Aaron Severson

In fourth gear, the AOD’s direct clutch remains engaged, but the forward clutch disengages and the overdrive band holds the front sun gear stationary. With the planet carrier driving and one sun gear held, the transmission becomes a completely mechanical overdrive with a ratio of 0.67:1. The yellow arrows in this diagram illustrate the power flow in fourth gear. (author diagram)


Ford took a different approach with the ATX transaxle, which became optional on the new FWD (Mk3) Ford Escort and Mercury Lynx for 1981 and the Ford Tempo/Mercury Topaz for 1984. Unlike the AOD and the earlier GM split torque transmissions, the early ATX used a separate planetary gearset specifically for torque-splitting purposes.

Color diagram of a 1981–1985 Ford ATX transaxle with split torque converter © 2017 Aaron Severson

Although the Ford ATX was actually an automatic transaxle for transverse engine/FWD applications, we’ve omitted most of the differential components for clarity. From this angle, the differential would be in front of the unit, driven by the input gear, which in turn was driven by the planet carrier (both shown in purple). The version of the ATX used in the larger Ford Tempo and Mercury Topaz had the same layout, but used additional plates in all three multi-disc clutches for greater torque capacity. (author diagram)

Known in Ford parlance as a “splitter gear,” the additional gearset was located within the torque converter torus housing, between the turbine and the flex plate. It was a simple planetary gearset with a single annulus (with 78 teeth) and three planet pinions surrounding a sun gear with 48 teeth. The flex plate and torus housing drove the splitter unit annulus at engine speed while the transmission input shaft, driven by the turbine, drove the splitter sun gear.

As we’ve explained in the preceding pages, this arrangement served to demultiply hydraulic slippage in the torque converter. Since annulus speed (VA) was engine speed while sun gear speed (VS) was turbine speed, the speed of the splitter gearset’s planet carrier (VC) was therefore:

VC = VS + ((VA – VS) / (1 + sun gear teeth / annulus teeth))

… or:

VC = VS + ((VA – VS) / 1.615)

For example, if the engine were turning 2,000 rpm and there was 100 rpm of converter slippage, carrier speed would be 1,961.9 rpm, demultiplying hydraulic slippage from 5% at the turbine to about 1.9% at the carrier.

1981–1985 Ford ATX transaxle split torque converter diagram © 2017 Aaron Severson

In split torque ATX transaxles, the torus cover, which is bolted to the flywheel, drives the impeller, the oil pump shaft, and the annulus of the “splitter” gearset (all red in this diagram). The turbine drives the main shaft and the splitter sun gear (both medium blue). The planet carrier of the splitter gearset drives a secondary driveshaft (both light green), which drives the intermediate clutch. The yellow power flow arrows illustrate the way input torque is split at the flywheel. (author diagram)

The splitter gearset’s planet carrier drove a hollow sleeve shaft (which for reference we’ll call the “splitter shaft”), passing through the main transmission input shaft. (The sleeve shaft was also hollow because it contained the solid driveshaft for the transmission oil pump, which was located on the opposite side of the transaxle from the torque converter.) Both input shafts carried power to the main planetary transmission, a Ravigneaux gearset with a single annulus (with 86 teeth), two sun gears (one with 52 teeth, the other with 29), and three short and three long planets (each with 17 teeth) on a common carrier. The carrier drove the differential input gears.

1981–1985 Ford ATX transaxle showing power flow in 1st gear © 2017 Aaron Severson

In first gear, the split torque ATX sends all power through the converter turbine to the main shaft (medium blue) and a one-way clutch (fuchsia) to the smaller rear sun gear (dark red). The larger front sun gear (orange) is held stationary by the low band, forcing the carrier and differential input gear (purple) to rotate forward at reduced speed. As the yellow power flow arrows indicate, in first gear, the splitter gearset transmits no power (although the splitter gearset goes rotate). (author diagram)

The splitter shaft was ineffective in first and reverse, rotating idly while the main shaft drove the smaller input sun gear at turbine speed. In second gear, the multi-disc intermediate clutch engaged to connect the splitter shaft to the annulus of the Ravigneaux gearset. Since the larger rear sun gear was held by a brake band, this caused the smaller sun gear to overrun the input shaft and spin idly on its one-way clutch. Since the splitter shaft was now driving, hydraulic slippage was demultiplied by 61.9% (1 / 1.615).

1981–1985 Ford ATX transaxle showing power flow in 2nd gear © 2017 Aaron Severson

In the ATX transaxle’s second gear, the low band remains engaged, holding the larger rear sun gear (orange) stationary while the intermediate clutch engages, allowing the splitter shaft (green) to drive the rear annulus (dark blue). The one-way clutch allows the smaller front sun gear to overrun the main shaft, which continues to turn idly. As the yellow power flow arrows illustrate, this sends all power through the splitter shaft in second gear. (author diagram)

In the direct drive third gear, the multi-disc direct clutch engaged, again allowing the main input shaft to drive the smaller sun gear, and the brake band was released, allowing the large sun gear to rotate freely. The intermediate clutch remained engaged, so the splitter shaft continued to drive the annulus at the same time. With both the annulus and small sun gear driving, the planet gears overdrove the large sun gear, forcing it to rotate faster than the engine. This in turn forced the planet carrier to rotate faster than either input shaft, albeit still slightly slower than the engine.

1981–1985 Ford ATX transaxle showing power flow in 3rd gear © 2017 Aaron Severson

In the ATX transaxle’s third gear, the low band releases, the intermediate clutch remains engaged, and the direct clutch engages, locking the main shaft to the smaller front sun gear (dark red). With the annulus (dark blue) driven by the splitter shaft and the small sun gear driven by the main shaft at turbine speed, the large sun gear (orange) is overdriven. The planet carrier (purple) then “resolves” the speed difference, rotating slower than the large sun gear, but faster than either the small sun gear or the annulus. (author diagram)

Although the math is again very cumbersome, this arrangement demultiplied converter slippage by a whopping 93.4% in third. For example, if engine speed were 2,000 rpm with 100 rpm of converter slippage, carrier speed (discounting mechanical losses) would be 1,993.4 rpm, reducing hydraulic slippage from 5% at the turbine to a mere 0.003% at the differential. That was efficient enough that the ATX could forgo a lockup clutch without a noticeable sacrifice in fuel economy, an important consideration for Ford’s cheapest U.S.-market models.


Ford’s split torque revival was relatively brief. Later versions of the ATX transaxle abandoned the splitter gear and dual input shafts for either a conventional hydraulic lockup (which Ford abbreviated FLC, for “full lockup clutch”) or, in some applications, a centrifugal lockup clutch (CLC) similar to that of the C5 transmission. The change coincided with the availability of more powerful engines in the Escort/Lynx and Tempo/Topaz lines, which suggests that the rationale may have been to facilitate increases in the transaxle’s torque capacity. (Tempo/Topaz versions of the ATX already had extra clutch plates in each multi-disc clutch.)

Similar concerns led to the elimination of the split torque feature of the four-speed AOD in the early nineties. The AOD was adequately strong for early eighties engines, but as power and torque increased throughout the decade, the secondary input shaft became a notable weak link. When the electronically controlled AOD-E debuted in 1991, it had only a single input shaft with no third-gear torque split. The same was true of the closely related 4R70 and 4R70W that replaced the AOD-E in 1993.

By then, all or nearly all domestic and most non-U.S. passenger car and light truck automatics had lockup clutches, most of them hydraulically operated and electronically controlled, differing only in minor details. The centrifugal lockup clutch eventually went the way of the split torque units, since its purely mechanical operation didn’t offer the fine-tuned control of electronically controlled electro-hydraulic clutches, whose operation can be better tailored to different operating conditions. For example, an electronically controlled clutch can be programmed to remain unlocked during warm-up or if either the engine or transmission is in danger of overheating, which a centrifugal clutch cannot.

This period of comparative orthodoxy — mostly four-speed overdrive automatics with electro-hydraulic lockup clutches — turned out to also be relatively brief. Subsequent automatic transmission design has diverged along several paths, including relatively conventional planetary transmissions with an ever-growing number of gears, at least three types of continuously variable transmissions, and dual-clutch semiautomatic transmissions. All these are beyond the scope of an article that is already considerably longer and more complicated than we originally intended, so we’ll just say that for automatics that have torque converters, computer-controlled lockup clutches are now the established norm and are likely to remain so.

As for split torque layouts, those have become common for hybrid electric vehicles, but that subject, like many others, will have to wait for another day.


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Hildebrand, assignors to Ford Motor Company, U.S. Patent No. 4,224,838, “Four Speed Ratio Automatic Transmission With Compact Gearing,” filed 6 December 1978, issued 30 September 1980; “Saturday Mechanic Looks at the ’82 Fords,” Popular Mechanics Vol. 156, No. 4 (October 1981): 128–129, 169–170; Otto Schwab and Siegfried Krauss, assignors to Zahnradfabrik Friedrichshafen AG, German Patent No. 1,160,257B, “A Fluidic Torque Converter Arrange in Its Core Area Centrifugal Friction Clutch,” filed 20 August 1960, issued 27 December 1963; Senate and House of Representatives of the United States of America in Congress assembled, “Energy Policy and Conservation Act,” Public Law 94–163, Title V (Part A – Improving Automotive Fuel Economy), 22 Dec. 1975; Mort Shultz, “Car Clinic,” Popular Mechanics Vol. 150, No. 6 (December 1978): 55; Dennis Siamanitis, “Ford’s New Escort: Some Technical Tidbits,” Road & Track Vol. 31, No. 11 (July 1980): 77–80; Russell Earl Silberschlag, assignor to Borg-Warner Corporation, U.S. Patent No. 4,117,918, “Wedging Centrifugal Clutch with Torque Limiter,” filed 28 December 1976, issued 3 October 1978; and U.S. Patent No. 4,305,493, “Friction Shoe Assembly for a Speed Responsive Centrifugal Clutch Assembly,” filed 26 December 1979, issued 15 December 1981; Jim Smart, “How to Build the Perfect AOD – AODetail,” Mustangs and Fords July 2014, www.mustangandfords. com; Vincent M. Staub, Jr., assignor to General Motors Corporation, U.S. Patent No. 4,317,510, “Torque Converter Having a Viscous Drive Portion,” filed 24 July 1980, issued 2 March 1982; Edwin Storm’s Free Car Brochures website at the Old Car Manual Project (; Studebaker Corporation, Studebaker Automatic Drive Shop Manual (South Bend, IN: Studebaker Corporation, ca. 1952), and “The New Studebaker for 1951” [brochure D130-11-50], November 1950; Hans Tore Tangerud’s Autoblog website (; William K. Toboldt and Larry Johnson, Goodheart-Willcox Automotive Encyclopedia (South Holland, IL: The Goodheart-Willcox Company, Inc., 1975); “Transmissions,” Popular Mechanics Vol. 115, No. 1 (January 1961): 157–158; United Motors Service Division, The Hydra-Matic Transmission 1946-1955: On-the-Car Adjustment Service Manual (Detroit, MI: United Motors Service Division of General Motors Corporation, 1956), and Hydra-Matic Controlled Coupling Transmission Service Manual (Bulletin A-3755) (Detroit, MI: United Motors Service Division of General Motors Corporation, 1 November 1957); U.S. War Department, Ordnance Maintenance: Hydra-Matic Transmission and Propeller Shafts for Light Tanks M5, M5A1, and 75-MM Howitzer Carriage (War Department Technical Manual TM 9-1727C (Washington, DC: U.S. Government Printing Office, 5 February 1943); Matthias Wandel, “Planetary gear ratio calculations,” Woodgears, gear/ planetary.html, accessed 11 August 2016; Bill Williams, “1948 Packard Station Sedan,” Special Interest Autos #17 (June-July 1973), reprinted in The Hemmings Motor News Book of Packards: driveReports from Special Interest Autos magazine, eds. Terry Ehrich and Richard Lentinello (Bennington, VT: Hemmings Motor New, 2001): 51–56; Gary Witzenburg, “The ’82 Fords,” Popular Mechanics Vol. 156, No. 3 (September 1981): 89–91, 168; and Walter A. Woron, “Motor Trials: Packard 200 Is the One to Beat for Comfort and Performance,” Motor Trend Vol. 3, No. 11 (November 1951), reprinted in Packard Gold Portfolio 1946–1958 36–37, 43.

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  1. Well Aaron . . another masterpiece. This and the GM automatic history are probably the most definitive descriptions of these technologies on the Internet, excepting pure design and engineering treatises. Well done and thank you; this must have been an enormous amount of work.

  2. I have yet to read this monumental work in depth. Whether it will add to my working knowledge is debatable, but my brain will benefit from the workout.
    Aaron is probably now the best informed person in the world regarding the history of automatic transmission development

    1. I appreciate the compliment, but I’m really not! This is a remarkably broad, convoluted, and idiosyncratic field and there’s a LOT I don’t know. For people who want a broader overview, I would recommend a book by Philip G. Gott entitled Changing Gears: The Development of the Automotive Transmission, published by the SAE as part of their Historical Series in 1991. (At this point, an updated, expanded edition wouldn’t go amiss, given all the subsequent development in CVTs and automatics with five or more speeds.)

  3. Smashing read, great job!

  4. I can’t imagine the hours of work which you must have put into understanding these various transmissions, to say nothing of writing up a description that a simpleton like me could (mostly) understand. Another fascinating article, thank you for all your effort!

    One thing I’ve always wondered about was if any manufacturers looked into Wilson pre-selector gearboxes as a basis of an automatic. Wilson pre-selectors were pretty well established technology, although not common, by the late ‘30s. Obviously some sort of mechanism would have been required to determine what gear to select and when to actually shift, but starting with a Wilson ‘box at least some of the problems would have been solved. But I’ve never heard of anyone going that route.

    1. The GM team that designed Hydra-Matic was certainly familiar with the Wilson preselector. In fact, Cadillac’s chief engineer ordered an early Daimler Double Six with the Wilson and Laurence Pomeroy’s Fluid Flywheel for evaluation purposes. However, Wilson gearboxes were quite bulky and complex because the nature of their operation required a separate set of epicyclic gears for each ratio, including reverse. With automated hydraulic operation and combinations of brakes and clutches, it was possible to get the same results more efficiently.

      1. Interesting—thanks for the information!

        1. I haven’t studied the Wilson preselectors in any great detail, but if you’re curious, the applicable U.S. patents are US1404675 and US1796904. As you’ll see if you look at the first one, the original iteration had three speeds forward and one reverse, for which it requires four epicyclic gearsets. A Simpson gearset (which I’ll be discussing in great detail in the next few days) provides the same number of ratios from only two gearsets, and a single Ravigneaux gearset can give you four forward speeds and reverse if you have enough clutches. So, you can see how those would be preferred from a standpoint of cost and packaging!

        2. For comparison, a four-speed Wilson pre-selector has four planetary gearsets, four sets of brake bands, and a cone clutch, which is a lot of pieces.

  5. sir im having an issue with my 93 f150 aod. its the mechanically controlled aod. works great no real problems. but the question is when i put my buddys obd code finder on it. the only readings i got was for all the electronically controlled aod. there were around 6 defaults that popped up. i called a trans shop and he had no answer,but it sounded strage to him. if someone would have put in a used ecu ,fron a electric controlled aod. would it work,yet throw out aod default codes. i take it your a writer and not a trans guy ,but maybe someone could answer the question.

    1. I’m not able to provide repair or maintenance advice, sorry!

  6. Thanks for this. New to the site and found it because of this read. As the new owner of a 92 Alante with the viscous clutch I wondered what the difference was. It does drove different than a lock up. It’s weird it’s not noticable as even a new soft engaging lock up but it acts somewhat like it has one. I feel better and it makes more sense now.

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