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A Guide to the Eaton G80 Locking Differential

Back in 2023, the Ford Motor Company sold 750,789 examples of the F-Series in the United States of America. Be that as it may, remember that General Motors sells full-size trucks under two brands. Chevrolet Silverado and GMC Sierra deliveries totaled 850,885 units, and by design, most of them were specified with a mechanical locking differential supplied by the folks at Eaton.
Eaton MLocker mechanical locking differential (a.k.a. GM G80 mechanical locking differential) 47 photos
Photo: Eaton
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Known as the MLocker in the American-Irish corporation's vernacular, the G80 locking diff rolled out in the 1970s. Building on the success of the Posi differential, the MLocker was available as an optional extra in the Rounded Line C/K series of trucks. Better known as the Square Body, the third-gen C/K was introduced for the 1973 model year.

Originally designated G86, the Eaton Automatic Differential Lock would take on a new regular production order code for the 1974 model year, that RPO code being G80. Adding to the confusion, G80 used to mean Positraction axle prior to the Eaton ADL's introduction. The MLocker is listed under said code in the order guide for the 2024 model year Chevrolet Silverado 1500 as standard equipment for the Custom Trail Boss, LT, RST, LT Trail Boss, LTZ, as well as the High Country trim level.

Optionally available on the WT and Custom, the G80 rear locking diff cannot be had on the ZR2. Instead, customers are presented with two full-locking differentials, as in G93 for the front unit and G94 for the rear unit. The mid-sized Colorado and its GMC-branded cousin also come or can be had with the MLocker, depending on trim level.

Eaton MLocker mechanical locking differential \(a\.k\.a\. GM G80 mechanical locking differential\)
Photo: Eaton
Bear in mind the truck-specific G80 shouldn't be confused with the limited-slip differential fitted to cars. If we're talking about old Camaros and Firebirds, then G80 stands for limited-slip diff. GM dubbed it Positraction for the Chevrolet brand, Safe-T-Track for the Pontiac division, and Anti Spin for the Oldsmobile brand. G80 is also used to designate the limited-slip differential of the 2016 to 2024 Camaro, and G96 is the regular production order code for the Camaro's electronic limited-slip diff.

Turning our attention back to the G80 rear locking differential for body-on-frame pickup trucks, General Motors also refers to this fellow as Gov-Loc. Care to guess why? To put it as simply as possible, the MLocker combines the clutch plates of a limited-slip differential with the positive engagement of a mechanical locker. This is made possible by a governor with mechanical flyweights, hence the aforementioned Gov-Loc designation.

The Loc part of Gov-Loc is a reference to how the G80 automatically locks if one wheel starts to spin. This, in turn, enables both rear wheels to propel the pickup truck. The G80 helps a rear-drive truck go places traditionally reserved for 4x4s. Obviously compatible with 4WD, the G80 enhances the off-road capability of four-wheel-drive trucks.

2004 Chevrolet Silverado GM G80/Eaton MLocker mechanical locking differential
Photo: Van Svenson on YouTube
Most of the time, the rear automatic locking differential acts as an open differential. The locking mechanism is triggered when the MLocker detects a difference greater than 120 revolutions per minute between the rear wheels. Unlike an electronic locker, the G80 was designed to engage and disengage automatically. No input is required from the driver thanks to the flyweight-type governor.

GM's truck-based sport utility vehicles featured the G80 prior to the switch to the T1XX platform for model year 2021. Since gaining independent rear suspension, the Chevrolet Tahoe, Suburban, GMC Yukon, Yukon XL, Cadillac Escalade, and Caddy Escalade ESV transitioned to the G86 mechanical LSD and the G96 electronically-controlled LSD.

Although the MLocker has undergone design improvements over its tremendously long production run, the concept has remained constant. Some believe that the MLocker is awfully simple, but a helluva lot of engineering went into the development of the locking-type differential.

Eaton MLocker mechanical locking differential \(a\.k\.a\. GM G80 mechanical locking differential\)
Photo: Eaton
As opposed to GM's 120 revolutions per minute, Eaton claims that lockup occurs at wheel speed differences in excess of 100 revolutions per minute. Affectionately nicknamed Gov-Bomb by those who experienced a blown diff, the G80 locks at speeds under 20 miles per hour (make that 32 kilometers per hour in the metric system). At speeds above 20 miles per hour, the latching bracket swings away from the flyweight mechanism to prevent lockup.

According to an exploded view illustration of the Eaton MLocker, the differential comprises the following components: engagement mechanism, lockout bracket, pinion shaft, reaction block, side gear, pinion gear, spherical thrust washer, cam-faced side gear, cam plate, wave spring, clutch pack, guide clips, retaining ring, and shim pack. GM recommends using SAE 80W-90 GL5 gear oil.

Technical service bulletin #91-4-109 clearly states no additives whatsoever. However, the bulletin in question does recommend additives for the limited-slip differential of the Camaro, CT4, and CT5. Based on the GM Alpha platform, said vehicles take SAE 75W-90 GL5 differential oil. Coincidentally, the seventh-generation Chevrolet Corvette is rocking the same limited-slip diff.

GM G80 mechanical locking differential \(a\.k\.a\. Eaton MLocker mechanical locking differential\)
Photo: GMC on YouTube
Eaton says the MLocker provides best-in-class traction, and Eaton further states that its mechanical locking differential is a maintenance-free design. This type of rear axle can very well survive 300,000 miles on the original gear oil. If you want your truck to be as dependable as possible in the long run, then change the gear oil every 50,000 miles (80,000 kilometers) or so, especially in case of severe service.

General Motors will drop the G80 limited-slip differential from its car lineup in the coming years due to Cadillac going fully electric by 2030. The sixth-generation Camaro ended production in December 2023, and multiple unverified reports are suggesting an electric vehicle for the seventh gen. In other words, only the G80 mechanical locking differential will soldier on in the Detroit-based automaker's pickup trucks.

"The G80 locking axle provides a greater traction advantage than limited-slip differentials in most situations, while its automatic engagement requires no driver involvement, unlike some competitors’ electronic lockers, which require driver activation," said executive director of global gasoline engines Jeff Luke in 2015. Back then, he was executive chief engineer at GM. "The G80’s simplicity, durability, and sure-footed grip have been proven with generations of customers," added Luke.

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 Download: Eaton MLocker mechanical locking differential (a.k.a. GM G80 mechanical locking differential) (PDF)

About the author: Mircea Panait
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After a 1:43 scale model of a Ferrari 250 GTO sparked Mircea's interest for cars when he was a kid, an early internship at Top Gear sealed his career path. He's most interested in muscle cars and American trucks, but he takes a passing interest in quirky kei cars as well.
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