GM Transmission ID Guide: Powerglide, Turbo 350, And Turbo 400

When repairing or restoring your Chevrolet car or truck, identifying the transmission you’re running is a key element to avoid the automotive version of quicksand. General Motors has produced a myriad of versions of its fabled automatic transmissions over the years, although three models make up the bulk of pre-ECM GM slushboxes; the Powerglide, the Turbo-Hydramatic 350 (Turbo 350), and the Turbo-Hydramatic 400 (Turbo 400). Let’s take a brief look at the backstory and origins of each transmission, and how you can visually identify them as well as locate ID tags and identification numbers.

The easiest method of GM automatic transmission identification is to examine the transmission pan. Evolutionary designs came with unique pan shapes, which are clear indicators of which transmission you are dealing with. Once you identify the pan, count the number of pan bolts for confirmation. 

GM produced many transmission variants over the years, we included some of them here, but our main focus is identifying the fluid pans for Powerglide, Turbo 350, and Turbo 400.

The length of the unit can also tell you a few things. Not only is the diagram below helpful for identification purposes, but is a great visual source to see if you have enough room for a swap you might be contemplating. For example, replacing a Powerglide with a Turbo 350 is an easy swap. This is because the transmission lengths are the same and the positioning of the cross member can be easily modified, you only need to change the transmission mount, add transmission cooler lines, and update the shift indicator to a Turbo 350 pattern to complete the exchange.

Turbo 400 is the biggest, Turbo 350s and Powerglides smaller.

Never underestimate the mighty post-war Powerglide.  This two-speed box thrives after 71 years and still sees use in motorsports and classic cars. There are not too many gearboxes that can make that claim. When introduced in upper-level Chevrolet models in 1950, the Powerglide represented the first automatic transmission offered by the Big Three. Ford didn’t offer its automatic transmission until 1951, and Mopar buyers had to wait until 1954.

The Powerglide was Chevrolet’s main automatic transmission through the mid-1970s. A mechanically robust transmission, it was used in other General Motors vehicles as well. The transmission underwent little change, the major difference was a switch from cast iron to an aluminum casting in 1963.

The Mighty Two-Speed Powerglide 

The Powerglide on the left is a cast-iron unit from a 1958-62 Chevrolet. The one on the right is an aluminum-case Powerglide.

In 1962, aluminum units were only used with the 327 engine, but by 1963, all Powerglides had aluminum cases. The 1956 through 1962 Corvettes only used the aluminum versions. Identifying a Powerglide is a matter of locating the source code on the transmission block and decoding it to find the unit’s year of manufacture and of course, the 13 bolt holes on the pan. 

Typical Powerglide fluid pan

Here’s what to look for:

  • Casting numbers on the case and extension housing.
  • Powerglide transmissions were cast with the word Powerglide along the body
  • Date Casting Codes
  • Assembly Date Code Stamping – can be stamped anywhere…
  • Chassis VIN Number stamping or “source serial number”- beginning in 1962

See partial VIN stamped on housing as well as the date code on the fluid pan.

Prior to 1967, transmission ID numbers contained the plant prefix code, month and date of production (expressed numerically), and a shift code (D = Day, N = Night). From 1967 on, the ID number contained the transmission type or plant prefix, Date (coded below), and a shift code. The constants in decoding the transmission ID number are the date the transmission was produced.

  • Pre-67 Example: C213N – (C = Cleveland Powerglide, February 13, Night Shift)
  • Post-67 Example: P9E03 – (P = TYPE, 9 = year (1969), E = Month, 03 = Day of Month)

The transmission identification number or source serial number (chassis VIN) is usually located close to the transmission code. This number will contain a division identification number, the model year, and the assembly plant and production sequence (last six digits) of the vehicle identification number (VIN) stamped onto the transmission.

A Turbo Of A 350

Responding to market demand for a three-speed automatic, The Turbo-Hydramatic 350 was introduced in 1969 as a replacement to the Powerglide in Chevrolet cars and trucks with six-cylinder engines or low-horsepower V8 engines. It is widely known as one of the best automatic transmissions ever built. Jointly developed by Chevy and Buick, it is also known as the CBC (Chevrolet-Buick combined) 350.

The Turbo 350 quickly became known for its strength, versatility, and compactness. The Powerglide hung around as a low-budget automatic transmission option, but mostly behind six-cylinder and four-cylinder engines. In 1974, the two-speed box then went to the great scrap heap in the sky. The Turbo 350 transmission was prevalent in nearly all GM, rear-wheel-drive cars and trucks through 1984. It was phased out of use and superseded by GM’s 700R4 starting in 1982.

Typical Turbo 350 fluid pan. Remember, check for a nearly square shape and 13 bolts.

Other than the shape of the fluid pan, another key way to identify the Turbo 350 is by the vacuum modulator. This modulator is stationed at the transmission’s side, and there will be a vacuum line connected to it. 

Some transmissions will have a code stamped onto them while others have a tag riveted to the case. Finding the ID tag can be extremely difficult on the Turbo 35o particularly because, it’s probably been lost. If it is still attached, the ID can be found in several places on the transmission:

  • Near where the shift linkage attaches to the body
  • Just above the housing on the right side of the pan
  • On the passenger side on a flange by the bell housing

If you’re under a car trying to read this, it can be tough. You’ll not only need to hunt for, and hopefully find the tag, but it needs to be free of enough gunk to be legible.

Above is the transmission ID tag on a 1970 Corvette. Note partial VIN stamped at the bottom.

You will have to decipher the numbers on the ID tag in order to properly tell exactly what kind of transmission you have.

  • MV4 – This code identifies your turbo 350 as a “C” model, which had a locking torque converter. This code was used for ten years from 1976 to 1986.
  • MX2 –  Another 350C code. Found on ’76 through ’84 models
  • MX3 – Another Turbo 350 C code. 1976 through 1981 model years.
  • MX5 – Turbo 350C,  1982 and 1983 only.
  • M33 – This standard Turbo 350 code identifies it as a standard Turbo 350. This version did not have a locking converter.  Manufactured from ’76 through ’81.
  • M38 – Standard Turbo 350 manufactured from 1976 to 1981.

The Mac-Daddy 400

The Turbo 400 was GM’s heavy-duty three-speed transmission used from 1964 to 1990. It was standard equipment on large displacement, high-torque engines, and is generally found in trucks, and full-size GM rear-wheel-drive cars.

The Turbo 400 has a very distinct fluid pan.

The 400 transmission has a main case of cast-aluminum alloy with a length of 24-3/8-inches long. Its case is essentially smooth. The fluid pan shape is irregular, being likened to distorted Texas state borders. It also uses the same vacuum modulator as the Turbo 350. The Turbo 400 is the largest of the GM auto transmissions, but still surprisingly compact in light of the immense power they can handle.

Another quick way to identify a Turbo 400 is to look at the kick-down mechanism. Unlike the Turbo 350, instead of a “kick down” cable, it uses an electrical slide switch, which is controlled by the throttle linkage. Generally, in ’68-’71 vehicles, the switch is located on the carburetor. On ’72-’77 vehicles, the switch is located over the accelerator pedal.

The "kick down" switch locations for the Turbo 400

There are two significant variations of the Turbo 400. The Turbo 375 was a version of the transmission used from 1972 through 1976, in small displacement cars. It is identified easiest by its “375-THM” designation cast in the underside of the tail housing. The Turbo 475 was an extra-heavy-duty version and was found in larger trucks from 1971 on.

In the early ’90s, GM rebranded all of its transmissions. They created a standardized naming system across the product line and changed the name of the Turbo 400 to 3L80. The 700R4 became the 4L60 and so on. There is no difference other than the name.  3L80 stands for three gears, longitudinal mount (rear-wheel drive), and the 80 signifies that it can handle 8,000 lbs. gross weight.

Steel tag on the passenger side:
CD-68-315953
(C: Chevrolet, D: 327, 68: 1968, 315953: sequential production number)

Serial number stamped on the case, above the oil pan, driver side:
18L165790
(1: Chevrolet, 8: 1968, L: Van Nuys, CA, 165790: last 6 digits of VIN)

Turbo 400 transmissions were partial VIN stamped either above the pan rail on the driver’s side (very difficult to see with the exhaust system installed) or on the driver’s side of the bell housing as indicated in the picture.

Additional Serial Number Info of the Turbo 350 and Turbo 400

On the Turbo 350, the VIN will be stamped on either the driver’s side housing (near the shifter), on the right side of the housing just above the pan, or on a boss behind the bell housing flange on the passenger side of the transmission. On the Turbo 400, the VIN is stamped on a machined surface just above the pan on the driver’s side.

Prior to 1967, transmission ID numbers contained the plant prefix code, month and date of production (expressed numerically), and a shift code (D = Day, N = Night). From 1967 on, the ID number contained the transmission type or plant prefix, Date (coded below), and a shift code. The constants in decoding the trans ID number are the date the transmission was produced.

Pre-67 Example: C213N – (C = Cleveland Powerglide, February 13, Night Shift)
Post-67 Example: P9E03 – (P = TYPE, 9 = year (1969), E = Month, 03 = Day of Month)

Month code:
A = Jan, B = Feb, C = Mar, D = Apr, E = May, H = Jun, K = Jul, M = Aug, P = Sep, R = Oct, S = Nov, T = Dec

About the author

Dave Cruikshank

Dave Cruikshank is a lifelong car enthusiast and an Editor at Power Automedia. A zealous car geek since birth, he digs lead sleds, curvy fiberglass, kustoms and street rods. He currently owns a '95 Corvette, '76 Cadillac Seville, '99 LS1 Trans Am and big old Ford Van.
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