GM Turbo 400 (TH400 / 3L80) Identification & Tech Info
The Turbo-Hydramatic 400 (TH400, THM400, later called the 3L80) is GM’s heavy-duty three-speed automatic. It was built for rear-wheel-drive and four-wheel-drive cars, trucks and vans from the 1964 model year until the 4L80E replaced it for 1991. Use this page to confirm you have a TH400 and to find the rebuild, wear and upgrade parts we stock for it.
TH400 at a glance
| Item | TH400 / 3L80 |
|---|---|
| Type | Three-speed automatic, longitudinal (rear-wheel and four-wheel drive) |
| Model years | 1964 to 1990. Introduced in 1964 Cadillacs and as the Buick “Super Turbine 400”; Oldsmobile, Pontiac and full-size Chevrolet followed for 1965. It left most passenger cars around 1980 and stayed in GM trucks and vans through 1990. The U.S. military HMMWV (Humvee) kept the 3L80 later, which is why some parts listings run into the 1990s. |
| Replaced by | 4L80E (four-speed, electronically controlled) for the 1991 model year |
| Gear ratios | 1st 2.48, 2nd 1.48, 3rd 1.00 |
| Case length | 24-3/8 in., measured without the tail housing |
| Output shaft | 32 splines |
| Name | Renamed 3L80 under GM’s late-1980s naming system: 3 speeds, longitudinal, strength rating 80. A 3L80 is a TH400; the parts are the same. |
How to identify a TH400
- Oil pan. 13 bolts in an irregular, many-sided shape. A Turbo 350 pan also has 13 bolts, so go by the shape, not the bolt count: the TH350 pan is nearly square with one corner cut off.
- Kickdown. The TH400 kicks down electrically. There is an electrical plug on the side of the case, fed by a switch at the throttle linkage, and no kickdown cable. A TH350 uses a cable.
- Bellhousing. The bellhousing is cast as part of the case. Buick, Oldsmobile, Pontiac and Cadillac share one bolt pattern (often called BOP); Chevrolet uses a different one. Check the pattern before you swap a TH400 between engine families.
- Length. The case is 24-3/8 in. long without the tail housing. A TH350 case is 21-3/4 in.
Case connector styles
The plug on the side of the case changed over the years. Match the one on your case:
| Connector | Years | Part |
|---|---|---|
| 1 prong, small OD | 1964 | 34444 (OE 2977975) |
| 2 prong, with variable pitch (Switch-Pitch) | 1965–1967 | 34444B (OE 2984091) |
| 1 prong, large OD | 1968–1998 | 34444A (OE 24205373) |
| 2 prong, without variable pitch | 1970–1982 | 34444C (OE 8629502) |
Switch-Pitch units. Some mid-1960s Buick, Oldsmobile and Cadillac TH400s used a variable-pitch stator, controlled through a 2-prong case connector. A 2-prong plug alone does not prove a Switch-Pitch unit, because later non-Switch-Pitch cases also used two prongs. These units need their own converter and pump parts, so confirm before ordering.
Short shaft vs long shaft
Two-wheel-drive TH400s use a long output shaft inside a tail housing. Four-wheel-drive units use a short output shaft that sticks out about 2-1/2 in. from the case and drives the transfer case through an adapter; some trucks with the NP203 transfer case used an even shorter shaft. Measure the shaft and count the splines before ordering an output shaft, adapter gasket or tail-housing seal.
- TH400 / 4L80E adapter-to-transfer-case gasket (1284)
- 6-bolt transfer case adapter gasket (1043)
- TH400 extension housing seal kit with bushing, 2WD (EHK400-KIT)
- TH400 / 4L80E output shaft bushing, narrow style (1404G)
TH375 and heavy-duty versions
The TH375 is a lighter-duty TH400 used in the early-to-mid 1970s. It has fewer clutch plates and an output shaft sized for Turbo 350 driveshaft yokes, and it is marked “375-THM” where the tail housing bolts on. Check plate counts before ordering a friction module. A heavier-duty version, the TH475 (later 3L80-HD), went into larger trucks.
Common wear points
- Case bushing walk-out. The rear case bushing can work its way out of its bore. That leaves the output shaft unsupported and can ruin the governor gear. Sonnax makes a bushing designed to stay put: Sonnax no-walk-out case bushing (34006-SP) and the HD case bushing kit (34006-SPB).
- Early, soft shifts. Check the modulator and its vacuum line first; a cracked or softened rubber hose lets the modulator see low vacuum. Modulator, red stripe (1450) · modulator hose (44162).
- No kickdown. Check the throttle switch, the wiring, the case connector and the detent solenoid on the valve body. Detent kickdown solenoid (1414A).
- Worn pump bushing. Sonnax notes that a worn front pump bushing lets the converter hub and inner pump gear move off center, which damages the pump gears and housing. Sonnax HD pump bushing (34034T-01) · pump body bushing (1388B).
TH400 parts we stock
Rebuild kits
- TH400 / 3L80 gasket and seal overhaul kit (1291A)
- Build your own TH400 HP master rebuild kit (SMTH400)
- Build your own basic TH400 rebuild kit (BKTH400)
- Paper gasket and rubber sealing kit (1291PRC)
- Master bushing kit, 1965–1990 (1389)
- Thrust washer kit with selectives (1409)
- Torrington bearing kit, all years (1014B)
Clutches, bands and sprags
- Forward / direct GPZ friction (GPZ110)
- Forward / direct Red Eagle friction (AL031740)
- Forward clutch steel plate (511405)
- Rear (reverse) band (1023A) · front (intermediate) band (1023)
- TransGo upgraded intermediate clutch snap ring (400-SR-1)
- Low roller clutch (A34654) · intermediate sprag, 34 element (A34652AX)
- Direct clutch drum with 34-element sprag (1371B)
Pump, filter and pan
- Pump repair kit, 1964–1990 (PK400-kit)
- Pump body with gears (A34510) · pump gear set (A44530AC)
- Oil filter, felt (1290A) · high-flow screen filter (1290A-HP)
- Oil pan gasket, Farpak (1439AF) · cork / rubber (1439A)
Shift kits, governor and converter
- TransGo SHIFT KIT (SK400) · TransGo reprogramming kit (400-1&2)
- Superior valve body shift kit (STP-K400)
- Rebuilt governor (R34680) · governor gear repair kit (A34684)
- TH400 / 4L80E non-lock-up torque converter (GM6)
Browse by part type: rebuild kits · frictions · steels · bands · bushings · pumps · hard parts · valve body and shift kits · torque converters.
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