GM Turbo 350 Transmissions. Cliff Ruggles

seems that these days, because of the complexity of modern electronically controlled 4-, 5-, and 6-speed units, overhaul and repair has been limited to the dealership level. Few if any automotive repair shops will touch one, and in almost all cases a complete unit is ordered from a transmission rebuild shop or commercial remanufacturing facility. In the grand scheme of things, it saves time and money, but it doesn’t increase or improve the skills of the automotive mechanic. As with many other items, including smaller things, such as starters, alternators, master cylinders, brake cylinders, or calipers, the trend is simply to order a complete unit rather than take the time to completely and correctly rebuild the original factory components.

      Because many hobbyists are not always concerned with time and efficiency when making their automotive projects operational, they have the option to perform many of the repairs themselves. As far as the automatic transmission is concerned, these efforts save great expense. It is simply one of the areas of this hobby where you can get a better overall end result, learn a great deal about a somewhat complex automotive part, and have the satisfaction of doing it yourself. Knowing and understanding the design and function of the internal components, as well as the basic fundamentals of how an automatic transmission works helps a great deal.

      The automatic transmissions referenced in this book are all GM units that spanned more than 40 years of use in a wide variety of applications. Many similarities exist between some models, and some are simply based on the originals, or are highly improved versions. The TH350 doesn’t actually share any parts with the TH400, but the fundamentals for rebuilding it are the same, as is the basic procedure. The TH350 is slightly smaller and has lighter internal components, but uses the same basic principles to provide three forward speeds and one reverse.

Torque Converter

      All of the units covered in these pages use a torque converter that drives an oil pump located in the front housing, which is bolted to the case. The torque converter (which I cover in greater detail later in this chapter) is bolted directly to the engine’s crankshaft, and transfers all of the energy to the rear wheels. It routes these forces through the transmission’s internal components, and allows for enough slippage so that the vehicle can remain stopped at idle speed, and is efficient enough to provide very close to complete energy transfer at various vehicle speeds. Torque converters equipped with an internal clutch can actually be employed once the vehicle is in motion. They provide a complete or solid lock between the engine’s crankshaft and transmission’s input shaft, just like the clutch does in a manual transmission.

      The torque converter serves double duty. At the same time that it turns the transmission’s input shaft, it also drives the transmission’s oil pump. This provides oil flow and pressure at all times when the engine is running. Some oil is diverted to the transmission’s oil cooler, some is used to keep the torque converter full, and the rest is used to supply pressurized oil to various components in the transmission for lubrication and basic transmission function.

       The torque converter is bolted directly to the engine’s crankshaft and spins at engine speed. The hub on the converter has two notches that engage with the transmission’s oil pump to provide constant oil flow and pressure. The oil pump is the heart of the transmission; it supplies oil for cooling, lubrication, and to operate the various internal components for transmission function.

      All automatic transmissions contain clutch drums, planetary gears, sprags, or roller clutches, servos, accumulators, bushings, thrust washers, governors, and many have modulators. The basic arrangement of these components varies between models, but the end result is the same. The parts are designed to efficiently and effectively work together to use engine power to propel the vehicle over a wide variety of operating conditions.

Transmission Oil Pump

      The transmission oil pump is driven directly from the rear snout of the torque converter, which is bolted directly to the engine’s crankshaft, so the oil pump operates at engine speed. The oil pump uses a pressure relief valve to keep oil pressure within a specific range for best transmission operation. Oil from the pump is routed through the vehicle’s transmission oil cooler because heat is created in the transmission as well as inside the torque converter. Oil is also routed to various transmission internal components through the valve body at various sequences for gear selection; this helps keep the engine in its most efficient speed/load range over a broad variety of driving conditions.

       A pressure regulator valve is used to control transmission oil pump output pressure. The oil pump pictured here is from a TH400; the pressure regulator valve is located on the backside of the oil pump. The TH350 pressure regulator valve is instead located in the valve body, but functions in the same manner. Regardless of the location of the pressure relief valve, the oil leaving the oil pump must overcome spring pressure to maintain flow to the rest of the unit.

       The 700-R4 and 4L60 transmissions use a vane-type pump that is also a positive displacement pump. Hardened rings keep the vanes in contact with the outer portion of the pump. The clearance between the pump rotor and vanes is very close within the pump halves. The internal pump gear is also driven directly by the torque converter. Oil is picked up on the suction side of the pump and discharged on the output side. A pressure regulator valve is used in the pump similar to that of the TH400 to control output pressure.

       The TH400 and TH350 transmissions use a gear-type oil pump. The inner gear is driven directly by the hub of the engine’s torque converter. Note that the inner gear is offset in the housing. This creates a chamber that moves oil and forces it out of the pump.

       Early 700-R4 transmissions used a 6-vane oil pump assembly. This design was upgraded in 1987 to a 10-vane pump, then eventually to a 13-vane pump. The additional vanes do not increase pump output, but were used to smooth the flow of oil leaving the pump. The 10- and 13-vane pump assemblies can be installed into earlier units that originally came with a 6-vane pump. The later units also used greatly improved pump rings, as well as several improvements to the basic design of the pump itself, that increased durability and make it more reliable in long-term service.