Performance 700R4
Automatic Transmissions

Built for the 6.2L/6.5L Diesel
- Part I -


By Dr. Lee Swanger
TDP Member #0006

Strike up a conversation about automatic transmissions for our GMC and Chevy diesel trucks, particularly the '82 - '89 6.2L trucks, and you will hear one of two opinions: "That 700R4 is a piece of garbage. It overheats and can't survive in a diesel truck." or "The 700R4 is the ideal transmission for a truck - having a deep first gear ratio for more available torque when getting things rolling, and a generous overdrive and lock-up torque converter for improved fuel economy."

These two opinions can't both be right, or can they? My trusty '84 C-10 was still running its original 700R4 after 150,000 miles, and now was subject to the increased torque from my customized naturally aspirated 6.5L diesel engine. This hot new engine, pushing around 180 non-turbo horsepower out the flexplate, only emphasized the flare (slippage) I was experiencing on the 2-3 shift. My planned Nitrous Oxide upgrade in the future would only make the situation worse, like scattering tranny parts all over the secret "2Fast, 2Furious" dragstrip I had stumbled across just outside of Miami.

So I began researching what 700R4 upgrades were needed and what companies offered them before selecting a replacement transmission. There is a wealth of 700R4 information available on the Internet, and the links at the end of this article are the most useful ones I found. One caveat, however; the person publishing the information on any given website is probably NOT the technician who will actually be building your transmission. Further research is necessary on the reputation of any given supplier. The Diesel Page Forums are a good place to start. If you cannot locate the information you need by forum keyword search, just post your question and watch the opinions come pouring in.

This month in Part One, I will discuss the operation of the 700R4 and highlight some of the upgrades that are currently available to improve both performance and durability. Next month, in Part Two, we'll follow Fourth Gear Inc. as they build me a new high-performance upgraded 700R4 transmission.

In the Beginning, about 1982:

Historically, the new-for-1982 700R4 grew out of the venerable Turbohydramatic 350, with the addition of a fourth overdrive gear and a lock-up Torque Converter Clutch (TCC). The overdrive fourth gear, at a ratio 0.71 to 1, allows the engine in a car or truck to loaf along at 30% lower RPM when cruising on the highway, without changing the axle ratio. First gear is also deeper, with a 3.06 to 1 ratio in the 700R4. (The TH350 three speed offers a 2.52 to 1 low gear, and the TH400 low gear ratio is 2.48 to 1.) So getting a heavy load started is easier with the 700R4 due to the lower first gear.

The lockup torque converter, along with the overdrive fourth gear, was a brainchild of the oil crisis in the 1970's. By using hydraulic pressure to operate a clutch inside the torque converter, the fluid coupling could be bypassed, and the losses in churning the automatic transmission fluid inside the converter could be avoided. Another benefit was to cut down on the heat generated within the converter, helping the fluid, seals, clutches and bands to last longer.

Like many new products, the early 700R4's had some teething problems, and did indeed overheat, wear out, and leave drivers stranded more often than they should. Some of this was due to overstressed parts, but a lot of the problems were associated with the 700R4's new Throttle Valve (TV) and TV cable that replaced the vacuum modulator in the older automatic transmissions.

The 6.2L diesel engine (also introduced in 1982), with its lack of manifold vacuum, may have hastened the addition of the TV. Simply put, the TV tells the transmission how far you are pushing down on the accelerator, and raises the internal transmission pressures accordingly. If the TV cable is misadjusted, transmission pressures can be too low, and clutches will slip and burn themselves up.

The TV cable, friend or foe?

In our diesel engines, where the throttle position controls the amount of fuel injected into each cylinder, there is a very strong correlation between how far down the pedal is, and the amount of torque the engine is producing. To get to the rear axle and the tires, that torque has to be transmitted by the transmission. The transmission does that by using internal clutches and bands to join the correct gearsets together for the gear needed. The clutches and bands are applied by the pressure built up in the Automatic Transmission Fluid by the front pump inside the transmission. The TV (throttle valve) modulates or controls that pressure to suit the torque going through the transmission. If the pressure is too high, every shift will be a neck-snapper and tire chirper. That can be fun the first two or three times, but would get old real quick for running errands or commuting in traffic. So, the TV reduces the transmission fluid internal pressure at light throttle settings for more comfortable shifts.

 

 

 

The throttle valve (white arrow in picture above) sits in a bore in the valve body inside the transmission. It is pushed in by the a lever connected to the TV cable, shown by the white arrow in the following photo.

 

 

 

 

Conversely, if the transmission pressure is too low, the clutch packs will not be compressed with sufficient force to avoid slipping at high torque loads, such as towing a heavy load up a steep hill, or teaching the kid in the Honda Civic with the 5-inch exhaust tip a lesson. Slippage causes heat, which burns clutches and fluid, and grinds off debris to clog valve bodies, and all kinds of transmission failures can follow.

Unfortunately for some, GM made the TV cable easily adjustable, so owners tried to adjust it. Misadjustment, to try to soften up shifts by lowering hydraulic pressures, or tighten up shifts by raising hydraulic pressure, is probably the biggest source of problems with the 700R4. Don Marcone, 700R4 specialist and proprieter of Fourth Gear Inc, an Orlando, Florida supplier of 700R4 transmissions for street rods, Corvettes and trucks, says that lack of pressure on the clutches is the biggest problem with these transmissions.

 

The TV cable is adjusted on the engine, by releasing the slider, and then moving the accelerator to the wide-open position (with the engine off!) Setting the TV cable is straightforward, but must be done precisely as GM recommends. For an excellent explanation of the TV system and its adjustment, I recommend the article on TV 101 at the bowtieoverdrives.com website: www.tvmadeez.com/article/index.html, the source of the three pictures in this section.

 

 

Newer IS Better, in the case of the 700R4.

GM continued to upgrade the 700R4 with bigger input shafts, better clutches, improved accumulators, stronger servos, and finally the addition of an auxiliary valve body for the '88-'92 transmissions. During this time, the 700R4 transmission received a new designation - 4L60. As long as there is not an "E" on the end (as in 4L60E - the electronically controlled transmission) this is the final evolution of the 700R4 transmission concept.

A number of specialists have learned how to build a strong 700R4, and have built successful businesses on this transmission. One of the first was Jet Performance, with their Turbo700. They have been followed by BowTie Overdrives, also in Southern California. In addition, traditional transmission specialists like TCI, B&M Racing, Art Carr, and others offer upgraded 700R4 transmissions suitable for diesel truck applications. Most of the builders use only the latest 1988-1992 cores. One advantage for many applications is that the later transmissions will lock up the TCC (torque converter clutch) automatically in 4th gear with only an ignition-controlled +12VDC lead to the transmission. The TCC will unlock hydraulically below about 25 mph, or when the transmission downshifts out of 4th gear.

In addition, the aftermarket has supplied the need for parts even stronger than the latest GM parts for applications demanding very high horsepower and torque capabilities. Most builders offer a "Standard" upgraded 700R4 that will handle up to 450 ft.-lbs. of torque, and up to 450 horsepower. Heavy Duty high power upgrades can handle up to 600 ft-lbs. of torque and over 600 horsepower in applications ranging from street rods to Corvettes to our diesel trucks.

 

 

 

 

The general categories of parts that are upgraded include the clutches, with an upgraded set of nine clutch plates on the left, versus the OEM six clutch plates on the right.

 

 

 

 

 

 

 

The planetary gearsets are another component that is substantially improved in the latest versions. The improvements are largely internal in the shafts, bushings, washers, and oil flow control:

 

 

 

 

 

The Input Drum / Shaft assembly is another beneficiary of improvements. An original 1984 27-spline unit is on the left, a 1988 and up 30-spline shaft with a heavier drum is on the right, and the aftermarket TorqueDrive heat-treated and hardened unit, good for up to 600 ft.-lbs. of torque is in the middle :

 

 

 

 

 

 

 

Performance is improved with a bigger 2-4 servo (left), shown here with the 2-4 band:

 

 

 

 

A significant improvement in pressure control is achieved by replacing the early seven-vane front pump with the ten-vane version shown here. Note the uneven spacing of the vanes; this cuts down on noise and vibration inside the transmission:

In addition, experienced 700R4 builders will fine-tune each transmission with different springs, checkballs, accumulator pistons, and valve-body modifications.

GM must have made more than 40 electrical variations of the TCC (torque converter clutch) lockup control. Some are tied into the TPS, some respond to engine control module conditions, some lockup in 4th gear only, some in 4th and 3rd, and some even will lock up in 2nd gear under certain conditions. A good builder will be able to match a custom upgraded transmission to your truck or to your needs. The late model (1988-1992) transmissions will lockup in 4th gear with only a single +12VDC source of current, switched by the ignition switch.


If you select this simple operation, it is recommended by some specialists including Don Marcone at Fourth Gear Inc. that you put the hot lead on a toggle switch, so you can drive around town, empty without always locking up in 4th gear. But when you head out on the highway for some high-speed cruising, or if you will be towing, remember to flip the switch to lock up the TCC to prevent generating excessive temperatures.

Since I had removed my TPS to make way for a Nitrous Oxide microswitch, and disconnected the ECM (engine control module) to grab the engine speed sensor signal to drive my VDO tachometer, I had already modified my 1984 truck to select 3rd gear lockup with a rocker switch on the dashboard. This is to allow towing heavy loads in 3rd gear with the torque converter clutch locked, or for dynamometer runs in 3rd gear (direct drive, no overdrive) with the converter clutch locked. Following Don's advice, I added an additional rocker switch to disable the converter lockup for pleasant around-town driving including better accelerator response in 4th gear.

In PART II next month, we will visit 700R4 specialist Don Marcone of Fourth Gear Inc., near Orlando, and find out just what he does to assemble a custom transmission for my truck, then find out how it performs. Except for the pictures of the throttle valve components, all the photos in this article were taken at Fourth Gear Inc.   TDP

Lee Swanger
TDP Member # 0006


Information Sources:
Please visit the following links to learn more about the 700R4 and the companies that offer performance oriented versions. These links are not endorsements by The Diesel Page or the author. In addition, you can search TheDieselPage.com Forums to see what other members have experienced with products purchased from the listed companies.


About the Author:

Dr. Lee Swanger is a consulting engineer trained in mechanical engineering and metallurgy. Currently he is a principal engineer with a major engineering consulting firm, and a significant part of his work deals with large diesel engines in stationary power and marine propulsion.

Prior to his current position, Lee was Director of Research and Development for Clevite Bearing Company, and received his engineering degrees from Case Institute of Technology in Cleveland, Ohio and Stanford University in Palo Alto, California. Lee is a Registered Professional Engineer in Ohio, California, Virginia and Florida.

The opinions that Dr. Swanger expresses here are his alone, and may not be attributed to his past or present employers, or to The Diesel Page.


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