The 3-4 clutch failure is a common problem in the 700r4 and 4l60e transmissions. The 3rd accumulator check valve is
the first area I check for leaks if I have 3-4 failure. This is the check valve you see when you remove the 2-4 servo.
After I have checked it to determine if it was the cause of my clutch failure I replace it whether it was leaking or not.
This check valve is difficult to remove. I have found if I use a 3/8" NC starter tap and thread the capsule I can use a
small dent puller (3/8" NC threads), thread it into the valve and a couple of whacks with the slide hammer will usually
do the trick. To install the check valve use a 3/8" diameter steel rod and install the valve assembly 1 5/8" below the
worm track surface. The oil feed slots in the capsule must line up with the oil passage in the servo bore. I suggest
the first time you attempt this be very careful not to damage the case and purchase an extra check valve just in case
you make a mistake. The sonnax servo release check valve is an excellent part to use with your newly installed 3rd
accumulator check valve. The sonnax valve will allow you to increase the 3-4 apply oil and maintain shift timing. This
valve comes with complete instructions and is a must for high performance applications.
The next area to check is the 3-4 exhaust check valve is located in the input drum. It is the valve you see next to
the input shaft if the drum is on the table with the shaft pointed upward and you are looking at the top of the drum.
If this valve leaks the apply oil will exhaust causing the 3-4 clutches to slip. If this valve is slow to release it
will cause the clutches to drag on the 3-2 downshift. In either case you will experience 3-4 clutch failure. I
recommend the superior rapid release check valve. It is spring loaded and will exhaust fluid quickly. Also because
it is spring loaded it will help prevent centrifugal apply of 3-4 frictions during high rpm operation.
The 1-2 accumulators should always be checked for wear. The factory accumulator piston normally wears in the pin hole
area of the piston. The hole in the piston will become egg shaped causing fluid to leak past the pin. Another problem
that occurs when this happens is the piston cocks and scores the piston bore in the case. If you caught the problems
before the case becomes damaged I strongly recommend using the sonnax painless accumulator piston. The wide piston
design prevents cocking in the bore and the pinless design eliminates the pin hole. If the case is damaged i would use
the sonnax piston and sleeve kit.
The 3-4 clutch piston is another part you should not take lightly. The early aluminum style pistons require lip seals
to be installed. The late style steel piston has the seals molded to the piston. The late style piston gives a better
seal. A better seal will prevent a hydraulic leak. This steel piston can be retro fitted to all 700r4 and 4l60e
transmissions without any other changes. If you are doing a stock overhaul I recommend replacing all three pistons
in the input drum with the new steel type. If you do you will also need the new style overrun piston spring? If you are
using a reinforced input drum you will have to use the early style aluminum overrun and forward pistons.
We are now ready to choose a clutch setup. Over the years GM has changed this setup several times and has settled on a
seven friction six steel stackup. Numerous aftermarket companies have come out with a variety of high performance
combinations. The trend is extra frictions and steels. The problem is to increase the number of frictions you must
make the steels thinner. Thinner steels cannot dissipate heat and eventually fail. I like using a seven friction
setup similar to the factory setup. The difference is I use the .106" Kolene steels as compared to the factory .097"
steels. I prefer the Alto Red Eagle frictions which are approximately the same thickness as the factory frictions.
Using this setup I can use the heavy duty factory apply plate and a factory backing plate? The .144" backing plate
(marked "hd") is normally the plate you will need and sometimes you will need the .065" snap ring. I have found the
factory apply plate and backing plates are stronger than the plates supplied with the aftermarket kits. To make it
all fit you must use the 3-4 apply ring with the shorter legs marked "7" and the input ring gear with the 1 5/8" clutch
splines. Use the 5 factory load release springs unless you are using the Transgo high rev spring kit , in which case
you would leave the load release springs out.
I sell all the parts discussed in this guide in my online store. I offer information and tech tips on all parts I sell.
I carry a full line of parts for the 700r4 and 4l60e transmissions at very competitive prices.
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