A/T - 4L80E High Line Pressure/Broken Drum Cracked Case
TECHNICAL BULLETIN # 264TRANSMISSION: 4L80-E
SUBJECT: High line pressure
APPLICATION: GM Trucks, Isuzu, Jaguar
4L80-E
High Line Pressure
Broken Direct Clutch Drum
Broken Case
A broken direct clutch drum and/or case is usually the result of high line pressure (enough to "peg" a 400 psi gauge). This is usually caused by reverse boost oil that is introduced into the torque signal circuit. If this pressure can not escape through the torque signal circuit (via the separator plate feed hole and force motor) the result is uncontrollable line rise. There are several measures you can take to prevent or reduce this effect.
Note
If pressure is high only in reverse, follow steps one through four (these can be done in the vehicle). If pressure is high in all ranges and/or the unit has failed (due to high pressure), remove the unit and follow steps one through five.
[1] Replace the reverse boost valve.
The most common cross-leak point is the reverse/torque signal boost valve. When the boost valve bushing wears out, reverse boost oil is allowed to enter the torque signal passage. If you have an existing high line pressure problem replace the boost valve and bushing. The valve and bushing are sold separately. The part number for the valve is 8680549. The part number for the bushing is 8682856 (figure one). If your boost valve does not look like the one shown in figure one, you have a first design pressure regulator/boost valve assembly. If so, purchase the entire assembly. The part number is 8682998.
Note
The bushing will have to be purchased separately. Do not reuse the old bushing if the new PR assembly is used.
[2] Check the separator plate and valve body gaskets.
There are currently two designs of separator plates and gaskets. This change occurred when a wall was added to the case (figure two). If you would like more information concerning these parts refer to ATRA bulletin number 209.
Note
A separator plate/gasket/case mismatch can cause high line pressure in reverse. However once step three is performed, mismatches will not be a concern.
[3] Drill a 1/8"-3/16" hole through the case wall.
This will offer a larger exhaust for reverse boost oil that has cross-leaked into the torque signal circuit (figure three).
Note
First design case does not have this wall.
[4] Replace the force motor.
Use the second design force motor (figure four). There are two advantages with the second design force motor.
1. It offers a larger exhaust for torque signal oil.
2. Balance oil must pass through a screen on the first design force motor. If the screen becomes clogged, the internal valve will not regulate. The second design force motor does not have this feature.
Note
The second design force motor will work on all models. The first design force motor should not be used on vehicles designed for the second design. For this reason, always use the second design.
[5] Modify/check the stator support.
Enlarge the pressure regulator balance feed to .070"-.075" line (figure five). This is a good measure to ensure that there is adequate back-pressure on the pressure regulator valve. Also check for a cracked PR plug (figure six).
Make sure the air bleed (if equipped) has not been tampered with or is missing (figure seven). The orifice size is .020". If the orifice is missing or has been tampered with, plug the hole or replace the support.
Note
These modifications may offer a slight buzz during start-up when cold. This will not affect the transmission's operation.