4L80 Torque Converter Clutch Drag?
Let's talk about torque converter clutch drag when using a multi disc torque converter in a 4L80E transmission. We see this issue come up from time to time, especially (but not limited to) in 4L80E applications equipped with a transbrake valve body.
Frequently the first thing to get blamed is the torque converter, and while it is possible for a multi disc converter to get set up with too tight of internal TCC clearance, the reality is that the TCC drag issue most commonly goes back to the transmission side of things.
The simple fix for the majority of combinations that experience TCC drag is to install Sonnax part number 34200-14K. This is a specially designed lube regulated pressure regulator valve that can easily be installed into the transmission pump once the transmission pan has been removed. This makes for a fairly simple DIY install with the transmission still in the vehicle.
This valve addresses a number of issues in 4L80E transmissions, & one of the key benefits is a boost in converter TCC release pressure. What this means is that the converter will receive higher pressure TCC release oil to help hold the TCC off. Additional TCC release pressure helps insure that the TCC does not come into contact with the clutch apply surface inside the front cover at idle or low engine speed all of which helps to eliminate potential TCC drag issues.
If you perform this modification to your 4L80E & find that you're still combating a TCC drag issue, then the next diagnostic step we recommend is a line pressure test. Transmission line pressure can be tested by installing a 0 - 300 PSI mechanical pressure gauge with hose extension into the 1/4-inch NPT port located in the transmission case just above & to the rear of the transmission case.
Document your pressure findings, then shut the vehicle off. Disconnect the electrical harness from the case connector at the driver side rear of the case. Start the engine & perform your line pressure test again. If the line pressure doesn't change between the two modes of testing then chances are the EPC solenoid has failed & needs to be replaced. Excess line pressure can contribute to TCC drag issues as well, so addressing potential EPC issues is a crucial diagnostic step in combating this problem.
If you're running a transbrake valve body that either features no EPC solenoid or a modified constant pressure hydraulic circuit with a dummy EPC solenoid in place then this information regarding line pressure testing will not apply.
Aftermarket billet aluminum full manual transbrake valve bodies for the 4L80E will typically require the use of either a non-lock-up torque converter or a single disc lock-up torque converter only. The balance oil in the torque converter is changed in such a way when these valve bodies are installed that a multi disc torque converter will nearly always encounter a TCC drag issue when used with one of these valve bodies.
That being said, these billet valve bodies are an outstanding choice for any performance-oriented 4L80E that address & fix many other performance & functionality issues in the 4L80E. As such, the benefits of running such a valve body in a 4L80E in conjunction with a properly designed NLU or single disc LU torque converter to match your specific combination is certainly worth consideration; particularly in maximum effort applications.
A multi disc converter is really only necessary in applications seeking to lock the torque converter at wide open throttle whether that be for dyno testing purposes, roll racing, land speed racing, etc., or in applications that are seeking to tow heavy loads with the torque converter locked.
Otherwise, a single disc torque converter will serve the needs of normal cruising lock-up use perfectly regardless of power level. With modern torque converter technology non-lock-up options are also perfectly viable; particularly in maximum effort applications that can benefit from a spragless torque converter.