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Got a P1870 in your '96?
(Updated 10/1/2019 by Sherlock9c1 because the old "stake or block the valve" method would balloon the converter and wear the crankshaft thrust bearing.)
Why? In 1995, GM added a second control solenoid to the 4L60E Torque Converter Clutch (TCC) to allow them to better control the way the torque converter clutch lockup felt. The original on-off solenoid still enables lockup, and the new solenoid (the "apply" solenoid) controls a slide valve in the valvebody via "pulse-width modulation" (PWM)- what that means is that the solenoid is turned on and off rapidly but the time "on" and the time "off" are varied like a fuel injector (like "duty cycle") to control the amount of fluid released by the solenoid, and this controls lockup. Smooth shifts are easier on the drivetrain with little loss in performance.
That said, the valvebody valve acted upon by the PWM solenoid tended to wear the bore and cause problems. GM techs figured out that if the valve was staked in the full-open position, the code would go away. The downside of that fix was that now the torque converter becomes pressurized with transmission line pressure, which can go as high as 250psi at times. In a properly functioning 4L60E, lockup pressure should really be around 100psi or so. Given the size of the torque converter, this much higher pressure can physically distort the case of the converter and cause other problems.
Sonnax developed a repair kit for this, which includes improved valves and a special sleeve which not only eliminates future bore wear, but improves the valve structure themselves. It is P/N 77754 (pay attention as there are suffixes for the different aspects of this fix)).
One thing to note is that you will have to remove the valvebody and also purchase a reamer tool which is not cheap (~$85). You can always sell this tool on ebay once your done, or charge four friends $20 each and fix theirs while you're at it, then you're back to zero.
(Updated 10/1/2019 by Sherlock9c1 because the old "stake or block the valve" method would balloon the converter and wear the crankshaft thrust bearing.)
Why? In 1995, GM added a second control solenoid to the 4L60E Torque Converter Clutch (TCC) to allow them to better control the way the torque converter clutch lockup felt. The original on-off solenoid still enables lockup, and the new solenoid (the "apply" solenoid) controls a slide valve in the valvebody via "pulse-width modulation" (PWM)- what that means is that the solenoid is turned on and off rapidly but the time "on" and the time "off" are varied like a fuel injector (like "duty cycle") to control the amount of fluid released by the solenoid, and this controls lockup. Smooth shifts are easier on the drivetrain with little loss in performance.
That said, the valvebody valve acted upon by the PWM solenoid tended to wear the bore and cause problems. GM techs figured out that if the valve was staked in the full-open position, the code would go away. The downside of that fix was that now the torque converter becomes pressurized with transmission line pressure, which can go as high as 250psi at times. In a properly functioning 4L60E, lockup pressure should really be around 100psi or so. Given the size of the torque converter, this much higher pressure can physically distort the case of the converter and cause other problems.
Sonnax developed a repair kit for this, which includes improved valves and a special sleeve which not only eliminates future bore wear, but improves the valve structure themselves. It is P/N 77754 (pay attention as there are suffixes for the different aspects of this fix)).
One thing to note is that you will have to remove the valvebody and also purchase a reamer tool which is not cheap (~$85). You can always sell this tool on ebay once your done, or charge four friends $20 each and fix theirs while you're at it, then you're back to zero.
