We have a couple ways to go at this point.
First, (and is where I would start) would be to check for transmission diagnostic trouble codes. A basic scan tool in most cases wouldn't be able to read these codes, so you may need to have it done. By doing this, it should identify if there is an issue with a shift solenoid.
Second, try to flush the ATF system including the transmission and cooler. Something there could be causing the pressure to drop.
Third, check transmission line pressure. Here are the directions for doing this. The attached pic correlates with the directions.
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1999 Chevy Truck K 1500 Truck 4WD V8-5.3L VIN T
Line Pressure Check
Vehicle Transmission and Drivetrain Automatic Transmission/Transaxle Testing and Inspection Component Tests and General Diagnostics 4L60-E/4L65-E Automatic Transmission Line Pressure Check
LINE PRESSURE CHECK
pic 1
Line pressures are calibrated for two sets of gear ranges-Drive-Park-Neutral and Reverse. This allows the transmission line pressure to be appropriate for different pressure needs in different gear ranges:
Before performing a line pressure check, verify that the Pressure Control (PC) solenoid for the transmission is receiving the correct electrical signal from the PCM:
1. Install a Scan Tool.
Caution: Keep the brakes applied at all times in order to prevent unexpected vehicle motion. Personal injury may result if the vehicle moves unexpectedly.
Notice: The transmission may experience harsh, soft or mushy shifts for up to two days later.
2. Start the engine and set the parking brake.
3. Check for diagnostic trouble codes, including a stored diagnostic code for the PC solenoid.
4. Repair the vehicle if necessary. Include the following areas:
Inspect the fluid level
Inspect the manual linkage at the transmission
Install or connect the Scan Tool.
Install or connect the oil pressure gauge at the line pressure tap.
5. Put the gear selector in Park and set the parking brake.
6. Start the engine and allow the engine to warm up at idle.
7. Access the PCS Control test on the Scan Tool.
8. Increase DESIRED PCS in 0.1 amp increments. Read the corresponding line pressure on the pressure gauge. Allow the pressure to stabilize for five seconds after each current charge.
9. Compare your data to the Drive- Park-Neutral line pressure table below.
If your pressure readings differ greatly from the line pressure table, refer to the Diagnosis Tables.
The Scan Tool is only able to control the PC solenoid in Park and Neutral with the vehicle stopped. This protects the clutches from extremely high or low pressures in Drive or Reverse ranges.
Refer to the Line Pressure table. The pressures in the table assume an engine speed of 1500 RPM and a temperature of 66°C (150°F). The pressure will vary with a change in temperature.
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Let me know your thoughts so I can better help.
Take care,
Joe
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Thursday, May 6th, 2021 AT 10:33 AM
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