There is a service bulletin that relates to your concern. It specifies replacement of the switch. The switches are faced up, and often times debris in the fluid will settle on the switch itself. This can result in invalid signals, and they can be intermittent. Here is the bulletin;
#99-07-30-013: Tech - Service Engine Soon (SES) Light Illuminates (Repl Switch Assy) - (Jul 27, 1999)
Subject: Service Engine Soon (SES) Light Illuminates, DTC P1810 Set (Replace TFP Switch Assembly)
Models: 1999 Cadillac Escalade
1998-99 Chevrolet Camaro, Corvette
1998-99 Pontiac Firebird
1998-99 Chevrolet and GMC C/K, M/L, S/T, G Models
1998-99 Oldsmobile Bravada
1998-99 Holden Commodore
with Hydra-Matic 4L60-E Transmission (RPO M30)
Some customers may comment about the Service Engine Soon light illuminating. The PCM will have stored a Diagnostic Trouble Code P1810, Transmission Fluid Pressure Manual Valve Position (TFP) Switch assembly circuit malfunction.
Debris from aluminum shavings, bronze material, and certain clutch material may cause the pressure switches in the TFP switch assembly to stick or electrically short out.
A new TFP switch assembly with a plastic shield has been released. The plastic shield is designed to protect the pressure switch from debris. This improvement will reduce the likelihood of a DTC P1810. This product change went into production on April 8, 1999 (Julian date 098). The existing TFP switch assembly remains relatively unchanged except for the addition of the new shield, creating a new part number for the assembly. The shield fits over the existing TFP switch assembly and is not sold separately.
Follow diagnostics for the P1810 Diagnostic Trouble Code in the Transmission Section of the appropriate Service Manual. If the diagnostics indicate that switch replacement is necessary, use the correct switch P/N as listed below.
Service Parts Information
TFP Switch Assembly with Shield
Saturday, April 10th, 2010 AT 9:47 AM