The1994 & later 4×4 with 4L80E transmission and 1993 and later 4×4 with 4L60E transmission – On these models the DRAC is external, and is found behind the glove box, which must be removed for access to the DRAC, as shown in Figure 7. These utilize a 4WD Low Switch, located in the transfer case to signal the computer that 4W LOW has been selected. It does this by supplying 12V to the PCM when in 2WD or 4W HI modes. When 4W LOW is selected the 4W LOW switch grounds it’s 12V signal, changing it to ZERO VOLTS. The computer, recognizing this voltage difference, then changes its strategy which adjusts for correct shift timing and speedometer operation in order to compensate for the gear reduction that occurs when 4W Low is Selected.
Vehicles equipped with the 4L80E transmission may have a dummy speed sensor located in the output speed sensor location, there is no wiring for this speed sensor as it is there only to plug the hole!
Later models will have a plug in this location as the TOSS function on 1994 and later vehicles with the 4L80E transmission was eliminated. The VSS in the transfer case along with the 4W Low Switch takes over the functions of calibrating vehicle speed signal input, shift timing and speedometer operation and accuracy in all transfer case modes as shown in Figure 15. 1993 and later vehicles equipped with the 4L60E transmission will have the VSS in the transfer case ONLY and the 4W Low Switch located there as well. Refer to Figure 16 for the diagnostic tree and internal wire schematic, for 1994 and later “K” Trucks with the 4L80-E and 1993 and later “K” Trucks with the 4L60-E.
NOTE: Even though the vehicle you are working on is a 2WD model, the vehicle harness includes the wiring for the 4 Wheel Low switch, which if grounded for what ever reason, would create a stack shifting condition. Ensure that you check the status of the 4 Wheel Low switch parameter on your scan tool, and if it says “YES”, look for a grounded 1493 circuit.