Popping Heard Underneath I/P When Switching from Defrost to Heat (Replace Mode Valve

This bulletin is being revised to update the text. Please discard Corporate Bulletin Number 00-01-38-003 (Section 01- HVAC).

Some customers may comment that a popping sound is heard from underneath the instrument panel when switching from defrost to heat or from A/C to defrost.

The mode valve actuator may have a bind in the linkage.

1. Remove the left (driver's) side instrument panel sound insulator panel.
2. Remove the left (driver's) side knee bolster.
3. Disconnect the two vacuum lines from the mode valve actuator.

The mode valve actuator is behind the instrument panel just to the left of the radio and HVAC control. It has a blue vacuum line attached to the left side and a brown vacuum line on the back side.

4. Disconnect the mode valve actuator from it's attachment to the HVAC module case.

Notice: The mode door arm is plastic. Care must be taken to avoid damaging the mode door arm.

5. Disconnect the mode door actuator from the mode door arm.
6. Install the new mode valve actuator, P/N 52487750 (this is replacing P/N 52487554), to the mode valve actuator arm and snap into place on the HVAC module case.
7. Connect the vacuum lines to the actuator.
8. Install the driver's knee bolster.

Tighten
Tighten the screws to 2 N·m (18 lb in).

9. Install the left side instrument panel sound insulator panel.

Parts are currently available from GMSPO.

For vehicles repaired under warranty, use: