Diagnostic Aids

WARNING: This page does not describe the selected car, but rather 7 other vehicles, including the 2008 GMC Yukon XL, 2008 GMC Yukon, 2008 Chevrolet Tahoe, 2008 Chevrolet Suburban, and 2008 Cadillac Escalade EXT. However, it is still accessible from the selected car via links, so may be relevant.

Disconnecting the 16-way connector from the transmission will disable TCM and ECM communications.

The IMS is part of a 6 wire harness, which connects electrically to the control solenoid (w/body and TCM) valve assembly. Inspect the IMS, harness, connector, and control solenoid (w/body and TCM) valve assembly electrical pins for metallic debris. Use a terminal test kit for any test that requires probing the control solenoid (w/body and TCM) valve assembly harness connector or a component harness connector.

Use the Internal Mode Switch Logic table to compare scan tool reading with the transmission fluid pressure (TFP) switch. TFP switch states should correspond to the correct gear positions. Be sure to look for switch positions which are transitional, displays invalid or open. These are not normal IMS positions when shifter detents are properly engaged.

Use the scan tool IMS parameters to compare them to the ECM P/N parameter. When the transmission is in park or neutral, the ECM P/N switch parameter will read park/neutral. When the transmission is in a drive range, the P/N will read In Gear. Be alert for scan tool data parameters which frequently toggle or vary while testing. Switch positions which are transitional, display invalid or are open are not normal IMS positions when shifter detents are engaged.

Due to the properties of the stainless steel contact strips inside the switch, it is difficult to obtain meaningful resistance readings. For this reason, measuring the switch resistance may not help to conclude a faulty switch. However, the absence of all resistance where there should be would indicate a circuit failure.