IV. Code 39 Test Conditions

Once the ECM has given an enable command to apply the TCC, the ECM will continually check vehicle speed and engine RPM to determine if the TCC is operating properly. To do so, all enable conditions must be met and vehicle speed must be between 50 and 56 MPH at a steady throttle.

When these conditions are met, the ECM will cheek the engine RPM. If the RPM is greater than the value shown in the table below for a period of seven seconds or more, a Code 39 will set and turn on the "Service Soon" light (for 1982 vehicles, "Check Engine" light will tum on and Cruise Control will disengage) because a greater RPM indicates TCC apply has not occurred.

Standard Altitude
Axle Axle

E/K 1672 1784
C 1768 1928

If the conditions are not met, the BCM will not test for Code 39 even though an inoperative TCC condition may exist. For example, if the vehicle is being driven above 56 MPH, Code 39 will not be set even if the TCC is not able to engage. This can also make a Code 39 appear to be "intermittent" and difficult to reset, even if TCC is completely inoperative. Therefore, when testing the vehicle, it must be driven under the specific test conditions to reset a code. In addition, the BCM will not test for a Code 39 or any other code while in diagnostics.