Powertrain Controls - ECM/PCM

Diagnostic Trouble Code (DTC) 28:

Transmission Range Switch Circuit:

CIRCUIT DESCRIPTION
The Transmission Range (TR) pressure switch assembly consists of five pressure switches (2 normally closed and 3 normally open), and a Transmission Fluid Temperature (TFT) sensor combined into one unit and mounted on the valve body. The Powertrain Control Module (PCM) supplies battery voltage to each range signal. By grounding one or more of these circuits through various combinations of the pressure switches, the PCM detects which manual valve position has been selected by the vehicle operator. With ignition "ON" and engine "OFF," P/N will be indicated.

DIAGNOSTIC TROUBLE CODE (DTC) 28 WILL SET WHEN
Range signals "A" and "C" are both 0 volts ("ON") for two seconds.

ACTION TAKEN (PCM DEFAULT)
Drive Range 4 for shift pattern control, harsh shifts. The transmission Torque Converter Clutch (TCC) will be inhibited, and if in hot mode, no fourth gear will occur. DTC 28 will be stored in the PCM memory but will not turn "ON" the Malfunction Indicator Lamp (MIL).

DTC 28 WILL CLEAR WHEN
The fault condition(s) no longer exist, and the ignition switch cycled "OFF" then "ON."

DIAGNOSTIC CHART TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. This test checks the indicated range signal to the valve position signal actually selected.
2. This test checks for correct voltage from the PCM to the transmission external connector.
3. This test checks a short to ground in any one of the three valve position circuits.

DIAGNOSTIC AIDS
DTC 28 will set if the PCM detects one of two "illegal" combinations.

Correct & Incorrect Combinations Of Signals:

Refer to chart in accompanying image for various range switch combinations.

Refer to Transmission and Drivetrain for Transmission Range Pressure Switch Assembly Resistance Check or Functional Test Procedure. Automatic Transmission/Transaxle