Powertrain Controls - ECM/PCM

Diagnostic Trouble Code (DTC) 91:

Skip Shift Lamp Circuit:

CIRCUIT DESCRIPTION
The skip shift lamp display is in the instrument panel. When the skip shift lamp is illuminated, the Powertrain Control Module (PCM) is energizing the skip shift solenoid in the transmission and the gear shift can only be moved from 1st to 4th gear. This increases fuel economy. The lamp is controlled by the PCM and is turned "ON" by grounding CKT 375. The skip shift lamp will be enabled if all of the following conditions are met.

- Engine running.
- Vehicle speed greater than 15 mph but less than 21 mph.
- Throttle position less than 35%.
- Engine coolant temperature greater than 77°C (171°F).
- Barometric Pressure (BARO) greater than 76 kPa.

The PCM uses the measured RPM and the vehicle speed to calculate what gear the vehicle is in. It is this calculation that determines when the skip shift lamp should be turned "ON."

The skip shift (if all above criteria are met) will occur only once per upshift cycle. The skip shift system is not re-enabled unless vehicle speed drops below 1 mph.

DTC 91 WILL SET WHEN
The PCM detects the wrong voltage potential on CKT 375 for 26 seconds.

ACTION TAKEN (PCM WILL DEFAULT TO)
DTC 91 will be stored in the PCM memory, but will not illuminate the Malfunction Indicator Lamp (MIL).

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.

1. This should not turn "ON" the skip shift lamp. If the lamp is "ON," there is a short to ground in CKT 375 wiring or a fault in the PCM.
2. The PCM should ground CKT 375 and the skip shift lamp should come "ON."
3. This checks the skip shift lamp circuit up to the PCM connector. If the skip shift lamp illuminates, then the PCM connector is faulty or the PCM does not have the ability to ground the circuit.
4. For electrical diagnosis of the instrument panel or skip shift lamp bulb replacement refer to Instrument Panel.

DIAGNOSTIC AIDS
Inspect the harness and all related connectors including the PCM connections. These may cause an intermittent. Check for damaged or pushed out terminals.