Test Description

The numbers below refer to the step numbers on the diagnostic table.

1. 2. The 5-volt reference circuits for the sensors are connected together inside the PCM. Both DTCs P1635 and P1639 set at the same time indicates that a 5-volt reference circuit is shorted to a voltage.
2. 3. The 5-volt reference circuits for the sensors are connected together inside the PCM. When only DTC P1635 is set, this indicates a short to ground on one of the 5-volt reference circuits.
3. 5. This step isolates the 5-volt reference circuit from the sensor signal circuit.
4. 6. This step determines if the 5-volt reference circuits or the fuel level sensor signal circuit are only shorted when the fuel pump is enabled. You may have to enable the fuel pump a couple of times to see if the voltages increase. Inspect the fuel pump harness thoroughly for being shorted to the fuel tank pressure sensor or the fuel level sensor.
5. 8. The 5-volt reference circuits may be shorted to another PCM circuit. The shorted circuit may not be apparent when the PCM harness connector is disconnected. Testing continuity from each 5-volt reference circuit isolates the shorted circuit.

Fig 1: DTC P1635 (1 Of 3)

Courtesy of GENERAL MOTORS CORP.

Fig 2: DTC P1635 (2 Of 3)

Courtesy of GENERAL MOTORS CORP.

Fig 3: DTC P1635 (3 Of 3)

Courtesy of GENERAL MOTORS CORP.