GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2001 🢒 Regal 🢒 DTC 🢒 DTC P1635

Circuit Description

The PCM uses the 5-volt reference 1 circuit as a sensor feed to the following sensors:

    • The throttle position (TP) sensor
    • The manifold absolute pressure (MAP) sensor
    • The EGR valve pintle position sensor
    • The fuel tank pressure (FTP) sensor

The PCM monitors the voltage on the 5-volt reference 1 circuit. If the voltage is out of tolerance, the PCM will set DTC P1635.

Conditions for Running the DTC

The engine is running.

Conditions for Setting the DTC
    • The PCM detects a voltage out of tolerance condition on the 5-volt reference 1 circuit.
    • The above condition is present for longer than 10 seconds.

Action Taken When the DTC Sets
    • The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.
    • The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data.

Conditions for Clearing the MIL/DTC
    • The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed.
    • The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.
    • The DTC can be cleared by using a scan tool.

Test Description

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

  1. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.

DTC P1635 - 5 Volt Reference 1 Circuit

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

1

Did you perform the Diagnostic System Check-Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2
  1. Observe the Freeze Frame/Failure Records data for this DTC.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or as close to the Freeze Frame/Failure Records data that you observed.

Does the DTC fail this ignition?

--

Go to Step 3

Go to Intermittent Conditions

3
  1. Visually and physically inspect the PCM and engine grounds. Ensure that the grounds are clean and secure.
  2. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 4

4
  1. Turn OFF the ignition.
  2. Disconnect the EGR valve electrical connector.
  3. Turn ON the ignition, with the engine OFF.
  4. Using a DMM, measure voltage between the 5-volt reference circuit and engine ground.

Does the voltage measure near the specified value?

5 V

Go to Step 6

Go to Step 5

5

Does the voltage measure more than the specified value?

5 V

Go to Step 9

Go to Step 7

6
  1. Reconnect EGR valve.
  2. Disconnect throttle position sensor.
  3. Using a DMM, measure voltage between the 5-volt reference 1 circuit and engine ground.

Does the voltage measure near the specified value indicated?

5 V

Go to Intermittent Conditions

Go to Step 10

7
  1. Monitor DMM while disconnecting all other devices connected to 5-volt reference circuit one at a time.
  2. If voltage changes when one of the above components are disconnected, replace the component. Refer to appropriate replacement procedure.

Was a component replaced?

--

Go to Step 13

Go to Step 8

8
  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Check the 5-volt reference circuit for a short to ground or any sensor low reference circuit.
  4. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 12

9
  1. Turn OFF the ignition.
  2. Disconnect the PCM.
  3. Turn ON the ignition, with the engine OFF.
  4. Check the following circuits for a short to voltage:
    5. • The 5-volt reference circuit
    • The manifold absolute pressure (MAP) sensor signal circuit
    • The fuel tank pressure (FTP) sensor signal circuit
  5. If a problem is found, repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 12

10

Check for a short between the EGR control high circuit and the 5-volt reference circuit and repair as necessary. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 13

Go to Step 11

11

Replace the EGR valve. Refer to Exhaust Gas Recirculation Valve Replacement .

Did you complete the replacement?

--

Go to Step 13

--

12

Replace the PCM. Refer to Powertrain Control Module Replacement .

Did you complete the replacement?

--

Go to Step 13

--

13
  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text.

Does the DTC run and pass?

--

Go to Step 14

Go to Step 2

14

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK