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For 1990-2009 cars only
Makes 🢒 Geo 🢒 1997 🢒 Prizm 🢒 Engine 🢒 Engine Controls - 1.6L 🢒 Electrical/Accessory Load Idle-Up Signal Diagnosis

Schematic Reference: Engine Controls Schematics

Circuit Description

The Engine Control Module (ECM)/Powertrain Control Module (PCM) receives an idle-up signal when the rear defogger or the parking lamps are turn on. In return the ECM/PCM will adjust the idle to accommodate the extra engine load.

Diagnostic Aids

Use the scan tool and monitor the "Electrical Load," parameter, when the defogger or head lamps are turned "ON" the scan tool should read "ON." When the defogger or head lamps are turned "OFF" the scan tool should read "OFF."

If the rear defogger and the parking lamps are not operating correctly refer to 8A for diagnosis.

An intermittent malfunction may be caused by a poor connection, rubbed through wire insulation, or a wire broken inside the insulation. Inspect the harness connections for backed out terminals, improper matting. broken locks, terminal-to-wire connections before component replacement.

Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

  1. The OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

    2.

  2. This checks for electrical load idle-up signal to ECM/PCM.

Step

Action

Value(s)

Yes

No

1

Did you perform the On-Board Diagnostic (OBD) System Check?

--

Go to Step 2

Go to Powertrain Control Module Connector End Views

2

    Important:  If rear defogger or parking lamps do not operate, refer to 8A for diagnosis first.

  1. Install the scan tool
  2. Start and run the vehicle at idle with accessories off until normal operating temperature is obtained
  3. Monitor the RPM on the scan tool, turn rear defoggers switch "ON," than "OFF.

Does engine speed increase slightly when the switch is turned "ON"?

--

Go to Step 3

Go to Step 4

3

Monitor the RPM on the scan tool, turn the parking lamp switch "ON," than "OFF. "

Does engine speed increase slightly when the switch is turned "ON"?

--

System OK

Go to Step 5

4
  1. Turn the ignition switch OFF.
  2. Disconnect the ECM/PCM harness connector at the ECM/PCM.
  3. Check the electrical load-up signal circuit for an open or short between the rear defogger switch and the ECM/PCM.
  4. Repair as necessary.

Was a repair necessary?

--

Go to Step 7

Go to Step 6

5

  1. Turn the ignition switch OFF.
  2. Disconnect the ECM/PCM harness connector at the ECM/PCM.
  3. Check the electrical load-up signal circuit for an open or short between the parking lamp switch and the ECM/PCM.
  4. Repair as necessary.

Was a repair necessary?

--

Go to Step 7

Go to Step 6

6

Replace ECM/PCM.

Is the action complete?

--

Go to Step 7

--

7

Operate vehicle within the conditions under which the original symptom was noted. Does the system now operated property?

--

System OK

Go to Diagnostic Aids