GM Service Manual Online
For 1990-2009 cars only

High Idle Switch Diagnosis with RPO UF3

Circuit Description

The high idle system is used to provide additional engine power when needed for vehicle accessories. The engine control module (ECM) controls the high idle operation based on various sensor inputs and the high idle switch on the instrument console. This system is not activated by temperatures as is the high idle system for warm up, but the rest of the input and operation are the same. For more information on these parameters, refer to Manual High Idle System Description .

Diagnostic Aids

If the high idle system is inoperative but the switch circuit works, inspect for the following conditions:

    • The high idle switch option is not programmed--Refer to High Idle Switch Programming .
    • There are DTCs set--Diagnose these first.
    • There is a non-DTC fault in one of the following components or systems:
      • The brake switch
      • The clutch switch
      • The vehicle speed sensor

Step

Action

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 high idle switch parameter with a scan tool.
  2. Turn the high idle switch ON and OFF.

Does the high idle switch parameter change state?

Go to Diagnostic Aids.

Go to Step 3

3

Does the high idle switch parameter indicate ON all the time?

Go to Step 4

Go to Step 6

4

  1. Disconnect the high idle switch.
  2. Observe the high idle switch with a scan tool.

Does the high idle switch indicate OFF?

Go to Step 13

Go to Step 5

5

  1. Turn OFF the ignition.
  2. Disconnect the engine control module (ECM).
  3. Probe the high idle signal circuit of the ECM harness connector with a test lamp connected to battery voltage.

Does the test lamp illuminate?

Go to Step 11

Go to Step 14

6

  1. Disconnect the high idle switch.
  2. Connect a 3-amp fused jumper wire between the high idle signal circuit of the high idle switch harness connector and a good ground.
  3. Observe the high idle switch parameter with a scan tool.

Does the high idle switch indicate ON?

Go to Step 7

Go to Step 8

7

Probe the ground circuit of the high idle switch with a test lamp connected to battery voltage.

Does the test lamp illuminate?

Go to Step 9

Go to Step 12

8

Test the high idle signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 15

Go to Step 10

9

Test for an intermittent and for a poor connection at the high idle switch. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 15

Go to Step 13

10

Test for an intermittent and for a poor connection at the ECM. Refer to Testing for Intermittent Conditions and Poor Connections and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

Go to Step 15

Go to Step 14

11

Repair the short to ground in the high idle switch signal circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 15

--

12

Repair the open or high resistance in the ground circuit of the high idle switch . Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

Go to Step 15

--

13

Replace the high idle switch.

Did you complete the replacement?

Go to Step 15

--

14

Replace the ECM. Refer to Engine Control Module Replacement

Did you complete the replacement?

Go to Step 15

--

15

Operate the system in order to verify the repair.

Did you correct the condition?

System OK

Go to Step 2