Accessory Signal: Testing and Inspection

ELECTRICAL/ACCESSORY LOAD IDLE-UP SIGNAL DIAGNOSIS

CIRCUIT DESCRIPTION
The powertrain control module (PCM) receives an idle-up signal when certain engine electrical loads or engine accessory loads are present. The PCM increases the flow of air through the idle (bypass) air passage of the idle air control (IAC) valve when receiving an idle-up signal. The additional air flowing through the idle air passage causes an increase in the idle speed. The idle speed is increased in order to accommodate the increased engine electrical or engine accessory load. The PCM monitors the following systems for idle-up signals:
- The electric rear window defogger
- The exterior lighting system
- The A/C system-When A/C is selected from the HVAC control unit, the PCM receives a 12-volt signal. Once the PCM is aware of the request for A/C, the PCM determines whether engagement of the A/C compressor clutch is OK. In order to enable compressor clutch engagement the PCM provides a ground for the control circuit of the A/C compressor clutch relay. The PCM also increases engine idle speed in order to prevent a rough idle or a stalling condition when the A/C compressor is engaged.
- The power steering system-The power steering pressure (PSP) switch is normally open to ground. Turning the steering wheel increases the power steering oil pressure. Increased power steering oil pressure puts an additional load on the engine that may be noticeable at idle. The PSP switch closes when the power steering system oil pressure is high enough to cause an undesirable idle condition. The voltage signal from the PSP switch to the PCM indicates less than 1 volt when the PSP switch is closed. The voltage signal is near battery voltage when the PSP switch is open.

DIAGNOSTIC AIDS
Check for any of the following conditions:
- The High Electrical Load parameter on the scan tool can indicate if the PCM has received the rear defogger or parking lamps ON signal. The scan tool should indicate Yes when the rear defogger or parking lamps are turned ON and engine speed should increase slightly. Check for a malfunction in the IAC system or a short to ground in the Diagnostic Request circuit if the engine speed does not increase.
- A malfunctioning rear defogger or parking lamp (tail lamp) circuit may cause erratic electrical load idle-up circuit performance. Check for a properly operating rear defogger or tail lamp system.
- A PSP switch that does not close, or loss of ground at the PSP switch may cause the engine to stall when the power steering loads are high.
- The scan tool will display A/C Request Signal YES when A/C is selected and the HVAC blower switch is ON at any speed. The scan tool will display A/C Clutch ON whenever the PCM is allowing A/C compressor engagement. Therefore the scan tool will also display A/C Clutch ON when any defrost mode is selected.

An intermittent malfunction may be caused by a fault in any one of the idle-up signal circuits. Inspect the wiring harness and components for an intermittent condition.

TEST DESCRIPTION

Step 1 - Step 7:

Step 8 - Step 16:

Step 17 - Step 25:

The numbers below refer to the step numbers in the diagnostic table.
1. The Diagnostic System Check - Vehicle 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. This step verifies whether the PCM responds to the operation of the rear defogger. The IAC Motor Command Cycle percentage and the idle speed will increase when the rear defogger is turned ON if the idle-up system is OK.
3. This step verifies whether the PCM responds to the operation of the taillights. The IAC Motor Command Cycle percentage and the idle speed will increase when the taillights are turned ON if the idle-up system is OK.
4. This step verifies the operation of the A/C idle circuit. If the engine idle speed and IAC Motor Command cycle increase when the A/C is turned ON, the A/C idle circuit is functioning normally.
5. This step verifies the operation of the power steering pressure (PSP) switch circuit. If the engine idle speed and IAC Motor Command increase when the steering wheel is turned to 90 percent, the PSP switch idle circuit is functioning normally.
6. This step tests the PSP switch signal circuit for voltage from the PCM.
7. This step tests the operation of the PSP switch.
10. The A/C compressor clutch should not be engaged with the A/C switch Off. The PCM will not increase engine idle speed when the A/C compressor is supposed to be Off.
13. If the A/C compressor clutch engages, the compressor clutch, the relay, and the control circuit are all OK. The PCM will then need to be replaced because there was no increase in the IAC motor command percentage during A/C operation in Step 4.
22. This step tests for a short to ground in the Diagnostic Request circuit. A short to ground in the Diagnostic Request circuit can cause the PCM to ignore the idle-up signals to the PCM. A short in the Diagnostic Request circuit will cause the IAC Motor Command and the idle speed to remain fixed.