MODEL IDENTIFICATION
Vehicle model can be identified by fifth character of Vehicle Identification Number (VIN), stamped on metal pad on top of left end of instrument panel, near windshield. See MODEL IDENTIFICATION .
| Series (1) | Model |
|---|---|
| C | 2WD Pickup, Sierra & Suburban |
| G | RWD Van |
| K | 4WD Blazer, Pickup, Sierra, Suburban & Yukon |
| L | All-Wheel Drive Astro & Safari |
| M | 2WD Astro & Safari |
| P | Commercial Van/Motorhome |
| S | 2WD Blazer, Jimmy, Pickup & Sonoma |
| T | Bravada, 4WD Blazer, Jimmy, Pickup, Sonoma, Syclone & Typhoon |
| U | Lumina APV, Silhouette & Trans Sport |
| (1) Vehicle series is fifth character of VIN. | |
| (1) | Vehicle series is fifth character of VIN. |
MODEL IDENTIFICATION
DESCRIPTION
The ECM controls engine idle speed with the IAC valve. To increase idle speed, the ECM retracts the IAC valve pintle away from its seat, allowing more air to pass by the throttle bore. To decrease idle speed, it extends the IAC valve pintle towards it's seat, reducing bypass air flow. A Tech 1 "Scan" tool will read the ECM commands to the IAC valve in counts. Higher counts indicate more air bypass (higher idle). Lower counts indicate less air is allowed to bypass (lower idle).
TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
- The IAC tester is used to extend and retract the IAC valve. Valve movement is verified by an engine speed change. If no change in engine speed occurs, the valve can be retested when removed from the throttle body.
- This step checks the quality of the IAC movement in Step 1). Between 700 RPM and about 1500 RPM the engine speed should change smoothly with each flash of the tester light in both extend and retract. If the IAC valve is retracted beyond the control range (about 1500 RPM), it may take many flashes in the extend position before engine speed will begin to drop. This is normal on certain engines, fully extending IAC may cause engine stall. This may be normal.
- Steps 1 and 2 verified proper IAC valve operation while this step checks the IAC circuits. Each lamp on the node light should flash Red and Green while the IAC valve is cycled. While the sequence of color is not important if either light is "OFF" or does not flash Red and Green, check the circuits for faults, beginning with poor terminal contacts.
NORMAL GLOW PLUG CIRCUIT OPERATION
A normal functioning system should operate as follows
- With ignition switch in the ON position, engine not running and at room temperature, glow plugs are on for 4-6 seconds, then off for about 4.5 seconds. Following this on-and-off cycle, glow plugs are on for about 1.5 seconds and off for about 4.5 seconds. The glow plugs continue to cycle on and off in this time sequence for about 20 seconds.
- If engine is cranked during or after the above sequence, glow plugs will cycle on and off for a total duration of 25 seconds after the ignition switch is returned from the crank position, whether engine starts or not. Engine does not have to be running to terminate glow plug cycling.
Note. Glow plug on-and-off times vary with engine temperature, system voltage and/or ambient temperature. Lower temperatures cause longer duration of cycling.
DESCRIPTION & OPERATION
The tuning valve (rotary solenoid device) is electrically controlled by the PCM based on RPM and TP signal inputs, for accurate switching capability at optimum switching points to enable either a "split" or "single" plenum condition.
During the low and high end of the RPM range, the PCM de-energizes the valve and enables a "split" plenum condition that provides for peak torque along with increased peak horsepower.
During the mid RPM range, the PCM energizes the valve, thus enabling a "single" tuning plenum condition that provides for mid RPM range peak torque.