Here's a breakdown of why the IAC is unlikely the culprit and where to look next:
* IAC's Role: The IAC only controls the engine's idle speed. It doesn't directly affect transmission shift points. A faulty IAC would likely cause rough idling, stalling, or difficulty starting, not erratic shifting.
* Possible Causes of Your Symptoms:
* Transmission Issues: The quick 1-2 shift and RPM fluctuations at 35-40 mph strongly suggest a problem within the transmission itself. This could be due to:
* Worn or damaged transmission components: Internal clutches, bands, or solenoids might be wearing out.
* Low transmission fluid: Check your transmission fluid level and condition. Low or dirty fluid can cause erratic shifting.
* Faulty transmission speed sensor: This sensor provides information to the transmission control module (TCM) about vehicle speed, affecting shift points. A malfunction could lead to premature shifting.
* Transmission Control Module (TCM) Problems: The TCM controls shifting. A faulty TCM can cause a variety of shifting problems.
* Throttle Position Sensor (TPS): A faulty TPS sends incorrect information to the engine control module (ECM) about throttle position, which could indirectly affect shifting behavior.
* Mass Airflow Sensor (MAF): Similar to the TPS, a malfunctioning MAF sensor provides inaccurate information about air intake, potentially affecting fuel delivery and consequently shifting.
Troubleshooting Steps:
1. Check Transmission Fluid: This is the easiest and most important first step. Check the fluid level and condition. If it's low, add the correct type of transmission fluid (consult your owner's manual). If it's dirty or burnt, a fluid change and filter replacement are likely needed.
2. Scan for Diagnostic Trouble Codes (DTCs): Use an OBD-II scanner to check for any trouble codes stored in the ECM or TCM. These codes can provide valuable clues about the problem.
3. Inspect the TPS and MAF Sensor: These sensors can be visually inspected for damage, but testing with a multimeter is often required for accurate diagnosis.
4. Professional Diagnosis: If the above steps don't reveal the issue, take your vehicle to a qualified mechanic specializing in transmissions or auto electronics. They have the tools and expertise to properly diagnose the problem and perform the necessary repairs.
In short, while you recently replaced the IAC, it's highly unlikely to be the cause of your shifting problems. Focus your attention on the transmission and associated sensors. Ignoring transmission problems can lead to more extensive and costly repairs down the line.
