* Modulation: Vacuum controls the pressure applied to the clutch packs within the transmission, influencing shift firmness and timing. A leak weakens the vacuum signal, leading to sluggish shifts, harsh shifts, or even complete shift failure in some gears.
* TCC (Torque Converter Clutch): The torque converter clutch engages and disengages based on vacuum. A leak can prevent the TCC from locking up properly, resulting in slippage, poor fuel economy, and overheating.
While other issues can cause shifting problems (worn clutches, low fluid, faulty governor, etc.), a vacuum leak is a common and often overlooked culprit, especially in older vehicles. A thorough vacuum system check is warranted if you are experiencing shifting problems. This includes inspecting all vacuum lines for cracks, leaks, and proper connections, as well as checking the vacuum source (usually the intake manifold).
