An automatic transmission that has a disconnected or completely blocked modulator valve vacuum line would exhibit which of the following problems?

An automatic transmission that has a disconnected or completely blocked modulator valve vacuum line would exhibit which of the following problems?

• A. Control or main pressure would be lower than normal.
• B. The transmission would experience early upshifts.
• C. No upshifts would occur at any speed.
• D. Upshifts would occur at much higher speeds than normal.

Answer: (D)

The key idea is how the vacuum modulator sets when the transmission shifts. The modulator valve is driven by engine vacuum, and that vacuum signal tells the transmission to shift at certain engine speeds based on throttle/load. When the vacuum line to the modulator is disconnected or blocked, the valve sees no vacuum, which it interprets as a high-load condition. That makes the transmission hold gears longer and shift at higher engine speeds than normal. So shifts occur much later, at higher RPMs than usual. This isn’t about reducing overall main pressure, nor does it cause no upshifts or earlier upshifts—the fault pushes the shift points to occur at higher speeds.

The key idea is how the vacuum modulator sets when the transmission shifts. The modulator valve is driven by engine vacuum, and that vacuum signal tells the transmission to shift at certain engine speeds based on throttle/load. When the vacuum line to the modulator is disconnected or blocked, the valve sees no vacuum, which it interprets as a high-load condition. That makes the transmission hold gears longer and shift at higher engine speeds than normal. So shifts occur much later, at higher RPMs than usual. This isn’t about reducing overall main pressure, nor does it cause no upshifts or earlier upshifts—the fault pushes the shift points to occur at higher speeds.