What Is a Transmission Vacuum Modulator Valve?

by Greg Day
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The transmission vacuum modulator valve determines how much load is on the engine so the transmission can shift properly. It has a vacuum line that connects to the intake and measures the amount of vacuum in the engine.

High Load vs. Low Load

In order to determine if whether transmission should shift, the modulator valve measures the load of the engine's vacuum. Engine load is the measurement of how hard an engine works at any given moment. For example, coasting down hill will create a low engine load, while driving uphill with two passengers and a trunk full of luggage will create a high load. Thus, shift points for the transmission are figured according to the valve's measurement.

How It Works Within the Transmission

The modulator valve is a part of the transmission vacuum. Inside the canister of the vacuum modulator, the valve works with a diaphragm and a calibrated spring to measure load. When the diaphragm senses changes in engine vacuum because of load changes, the diaphragm pushes against the valve while the calibrated spring moves against the movement of the diaphragm.

Potential Problems

If the modulator valve is not working properly, the transmission will not shift properly. These problems can arise when the hose connecting the transmission to the engine vacuum cracks or burns, or collapses onto itself.

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