31. Transmission vacuum modulator, V12 cars TH400 transmission

The vacuum modulator sits on the side of the transmission, and connects to a vacuum line that runs up to the engine. The vacuum signal tells the transmission about engine load, so the transmission can shift at the appropriate time. There is a rubber diaphram inside the modulator, and over time the diaphram gets weak and eventually can break. Once broken, the engine will suck transmission fluid into the intake and you'll get clouds of white smoke out through the exhaust, particularly at high speeds. Transmissions with this problem will not shift correctly. Fortunately, they are easy to replace!

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