The transmission vacuum modulator on V12 Jaguars with the GM TH400 automatic transmission is mounted on the side of the transmission and connected to the engine with a vacuum hose. Its job is to sense engine load through manifold vacuum so the transmission knows when to shift and how firm the shifts should be.
Inside the modulator is a rubber diaphragm that eventually becomes brittle with age. When the diaphragm fails, engine vacuum can pull transmission fluid through the vacuum hose and into the intake manifold.
Common symptoms:
• Thick white smoke from the exhaust, especially at highway speeds or on heavy throttle.
• Transmission shifts late, harshly, or unpredictably.
A quick check is to remove the vacuum hose from the modulator. If transmission fluid is present in the hose, the modulator has failed and needs replacement.
Fortunately, the vacuum modulator is inexpensive, easy to access, and simple for most DIY Jaguar owners to replace with basic hand tools.
