Automatic Transmission Governors, Modulators, Parts

For an automatic transmission to shift at the proper time and with the necessary firmness, the load on the engine must be communicated to the transmission. On modern computer controlled vehicles, the transfer of engine load information takes place electronically, but on older vehicles a vacuum modulator or Throttle Valve (TV) cable is used to signal engine load to a throttle valve, and the governor and throttle valve control hydraulic pressure in the transmission. These components ensure that when the driver mashes the gas pedal, the hydraulic pressure in the transmission rises to parallel the increase in engine torque, so shifts are delayed and occur at higher rpm, shifts are firmer, and downshifting occurs if needed.

For best acceleration, the transmission must be held in gear longer and shifts must be firmer. However, if hydraulic pressure is not increased to match engine load, not only will performance suffer, but severe transmission damage can result. On electronically controlled transmissions, shift timing and firmness are controlled by a computer that uses internal programming and sensor input indicating throttle position, engine load, vehicle speed, etc. to manipulate fluid pressure and flow via electric solenoids in the valve body. Older, non-electronically controlled transmissions used a different method.

On non-electronically controlled transmissions, the transmission pump supplies fluid to the governor and throttle valve, which in turn direct fluid to shift valves in the valve body. The governor is driven by the transmission output shaft and builds pressure directly in relation to vehicle speed. Under light acceleration, the throttle valve does not have much effect on the shift valve and governor-controlled shifts are earlier and softer. Under hard acceleration, when engine load is high, the throttle valve will act against the shift valve to force governor pressure higher. This delays the shift and supplies higher pressure to the transmission bands and clutches, giving them the clamping power needed to control members of the planetary gearset and enact firm, hard shifts.

As stated earlier, engine load is signaled to the throttle valve by a vacuum modulator or throttle valve cable. The vacuum modulator is attached to the transmission and connected to the throttle valve with a shaft, and to an engine vacuum source with a rubber hose. The high engine vacuum that exists when the engine is under light load makes the throttle valve apply little pressure so the transmission shifts early, with a soft, smooth shift. When the gas pedal is depressed quickly and the engine is under heavy load, the low vacuum that exists under such conditions moves the throttle valve so the transmission shifts later with a firmer shift. The throttle valve cable provides a mechanical connection between the carburetor or fuel injection throttle body and the transmission throttle valve. If you need a replacement, or a replacement vacuum modulator, governor, or any other related components, we have the necessary components to restore proper shifting performance and vehicle operation.

Proper governor, vacuum modulator, and throttle valve cable (sometimes abbreviated TV cable or throttle cable) function is important for any vehicle, but especially so for a performance car or truck. When you’re putting more horsepower through the transmission and want to get as much of it to the wheels as possible, it’s crucial to be able to control shift timing and firmness and we have the performance components to help you do it. If you’re running different diameter tires or changing the rear end gear ratio, we have governor recalibration kits that will let you change shift points, so your engine can stay within its peak power rpm range. We also offer governor pressure solenoid upgrade kits, governor springs, and custom logo governor covers.

Part throttle shift points can be raised or lowered with our adjustable vacuum modulators. With just a turn of a screw inside the vacuum nipple you can alter the pre-load on the modulator diaphragm and fine tune shift points. Swapping the original transmission with a later model automatic transmission with overdrive is a common mod on muscle cars, street rods, and other older vehicles. But getting the TV cable adjustment right may not be possible when using the original cable on a non-stock application, especially with an aftermarket carburetor and intake manifold. We offer TV cable corrector kits that work with popular aftermarket carburetors, and enable the proper adjustment that will ensure optimal transmission performance and durability. We also have heavy-duty detent cables, kickdown kits, kickdown switch kits, and brackets to enable TV cable connection to specific carburetors.