How Does a Truck Governor Work?

There are several type of governors used in semi-truck world today. Mechanical governors use rotating flyweights and a control spring to help control diesel engine speed. Power assisted style governors rely on engine oil pressure to move an operating linkage that is tied to the throttle linkage. Hydraulic style governors use a valve plunger that is controlled by pressurized oil and is connected to the fuel control mechanism. Pneumatic governors use the vacuum created by the engine to control a diaphragm that is connected to the fuel control linkage. Electronic governors are directly wired to the ECM or an electronic control box in older applications and generally cannot be modified without specialized computer equipment.

Governors used on today’s Semi-trucks fall into two basic categories: speed limiting governors and variable speed or all range governors. Speed limiting governors prevent the truck driver from pushing the vehicle past a certain speed. The most common use of speed limiting governors are to keep down insurance cost, and to protect the truck and the cargo. Studies performed by the American Transport Research Institute in 2007 reported that 69% of companies that made use of speed governors based the maximum speed setting on the posted speed limits in the particular area of operation.

Range limiting governors prevent the engine from traveling past engine speeds that will cause internal engine damage or dropping below a certain speed that will cause the engine to run rough. Generally this type of governor will allow the driver to push the throttle to 100% in any gear without allowing the engine to exceed its maximum speed. Range governors are used only as a means of protection against engine damage and will not hamper the driver from doing what he wants in any way with the exception of blowing up the engine, of course. As technology has increased most mechanical governors have been replaced by electronically controlled units to help prevent tampering. On this type of governor the ECM or electronic control box relies on the signal from the crankshaft position sensor to determine engine speed and will shut off pulses to the fuel injectors to prevent the engine for traveling past the red line or will increase fuel flow if the engine begins to drop below idle speed to help prevent stalling.

Speed limiting governors are widely used by companies that lease or provide trucks to hired drivers. These types of governors are tied into a central control box or on newer trucks are tied into the engine control module. On passenger vehicles these governors are set to the stock rating of the installed tires when the vehicle was built, or to the government mandated maximum speed depending on the country or market. In the truck world however, these governors are controlled by the operating company and are often set just above the maximum speed where the truck will be operating. The operation of this type of governor is pretty simple, the ECM relies on signals from the transmission differential or the wheel speed sensors and shuts off pulses to the ignition or fuel injectors until the vehicle drops below a set speed. In most cases the maximum speed a truck can travel is 85 MPH even though the engine and transmission are capable of traveling much faster.