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Your Guide to Solenoids in Semi-Trucks

Even though solenoids are found in many household items such as washing machines, dishwashers, doorbells, sprinkler systems, they are most known for the roles they play in allowing automobiles to run smoothly. Many car enthusiasts know of all the different solenoids found in cars and trucks, but how about in semi-trucks? Semis are much larger than standard vehicles and therefore house a greater number of solenoids than traditional vehicles.

It takes many components to make trucks operate properly, decrease emissions, and increase efficiency. Solenoids are an important component of semis. Let’s take a different look at the different kinds of solenoids and their functions.

The Purpose of Solenoids

Solenoids are a simple concept; in that they work by converting electrical energy into mechanical energy. This happens by creating a magnetic field which then produces the desired motion. Solenoids operate either a push or pull operation and some can actually do both. Simply put, solenoids signal for valves to open or close as well as to tell certain components to come on or shut off.

In some cases, such as in the transmission, there are multiple solenoids that work together to execute a function.

Starter Solenoids

The starter solenoid is the most recognized solenoid and is used to initiate the engine. This type of solenoid installs in either one of two ways. The first and most common involves attaching it directly to the starter. The second includes detaching it from the starter. In a detached starter solenoid, it instead attaches under the fender well or on the firewall. This solenoid sends a powerful electrical current through the starter motor, which prompts the engine to turnover. This allows the motor to start under its own power.

Transmission Solenoids

This type of solenoid is a valve that is used to control the flow of transmission fluid throughout the engine. This, in turn, allows the truck to shift gears. The transmission solenoid typically installs in the transmission valve body, control unit, or control module. It is important to understand that trucks do not have just one transmission solenoid. There are several solenoids in the transmission for each gear that allows for shifting. The truck’s engine control unit or transmission control unit senses the speed of the truck and sends a signal to the appropriate shift solenoid in the truck to allow upshifting or downshifting of gears as needed.

Four-Wheel Drive Solenoids

Four-wheel drive solenoids are responsible for engaging and disengaging the four-wheel-drive feature of the semi. The size of the truck will depend on the number of solenoids required to initiate the four-wheel drive. You will find an even number of solenoids on each side of the truck in the housing. A failure in any one solenoid will cause the four-wheel-drive feature to either work improperly or not at all. A point of interest is that solenoids are not found in trucks that you have to engage and disengage the four-wheel-drive feature manually.

Solenoids for Doors and Hoods

Door solenoids are powerful enough to open even the toughest doors and hoods of semi-trucks. The number of solenoids per door or hood will depend on the size of each component. Solenoids allow for easy opening of each component by either the push of a button or the flip of a switch. There are solenoids available that are anti-rust and will last much longer than cheaper quality solenoids that are subject to rust, cracks, and leaks.

Window Solenoids

Manual windows will not have solenoids, but semis that have automatic windows will have solenoids. The function of the solenoids is to signal the windows to move up and down. This can either be by a remote, button, or a switch.

Fuel Injection Solenoids

The bottom line is that your semi will not run if the engine is not receiving fuel. The fuel injection solenoid’s function is to tell the injectors when to release fuel to the engine. There are different types of fuel injection solenoids available that are able to reduce emissions and increase the efficiency of the vehicle.

Diesel Fuel Shut-Off Solenoid

A diesel fuel shut-off solenoid is responsible for the regulation of fuel to the engine. It contains an inlet pipe, which then transports the diesel fuel from the fuel line to the solenoid valve. This type of solenoid is typically installed as part of the main electrical system. This means that abnormal temperatures or malfunctions will be monitored and if something goes wrong, the electrical current to the solenoid is shut off. The valve then closes and prevents fuel from reaching the engine.

Solenoids Power Accessories

Many vehicle accessories are powered by solenoids, and semis are no exception. Unlike traditional vehicles, semis must have the ability to perform unique functions. For instance, dump trailers must have the ability to lift and lower the bed. This is accomplished with the use of solenoids. In fact, solenoids are used to power many special features of semi-trucks.

How Many Solenoids Do Semi Trucks Have?

There is not a one size fits all answer to this question. The fact of the matter is that the number of solenoids will depend on what size semi you have and what features are on it. For example, is it a four-wheel drive? Does it have power or manual windows and doors? What kind of special functions are installed on the semi? The answer to all of these questions will impact the number of solenoids on your truck.

Turn to the Solenoid Specialists

Johnson Electric is the premier expert in solenoids for semi-trucks. Our specialists can walk you through all your options and help you decide on the best option to fit your needs. If you aren’t quite sure what you need, we are more than happy to give you professional advice on what might work best for your needs. Contact us for more information on our line of high-quality solenoid products.


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