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An Overview Of Commercial Motor Vehicle Control Systems – Part II

Welcome to Part II of our series! In Part I, we introduced you to the basic primary controls of commercial motor vehicles. After all, if you want to embark on a rewarding and lucrative career as a professional truck driver, you’ve got to know how to operate the vehicles you intend to make a living with.

Today, we will finish outlining the braking control systems and then dive into transmission and clutch control systems. So without further ado, let’s start braking control system, part two!

Baking Control Systems Continued

In Part I we covered basic braking controls and antilock brake systems (ABS). Now we will get into auxiliary brakes and retarders and the interaxle differential lock.

Remember when we mentioned the service brakes in Part I? Well, auxiliary brakes and retarders help to reduce the vehicle’s speed without using the service brakes. These control systems keep service bakes from becoming overheated, but they should only be used on dry road surfaces.

These are the four basic types of auxiliary brakes and retarders:

  • Exhaust brakes: Exhaust brakes are the simplest type of heavy commercial motor vehicle (CMV) speed retarder. They function through a valve installed in the exhaust manifold that prevents exhaust gasses from leaving the chamber. This builds up pressure in the engine and prevents the engine from increasing speed. They are usually controlled by an on/off switch on the accelerator or clutch.
  • Engine brakes: Engine brakes are designed to alter the engine’s valve timing, which essentially turns the engine into an air compressor. They are built into the head of the engine and are operated using a switch mounted on the dashboard.
  • Hydraulic retarders: Hydraulic retarders operate on the driveline. They are generally mounted between the engine and the flywheel. Their operation is made possible by a directed flow of oil, which presses against the stantor vanes. They are usually activated through the use of a hand lever or accelerator switch.
  • Electric retarders: Electric retarders act by using electromagnets to slow the rotors on the drivetrain. They are also operated by a switch located somewhere in the cab.

The final piece in the brakes puzzle are interaxle differential locks. These braking control systems lock and unlock the rear tandem axles. In the unlocked position, the axles will turn independently of each other (on a dry surface).

When locked, the interaxle differential lock will equalize the power delivered to the axles in order to prevent traction-less wheels from spinning.

Transmission Control Systems

There are a variety of transmission applications depending on the truck being used. Even so, most transmissions fall into three categories: manual, semi-automatic and automatic.

  • Manual: A manual transmission requires the use of a clutch and gear shift lever. You will need to engage the clutch and manually change the gears in these applications. In manual applications you may have a gear shift lever that includes a range selector lever. You would use this to switch between a low and high range of gears. You may also encounter a splitter valve, which allows you to split gears if you are using a 13 or 18 speed transmission.
  • Semi-automatic: Next up are semi-automatic transmissions. These layouts include a clutch and gear shift lever. You may also encounter a control pad or panel with specific buttons on it – rather than a gear shift lever. Some shift may be handled by an on board computer and others may be handled manually by the operator.
  • Automatic: Fully automatic transmissions – which once were rare, but are now becoming more popular with technological advances – will generally only have a gear shift lever or control panel or pad. All gear changes are carried out by an on-board computer and hydraulic systems. On some models, a clutch may not be needed.

Well, we hope you’ve enjoyed Part II of our series on vehicle control systems. Join us in Part III where we finish transmission systems by delving into clutch controls, then finish out the series with a primer on secondary vehicle control systems.

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