Gauges and Instruments on a Semi Truck's Dashboard

Interior view of dashboard of semi truck
••• Comstock / Getty Images

The dashboard on a semitrailer, with all of its gauges and instruments, allows drivers to monitor more than just the engine’s performance. Much like pilots in airplanes, truck drivers pay close attention to their instruments to know the precise status of their vehicles.

A CMV's owner's manual is a helpful tool for finding these switches and gauges and also for determining what their particular warnings mean. Every time you are in a new CMV, take the time to become familiar with the dashboard. Locate the owner's manual and read it thoroughly before operating the vehicle so that if you encounter a problem, you can quickly learn about and perhaps diagnose the issue.

Gauges and Instruments

Commercial motor vehicles (CMV) are unique to the specifications of their manufacturers. However, you can expect to find a standard set of instruments and gauges on all semitrailers.

  • Voltmeter: This displays the charge condition of the battery. Three areas on the volt battery display show its status. The first red area shows if the battery is undercharged, The middle green area shows if the battery is normal. The right-hand red area shows an overcharge condition.
  • Engine Temperature Gauge: This displays the engine-cooling temperature and may be marked "Water Temp" or "Temp." A normal reading falls between 165 and 185 degrees Fahrenheit.
  • Engine Oil Pressure Gauge: This will display a higher pounds per square inch (psi) when the oil is cold. As the engine warms up, the psi will fall. Normal readings run from 30 to 75 psi.
  • Pyrometer Gauge: This displays the reading for the engine exhaust temperature. The gauge should have the safe temperature range printed on it. A high temperature is indicative of a problem such as a leak or clog in the air intake or exhaust system, a fuel ignition problem, or a wrong gear ratio for load, grade, or altitude.
  • Transmission Temperature Gauge: A reading from 180 to 250 degrees Fahrenheit is desired.
  • Rear Axle and Forward Rear Axle Temperature Gauge: This should display a normal range from 160 to 220 degrees Fahrenheit.
  • Tachometer: The "tach" displays the engine's revolutions per minute (RPM). Multiply the number on the tachometer by 1,000 to calculate your RPMs.
  • Speedometer: This displays your current speed, either in either miles per hour or kilometers per hour.
  • Odometer: This displays your total miles driven and on some models the total engine miles overall.
  • Throttle: The throttle is used to set engine speed or RPMs. It may be used in cold weather to keep the cab warm while the engine is idling. Each model of truck is different and if there is no throttle control, that means it has been replaced with electronic cruise control.
  • Ignition Switch: It turns on the power unit's electricity.
  • Fuel Gauge: This displays the power unit's current fuel supply.
  • Fuel Filter Gauge: This shows the condition of the fuel filter. There are two color sections: white and red. The red zone indicates a clog in the fuel filter.
  • Air Filter Indicator Gauge: This monitors the temperature outside of your vehicle.

Common Switches

There are buttons, switches, and levers for adjusting and controlling windshield wipers, air conditioning controls, and other typical features found in most vehicles. Additional switches common in most CMVs include.

  • Inter-axle Differential Control: A power unit will have this particular switch if it has dual rear axles with inter-axle differentials. When unlocked, each axle shaft and wheel can turn at different speeds as the vehicle turns.
  • Cold Start Warm-Up Switch: This is specifically for cold weather when the engine may be difficult to start. Turn on this switch to preheat the engine.
  • Exhaust Brake Switch: Turn this on when you need extra slowing power. It is especially useful when on steep grades or when heavily loaded.