Dashboard on a Semi Truck: Gauges and Instruments

A List of the Gauges and Switches You Can Find on a CMV

Red truck on the road at sunset
Credit: Jetta Productions / Getty Images

The dashboard on a semi-trailer truck, with all of the gauges and instruments, serves to to allow drivers to monitor the engine’s performance. 

The Dashboard: Semi Truck Gauges and Instruments

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

Voltmeter

The voltmeter displays the charge condition of the battery.

There are three areas on the volt battery display to 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 gauge may be marked Water Temp or Temp. This gauge displays the engine-cooling temperature. A normal reading will fall between 165 and 185 Fahrenheit.

Engine Oil Pressure Gauge

The engine oil pressure gauge will have a higher psi (pounds per square inch) when the oil is cold. As the engine warms up, the psi will fall. Normal readings run from 30 to 75 psi.

Pyrometer Gauge

The Pyrometer 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.
  • Fuel ignition problem
  • In wrong gear ratio for load/grade/altitude

    Transmission Temperature Gauge

    The transmission temperature gauge should have a reading from 180 to 250 degrees.

    Rear Axle and Forward Rear Axle Temperature Gauge

    This gauge will have a normal range from 160 to 220 degrees.

    Tachometer

    The tachometer displays the engine's revolutions per minute (RPM). Multiply the number on the tachometer (also known as tach) by 100 to get your RPMs.

    Speedometer

    The speedometer displays your current speed, either in miles per hour (MPH) or kilometers per hour (KPH).

    Odometer

    This gauge displays your total miles driven and on some models the total engine miles overall (i.e. if you have replaced the odometer at some point)

    Throttle

    The throttle is used to set engine speed or RPMs. The throttle may be used in cold weather to keep the cab warm while the engine is idling. **Note: On newer models, the throttle is replaced by cruise control**

    Ignition Switch

    The ignition switch turns on the power unit's electricity.

    Fuel Gauge

    The fuel gauge displays the power unit's current fuel supply.

    Fuel Filter Gauge

    This gauge shows 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

    The air filter indicator gauge monitors the temperature outside of your vehicle.

    Switches Commonly Found on a CMV Dash

    There are buttons, switches and levers for adjusting and controlling windshield wipers, air conditioning controls, and a whole host of features on most CMVs, such as: 

    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 and Warm Up Switch Cold Start

    This switch is specifically for cold weather when the engine may be hard to start. Turn this switch on to pre-heat the engine.

    Exhaust Brake Switch

    This switch is turned on when you need extra slowing power. This is especially useful when on steep grades or when heavily loaded.

    Additional Warning Lights

    • Left and right hand turn signals. A light should come on (typically shaped as an arrow facing either left or right) when the signal is deployed.
    • Water level warning light
    • Oil level warning light
    • Low air warning light
    • Low vacuum warning light. A warning light will emit when something is wrong with oil level, water level, low air pressure, and/or low vacuum pressure.
    • High beam light. A light should come on when high beams are deployed. **Note: The light switch may either be on the dashboard, a turn signal handle or on the floor near your pedals.**
    • Differential lock warning light. This light remains on as long as your differential is in the locked position
    • Charging circuit warning light. This light warns you if your battery is not charging
    • Cab lock light

    The CMVs owner's manual will, of course, be a helpful tool to find these switches and gauges, and also to determine what their particular warnings mean. Every time you are in a new CMV take time to become familiar with the dashboard. Locate the owner's manual so that if you have a problem you can quickly reference and/or diagnose the problem.

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