Oil pressure is probably the most important gauge. At engine start, you should get a reading within 10 to 15 seconds. If you do not, your shop must be notified immediately. At idle, it should read around 20 psi and 40-60 psi while operating. A fluttering gauge means there is air in the system either from a low oil fluid level or too high a level, causing the oil to aerate.
The voltmeter lets you know the condition of the electrical system. Most fire trucks operate on a 12-volt system. Normal readings should be 13 to 14 volts—in other words, always slightly charging or at least maintaining at 12 volts. If it drops below 12 volts, the electrical system will eventually fail. The amp meter relays the charging system condition via rate of “charge or discharge.”
It should be slightly above zero in the “green” zone. Too much charge means that the electrical system is trying to compensate for a system that is discharging. A reading below zero means that the system cannot keep up with the demand. Either way the electrical system will eventually fail. Most apparatus have a “load manager,” which should always remain on. The load manager sheds unnecessary electrical components when activated. It will start with those items that draw the most current such as the air conditioner or heater and subsequently the next most demanding loads draining power from the electrical system.
Air brake readings can vary. The normal operating range can be anywhere between 80 to 120 psi (preferably 100 to 120 psi). At 60 psi, you should get a low pressure warning. At 20 to 40 psi the parking/emergency spring lock brake should activate. Follow your department’s procedures for performing a “CDL” brake test to assure the brake system is functioning within normal parameters.
The fuel level is self-explanatory. My department made it a point to not let the fuel level drop below ¾. Never run until empty. These gauges do fail and can read inaccurately. The last thing you need is to run out of fuel when the gauge is still reading half full.
Engine coolant should be 165°F to 185°F for Detroit Diesels or 180°F to 200°F for most others. Turbo-charged engines tend to run hotter, especially in warmer climates, climbing to as high as 215°F. If it ever hits 220°F, call maintenance.
The transmission’s temperature range is 160°F to 200°F. If the transmission temperature gets hotter than 200°F, pull off the road, park the truck and place in high idle until the transmission cools down. Notify maintenance.
Diesel exhaust fluid gauge also is self-explanatory. It should be refilled on regular basis before reaching ¼. If not, the engine will begin to “derate” (reduce the power rating), and performance will be compromised. Worst case scenario, permanent damage will occur, and component parts will need replacement.
As a final word of advice, engage the “high” idle when parked at the scene with the truck running and emergency response lights on. This prevents the oil in the engine from “sloshing.” The engine maintains a better oil pressure for better lubrication to the engine and maintains a higher output for the electrical system without draining it. Always refer to your truck’s chassis manual for normal operating parameters and follow your department’s SOPs and shop guidelines.
This article was written for & published by Fire Apparatus & Emergency Equipment.