Truth in numbers
Find out from the owner’s manual, the engine maker or your employer’s maintenance department what your engine’s coolant temperature should be, and keep an eye on that gauge.
Two more important gauges are those that measure pressure in the truck’s primary and secondary air tanks. Big truck suspensions, seats and, in some cases, starters all run on compressed air. But so do the truck’s brakes, and this is where a lack of knowledge about gauges can result in serious injury or death.
“My air pressure gauges read about 120,” Gray says. “If they start to fall below that, then I know something isn’t right. Either the governor that tells the compressor to pump to 120 psi is bad, or maybe the compressor is bad.
“Normally a leaky air line won’t lose enough air so the pressure falls too low unless the line is broken completely in half, and then you can get a connector and put it back together.”
The air pressure gauges tell you if you can stop the truck. Check them continually. If the needle drops down and comes back up continually while the truck is running, or if it drops down and does not come back up, have the system checked and repaired before you take the truck on the road.
Another important gauge is the tachometer, which tells how fast the engine is turning: information mostly applied to shifting gears.
Experienced drivers often know when to shift without using the tach. “I started out with an old Mack B61 back in 1965,” Lawrence says. “The tach had numbers, but we’d listen to the sound of the engine to know when to shift.”
Drivers who have yet to develop this technique should shift with the tach. Under normal conditions, shift at about 1,500 rpm. For smoother, grind-free downshifting, check the engine rpm. Pull the transmission out of the higher gear, then rev the engine to at least the same rpm before dropping into the lower gear.
The tach can also increase fuel mileage. Shifting and cruising at lower rpm uses less fuel. But engine make, road conditions, weight and gearing will all factor into the best cruising rpm, so ask your employer at what rpm the truck should be run.
Another gauge that gives crucial information is the voltmeter. It tells how much electricity the truck’s alternator is making. It should read at least 13.5 volts.
“A lot of drivers don’t know this,” Lawrence says. “If your alternator’s not producing 13.5 volts, then you’re not putting enough power back into the battery, and you’ll eventually get to where your truck won’t start,” he says. “You need at least 13.5 volts to keep ahead of the draw on the batteries.”
These five gauges – oil pressure, engine coolant temperature, air pressure, tachometer and voltmeter – will be found in almost all trucks, and in all trucks the information they provide can be the difference between late and on-time, rolling or stranded, high or low repair bills, and even life or death. It’s part of a professional truck driver’s job description to understand and use information the gauges provide.