24-Volt Onboard Air Systems

Heavier commercial trucks, a good deal of diesel equipment, and many marine and military vehicles. Nearly every pickup, SUV, and passenger car runs 12 volts instead, so for an ordinary truck you want the 12-volt systems rather than anything on this page. Both systems here state it outright on their listings: they require a 24-volt electrical system. Checking the voltage at your battery bank before ordering takes a minute and saves a return, because the two are not interchangeable in either direction.

Because current falls as voltage rises for the same amount of work. The 24-Volt HornAir 232 and 24-Volt HornAir 545 compressors are each rated at a maximum of 13 amps, where a comparable single-compressor 12-volt system draws 25. That lighter current is the practical advantage of running 24 volts: thinner wire carries it, the fuse is smaller, and voltage drop over a long cable run matters less. Both systems still ship a 40 amp relay, sized with headroom well above the actual draw.

It will not work properly, and the mismatch is not limited to the motor. The compressor, the 110/150 PSI pressure switch, and the 40 amp relay are all 24-volt parts, which is why both listings state plainly that the unit requires a 24-volt electrical system. On a 12-volt vehicle fit a 12-volt onboard air system instead. Going the other direction is worse than underperformance: feeding 24 volts into a 12-volt compressor will damage it.

Tank size and duty cycle separate them. The 24-Volt HornAir 232 carries a 2-gallon tank and a 33 percent duty cycle at 100 PSI, which suits smaller horns and light pneumatic work. The 24-Volt HornAir 545 steps up to a 5-gallon tank at a 100 percent duty cycle, so it keeps pace with air tools and topping off air springs as well as horns. Larger train horns want at least a 4-gallon tank, which points to the 545.