Virtually all semi-trucks and other heavy-duty equipment use compressed air to engage the brakes making the compressor system crucial for the safety of drivers and other road users. And just like it’s the case with all compressors and any other pressurized system, something needs to regulate the pressure, which in this case is the air governor. Luckily air governors don’t fail all too often, but when they do, here is what you can expect.
The symptoms of a bad air governor include the compressor taking too long to build pressure, the safety valves activating too often, the compressor cycling too often, or not cycling at all.
What Is an Air Governor in Diesel Trucks
We already mentioned that the brakes in semi trucks are not powered by hydraulic fluid as they are in passenger vehicles but by pressurized air. Now, the air system isn’t a closed one, so it’s not pressurized all the time naturally. Instead, the truck has an air compressor that builds pressure every time you start the engine and stores it in a tank.
Then, when you push the brakes, the pressure from the tank travels to the brake calipers or the drum brake cylinders and engages them. But since the compressor is powered by the engine, there is no easy way of shutting it down when the pressure in the system is too high or turning it on when it’s too low.
That’s why air governors are installed. Air governors are essentially just spring-loaded valves that maintain the pressurized air at 90 to 135 psi. When the pressure becomes too high, the air governor valve opens and sends pressurized air to the compressor valve, which keeps it open. And with a constantly open valve, the piston in the compressor can’t pressurize air anymore.
Then, once the pressure drops, the valve in the air governor closes, thus cutting the pressurized air passage that’s keeping the compressor valve open and allowing it to resume pressurizing air for the braking system. That quickly builds up the pressure, and once it reaches a high enough level, the air governor valve opens again, and the process repeats itself.
You can also have a look at this video which includes an air governor animation and an explanation of the whole air braking system by, among other things, a highly experienced commercial driver instructor Rick August.
Symptoms of a Bad Air Governor
The symptoms of a bad air governor largely overlap with other system faults like a faulty dryer or air compressor. But if you notice the air safety valve going off, it’s a clear sign that the air governor is faulty, and you should have the air system inspected as soon as possible.
Truck Not Building Air Pressure
Each time the truck engine is shut off, the air pressure from the tank is released. That means each you start it up again, the compressor needs to build up some pressure in the tank before you can start driving. Now, that usually takes less than a minute, but with a faulty air governor that’s keeping the compressor valve open or partially open, that can take up to five minutes or more.
Compressor Cycling Too Often
The air governor is designed to maintain the system pressure roughly between 90 and 135 psi. That means each time the pressure in the air tank comes near 90 psi, the compressor kicks in and pressurizes it back to 130 or so.
But if the air governor is faulty, it may start activating the compressor prematurely. So instead of the compressor cycling any time it nears 90 psi, it will start kicking in at 110 or 115 psi, thus noticeably increasing the compressor cycling frequency.
Compressor Not Cycling
The most common type of air governor failure is that it doesn’t disengage the air compressor at all, leading to high air tank pressure. But it can also happen that the air governor disengages the air compressor late but not late enough to activate the safety valves.
That results in a system that’s always pressurized to 120 to 140 psi, in which case the compressor doesn’t need to cycle at all or cycle much less frequently than what’s normal or what you are used to.
Air Tank Safety Valve Activating
The air tank safety valve activating is the result of a more severe problem we previously described. And that’s the air governor not cutting out when it’s supposed to, or in this case, ever. That builds up too much pressure in the air tanks, which have safety valves that release excess pressure to prevent them from exploding. The safety valve noise can be described as loud pops/bangs or like a machine gun.
What Causes an Air Governor To Become Bad
There are no clear reason why air governors fail other than age and normal wear and tear. More specifically, the air governor outside seals fail, after which they can’t build up enough pressure.
In other cases, the inside valve seals fail, resulting in an air compressor that’s always off. Also, the air governor sticking can happen because of internal failure or the air governor valve spring break. In the latter two cases where the spring breaks or the valve sticks, the air governor can’t disengage the air compressor, after which the safety valves will activate, or the compressor will stop cycling.
How Do You Fix a Bad Air Governor
Fixing an air governor is a job that requires special tools and skills, so it’s only done by highly equipped and experienced refabrication shops. So, your only option is to buy a new air governor and replace it or buy a remanufactured one instead, but there is no fixing it by the side of the road.
Luckily, even brand-new OEM air governors rarely cost more than $120, and you can pick up remanufactured ones for about 25 bucks. Moreover, they are pretty accessible in most cases, so replacing an air governor shouldn’t take more than an hour which comes to $75 at the national average. That brings the total cost to replace an air governor to $100 with a remanufactured one or $195 at most with a brand-new OEM one.
Q: How do you test an air governor?
To test an air governor, you need to monitor the air pressure gauge. Start the engine and watch the air pressure gauges climb to 120-135 psi, and if they keep rising past that pressure, the governor is faulty. But if they stop, pump the brakes until the pressure drops to 80 psi, and if the compressor engages and the gauges start climbing, the governor is good. But if the gauges drop below 80 psi, the governor is faulty.
Q: Can an air governor be adjusted?
Yes, virtually all air governors can be adjusted by tightening or loosening the adjustment screw on top of it. By tightening the screw, you are lowering the air pressure, and by loosening, you are increasing the air pressure.
Q: What controls the air governor?
The pressure inside the air tank controls the air governor. The air governor has an air pressure port directly attached to the air tank, and when the air pressure exceeds a certain limit, the spring in the air governor compresses and disengages the compressor.
Q: What is cut-out psi for an air governor?
The cut-out pressure for an air governor depends on the settings, but it should be roughly 30 psi higher than the pressure when the air compressor engages, which is 90 psi, so the cut-out pressure should be set at 120 psi. However, the exact pressure varies between truck models and state laws.
Q: Does the governor control the air compressor?
Yes, the governor controls the air compressor. Although the air compressor is always running when the engine is on, the air governor controls its intake valve, which determines whether or not the compressor is compressing air.
Ultimately, the bad governor symptoms are hard to miss, plus it’s pretty easy to test the governor in no more than a couple of minutes. Those symptoms include the air tank safety valve triggering, the compressor cycling too often or not often enough, and the compressor taking too long to build pressure after starting the engine.