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Old 11-20-2020, 05:53 PM   #29
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Originally Posted by 777 Driver View Post
For me, the problem of using the engine driven air compressor isn't so much a function of the compressor operation as it is an air governor problem.

The air governor has a user adjustable (within limits) cut-out pressure. That is the high pressure limit of air pressure in the air tank. When that limit is reached, the air compressor stops pressurizing the tank. On my coach, that was set at the factory at 125 psi. At that point, the air governor does not start replenishing any air use which decreases the tank pressure until the pressure drops to about 90 - 105 psi (the cut-in pressure).

Trying to inflate a tire to 125 psi presents a challenge because that may be at, or below, the cut-out pressure of the air governor.

As I understand the air system, when you shut your engine down and the engine driven compressor stops, the air governor sees pressure drop below the cut-in limit (90 - 105 psi) and opens to allow the compressor to recharge the air tank. However, since the compressor has stopped, no air passes to the tank. A check valve prevents air loss from the tank and pressure in the tank remains at whatever pressure (somewhere between cut-out and cut-in) it was when you shut down. If you have any leakage or otherwise release air from the tank (brake application, as an example), the pressure in the tank will be below the cut-out pressure.

When you start the engine, the compressor again begins pushing air and the air governor has opened (it sensed below cut-in pressure at shutdown) and the pressure in the tank will increase to the air governor cut-out pressure.

So, if at that point you hook up your air hose and attempt to inflate your tire that's below the air governor cut-out pressure, air will start flowing into your tire.

However, for sake of discussion, lets assume that you want your tire inflated to 125 psi and it's currently inflated to 118 psi. Further, lets assume that your air governor cut-out is set to 125 psi and your cut-in is 105 psi. So, your tank pressure is 125 psi (you just started, the compressor ran, the air governor was open to allow air to pass to the tank until pressure reached the cut-out pressure of 125 psi, you heard the air dryer "chuff"). At that point, you connect your hose to the tire and air starts flowing from the 125 psi tank to your 118 psi tire. Eventually, equilibrium will be reached where the tank pressure and the tire pressure are equal and, importantly, somewhere between 118 psi and 125 psi. Still below your 125 psi target, but, above the 105 psi cut-in. And, there you sit. The engine running, the compressor spinning, the pressure still well above the air governor cut-in pressure.

Until the tank pressure drops below the cut-in limit, the air governor will not open and the tank will not re-pressurize to the cut-out limit (125 psi).

My drive and tag tires require 85 psi; no problem with those. My steer tires require 120 psi. Although that's "doable", it's a bit of a tug of war between the cut-in and cut-out.

Here's a link to the service data for a commonly used air governor:

Take care,


Investigate SMC Pneumatics air doubler. We use our coach air as input to doubler. Output of doubler goes to our hose reel. Super fast filling. Some folks have found used ones on Amazon. Yes, you do have to start the coach. Some folks donít want to do that but Iíve never seen anyone push their coach out of a park so they must start them at some point!
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Old 11-20-2020, 06:14 PM   #30
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Originally Posted by MSHappyCampers View Post
I use the on-board compressor. As for portable compressors I consider the VIAIR 450RV the best out there. Pricey, maybe, but you get what you pay for!
X2, I have tried several compressors and recently purchased the VIAIR 450RV and absolutely love the machine. I did extensive research prior to the purchase and haven't regretted the purchase at all.
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Old 11-21-2020, 12:37 AM   #31
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I started out 8 years ago using onboard air. My compressor range is 90-120, and coach needs 110 Fr and 95 R, so I was always having DW hitting the brakes to trick the compressor back on. And it just seems ludicrous to me to run a 3-4-500 Hp diesel for an air compressor.
After a year I bought the HF 150PSI, often on sale at $99. Works great.

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Old 11-21-2020, 10:11 AM   #32
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No on-board compressor with our gasser, so I bought a twin-tank compressor and stripped the tanks off. Works great, just needs 120 volts @ 15 amps. I just so happen to have a genset, so that's covered. around 30 pounds, but will inflate a 22.5 tire in a very short time.
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Old 11-21-2020, 10:30 AM   #33
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Like others with on board air, I tried airing up with the coach compressor. My current coach needs 120 psi in the steer tires and that was a little tough with the compressor kicking off @120. I bought a cheap 2 1/2 gallon compressor at Menards that has worked great. It's 150 psi max works great and is easy to store. I had twin Viar 450's in my Jeep for airing up the tires after a trail run. One was just too slow, so I added another one to speed it up. I recently sold that Jeep, but kept one of the 450s. It sits in it's bag on a shelf in my shop. I'm sure I'll find a use for it sometime.
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Old 11-21-2020, 10:39 AM   #34
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Not all compressors at "Home Depot" are the same.... many are created for portable 90 PSI air tools so like your on=board engine compressor, cut-in/cut-off parameters may be an issue for inflating higher pressure tires...and like many have said, just don't like running my diesel engine in the camp grounds to pump up my tires--but opinions vary....
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Old 11-21-2020, 04:01 PM   #35
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Originally Posted by Lt Dan View Post
I carry a Porter Cable CMB515 150psi compressor that works great. It only weighs 20lbs and adds air to the tires pretty fast. Others carry 12v Viair compressors but I found them slower than the PC.
Same here, my PC is a few years older, but has the same numbers. Got it on sale at Lowes for $100. Much easier and faster than the on board when parked at home, I don't have to start the engine to air the tires.
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Old 11-21-2020, 04:51 PM   #36
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viair rv 400 in the kit with hoses, etc. no problem keeping trailer tires at 110psi.
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Old 11-21-2020, 05:16 PM   #37
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Its a very good idea to carry a compressor when you "RV"
The best value is to have a compressor that will have a higher cutoff pressure than your tires require. My pancake compressor cuts off at 120, but cuts back in at 100, I have used it to fill my 100psi tire and it takes a while.
I had an issue with the schrader valves or valve stem or valve core leak when I checked my tires. This happened twice on two different tires. Luckily I had a valve stem with me and a small valve core tool. The issue is when replacing them almost all of the air will escape from the tire. Once I used my pancake compressor and the other time I was in the mountains and used the onboard compressor.
Whatever you have to use, it's better than not having anything.
A valve stem leak or a slow leak will never happen in the perfect locations.
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Old 11-22-2020, 09:56 AM   #38
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450RV here also. I use for the coach and for the jeep when I go off road. I have 31" tires and run them at 18 PSI on trail and 37 off trail. After airing down for the trail it takes around 2 minutes to air up 31`" tires from 18 PSI to 37 PSI. I run 115 in my RV front tires and the 450RV does a great job. I pull the Jeep up coach and and hook the 450RV to the Jeep battery and there is enough air line to reach all the coach tires.

Bad enough running the 450RV at 8:00 morning in a RV park but having to run coach trying to get the onboard compress to air up my front to 115 pounds. The coach air compress will drop below 115 several times while trying to fill to 115. You have to release air to get down to 90-95 PSI so the compressor will kick back in and fill back up to 120.

I had a compressor that would go up to 150PSI and worked great but it was 120 volt and I need something that would work on 12 volt while on the trails.
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Old 11-23-2020, 12:25 PM   #39
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Originally Posted by mljackson413 View Post
Do people carry a compressor on board a Class A
for tire pressure?
What brand would you recommend?
Tires call for 110 and 90 psi.
I carry a Viair 450 RV compressor and it works great. I use it to check my tire pressure before I leave on every trip. It hooks up easily to my engine battery, has plenty of hose to reach all my tires (including my tow car) and even blows up other toys!!
Once I had a problem with one of the hose connections. I contacted Viair and they sent me a new hose right away. Their customer service was fantastic.
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Old 11-23-2020, 12:36 PM   #40
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Originally Posted by KanzKran View Post
That's surprising, as the smallest engine compressors I'm familiar with are more than 10 cfm, and usually more like 15-16 cfm at speed. That's waaaay more than any small portable can deliver, even with the coach engine idling.
Actually the onboard air can't reach the pressure my front tires need, or at least it couldn't till I had HWH install their Active Air that needs 135 psi to work correctly. Even then it would require stopping and bleeding air off so the pump would restart.
Why run a 650 hp diesel when a small compresser does the job?
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compressor, air

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