HomeMods, Tech Talk & TipsProjects & ModsInstalling an SMI Air-Force-One Towed Vehicle Braking System On A Late Model Spartan Mountain Master Tag Axle Chassis

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This post is for those of you with a diesel powered/air brake coach that are considering DIY installing an SMI Air-Force-One air braking system for your towed vehicle.

I had previously completed a successful SMI Air-Force-One (AFO) installation on a 2006 Freightliner chassis (Fleetwood Discovery), but when we recently upgraded to a 2013 Entegra Aspire on a Spartan chassis, I was uncertain of the best points to tap for air. Now that I have done the research and completed the installation, I am posting this article with the intention of passing along what I learned along the way and perhaps provide some encouragement for anyone who might be undecided if they can handle it or not.

Why you should have a tow vehicle braking system: While many coach owners don’t consider a tow vehicle braking system a necessity, perhaps assuming the tow vehicle doesn’t really add that much more work for the much larger coach brakes to do, the fact is it is the only safe way to go. Consider:

  • It’s the law in all US states and Canadian provinces. Although the specific weight limits and requirements vary by state, for the typical vehicle towed behind a coach, a braking systems is almost certainly a legal requirement. And even if somehow it is not required in your state, it will be in the next one you drive through.
  • Insurance: In the event of an incident, your insurance is most likely not going to cover you fully if you are breaking the law.
  • Wear & Tear: A braking system will substantially reduce the wear and tear on the vehicle front end and on the towing equipment. It will also minimize damaging stresses on your coach hitch and chassis. The warranties on all of the aforementioned items may also be affected negatively by not using a braking system.
  • Liability: One of the worst case scenarios is a tow vehicle breakaway, however a good tow vehicle braking system will include a “breakaway switch” that will trigger upon separation and stop your vehicle. A “brakeless” tow vehicle, should it become separated (it does happen), is at a minimum going to do severe damage to itself as well as your coach. Think about it, if the tow bar system fails but the vehicle is still attached with the safety cables, what happens when you try to stop the coach with that loose cannon bouncing around behind you? Now if the safety cables fail too, your vehicle becomes an unguided missile that could kill someone and/or cause a pileup. Is it worth the risk of the lifestyle destroying litigation that would surely result from such negligence?

AFO2.jpgThe SMI Air-Force-One System (AFO) is one of only a couple of RV tow vehicle braking systems that can provide true proportional braking to the tow vehicle from a Diesel powered coach with air brakes. With the AFO, SMI provides DOT and FMVSS compliant equipment to protect and isolate the air system on your coach.

Theafo_01.jpg AFO system also includes vacuum assist to your tow vehicle brakes resulting in a soft pedal. So only a small, permanently installed and totally unobtrusive actuator is required in the tow vehicle to apply the brakes or stop the vehicle in case of a breakaway. Most other systems are electrically operated and use a bulky brute force device that must be removed to drive the towed vehicle and often results in damage to the tow vehicle seats and other components.

There are two parts to an AFO install:

  • Towed vehicle installation
  • Coach chassis installation

This post is about the Coach Chassis installation ONLY. It is specific to a 2013 Spartan Mountain Master tag axle built for a 2013 Entegra Aspire. However, the procedure is essentially the same for all diesel coach’s although the details might vary slightly with chassis production date as well as coach make and model. Note that Spartan builds a custom chassis for each coach maker that can vary in components and features, even between coach maker models.  However, most Spartan Mountain Master tag axle chassis of a similar vintage are going to be close enough in regard to the braking system that my procedure will be accurate.

Update 5/2016: 2017 Entegra Anthem owners take note. In April 2016 while at Entegra in Middlebury I got a look at a 2017 Anthem chassis and the changes made to it. One of those changes was to move all three air tanks to the rear of the coach (all 3 used to be in the front). Guess where they are now? Yep, just above the tag axle area, about where I describe installing the AFO Coach Assembly. I only got a look at the new location from the top of the chassis and not underneath, but it looked like the space in there might be too limited to squeeze in the AFO Assembly. 2017 and up Anthem owners will need to check this out and possibly find another spot to mount the assembly. 2017 Aspire chassis are now very similar to the pre-2017 Anthem chassis with the 3 air tanks still mounted up front so the mounting location I describe for the AFO assembly should still be fine. Likewise I would assume the new 2017 Insignia is similar to the pre-2017 Aspire chassis and that location will be fine as well. 2017 Anthem owners please let me know where you are installing the AFO Assembly so I can update it here.

Some key points about my approach to this installation:

  • I spent a substantial amount of effort researching and planning my install.
  • No parts of my Spartan chassis were permanently modified in any way. No air lines were cut and the installation is 100% reversible to factory as-built condition.
  • I now have about 22,000 miles on my installation with 100% reliability and zero issues.
  • I did all the work without a pit or a lift, right in my own yard (well OK, the campgrounds yard)
  • My 2013 Spartan chassis has factory installed push-to-connect air fittings on many of the air brake components. There are no crimp or compression fittings, at least on the major brake components involved with this install. Push-to-connect air fittings are similar to the push-to-connect plumbing fittings you may have seen in the hardware store except they are DOT approved for air brake use. If your coach does not have push to connect fittings, this project is certainly still doable, but that is going to make the job more time consuming and may require special tools or cutting of the primary air line.
  • I talked to SMI at length about their AFO and good places to tap into the air lines. We discussed the “official” tap points that Spartan recommends as well as other locations that are equally viable and in this case are actually preferred. Note that for obvious reasons SMI does not officially condone any air tap points that do not meet the current Spartan recommendation.
  • Before doing my AFO install, I also talked to Spartan – twice in fact. Once on the phone and again in person at Charlotte, MI – and both times the service manager there stated that the Spartan “towed vehicle air kit” (search on “S-1628-001”) was redundant if I was using an SMI AFO and that the AFO was actually significantly easier to install than the Spartan equivalent kit. He clearly stated I did not need both and he recommended I go with the AFO especially if I was going to DIY it.
  • After doing my install, I subsequently attended a week long Spartan Owner Training in Charlotte, MI and discussed my AFO installation with Mike O’Neil the instructor, plus two other senior technicians. All applauded the AFO, my installation and choice of air tap points as being ideal to protect the air system on the coach.
  • I have read many of the forum discussions regarding the AFO kit and whether it voids the Spartan warranty or not. Suffice it to say that I have done the research, asked the questions, come to my own conclusion, gone ahead with the AFO install and essentially had it blessed by Spartan. I am totally confident in my installation, its safety and my warranty compliance. You will need to decide for yourself.

Can you do it yourself? Yes you can if you have moderate mechanical skills, can crawl around and under your coach, wriggle yourself into tight spaces, have a modicum of patience, pay unerring attention to safety, and don’t mind getting your hands – and most of the rest of your body – dirty. Now that I have done it once, the next time it would take longer to jack up and safely block the coach than it would to do the installation.

What tools are needed? Nothing special. A basic socket set and a set of wrenches or adjustable wrench. A PVC pipe cutter or similar tool that will give you clean square cuts on the plastic brake line is helpful. Wire cutters or similar to clip the excess from any Ty-wraps that you use. Major drilling should not be required, but is possible if you can’t find a good spot with pre-drilled holes to mount the AFO Assembly. You may need to do some minor drilling to mount the bracket with the quick connect fitting at the rear of the coach.

You MUST safely block up your coach!  It is of utmost importance that you do a good safe job of preparing the coach for safety before you crawl under it. You will primarily need access to the area between the forward end of the transmission and the back of the tag axle and you will need to wriggle up and over the drive shaft slightly for some operations.

It is beyond the scope of this article to describe to you in detail how to safely block up and prepare your coach, but obviously you must make sure you secure it in such a way that nothing (like releasing the jacks) will allow the coach to change its elevation!  If you don’t know how to safely block your coach or have any reservations for any reason, DON’T DO IT, hire somebody with the proper skills.

You MUST drain all your air tanks! Since you will be working with and disconnecting the primary coach air supply lines you should release the pressure from all your air storage tanks. There are usually three air tank drain lanyards. The lanyards are wire with a loop in the end that protrude thru the chassis rails. On my Entegra Aspire, one lanyard (silver color) is found in the rear upper part of the passenger side tag wheel well. The other two (red and green) are located together just about even with the forward edge of the front tire on the passenger side – look under the coach between the front tire and your entrance steps and up toward the chassis rails. On Entegra Anthems and other coaches with IFS (Independent Front Suspension) all three lanyards are probably in the front. The front mounted lanyards might be a bit difficult to get at if the front of your coach is too low, if that’s the case a stick with a hook will help. Gently pull each lanyard and you should hear air escaping, hold it open until all the pressure is released.

IMAG2568.jpgA note about “push-to-connect” air fittings: Push-to-connect air fittings are similar to the push-to-connect plumbing fittings you may have seen in the hardware store except they are DOT approved for air brake use. To connect brake line tubing to the push-to-connect fitting firmly push it in, you should feel two ‘clicks’ or two levels of slight resistance – that’s it! To remove the brake line tubing, push in on the release ring (toward the body of the fitting) then pull on the tubing and it should come apart without too much effort.

Disclaimer – Proceed at your own risk. While I believe my information to be accurate and valid please note that if you decide to follow my lead you do so completely at your own risk. I am not an authority on SMI, Spartan or air braking systems nor am I a professional in any of those fields. Any actions you take or modifications you make to your coach and/or towed vehicle require due diligence on your part and all results therein and thereafter are completely your responsibility.

Basics of the AFO coach assembly: The AFO kit may seem complex at first, especially if you (like me), are not intimately familiar with air brake systems. That’s OK, you don’t need to be an expert to do this install. Basically the AFO coach assembly protects and preserves your coach air supply should something detrimental happen to the air lines between it and your tow vehicle.

There are three air lines that need to be connected to the AFO coach assembly:

  • AFO3_anno.jpgA 1/4″ Delivery Air line that runs from the AFO Assembly to the rear of your coach where you install a bracket with a quick-connect fitting that connects the air supply to your toad.
  • An air tap into the coach Primary Supply Air. This is basically the full pressure air that is stored in air tanks on your coach. The tap is a ‘T’ that will be inserted into a 5/8″ air line or threaded into the Service Brake Air Relay. A 1/4″ air line is run from the ‘T’ to the AFO Assembly.
  • An air tap into the Metered Supply Air. This is the source of the “proportional” air that will get passed thru the AFO Assembly and apply your toad brakes proportional to your coach brakes. This tap is a 3/8″ threaded 90 degree fitting that will be installed in the Service Brake Air Relay. A 1/4″ air line is run from the tap to the AFO Assembly.

How I selected the air tap points and AFO Assembly mounting location:

Spartan/AFO instructions in the AFO manual say to mount the AFO Assembly near the primary drive axle and tap into the Service Brake Air Relay there. Here’s why I did not heed that advice:

  • IMAG2580.jpgIt is extremely tight around the primary drive axle. The tag axle and numerous other assemblies make human access here very difficult. Without a lift or pit I did not see any reasonable way to wriggle into that area, and even if I could, I don’t think I would be able to move my arms into position to do what needed to be done.
  • Tapping into the Primary Supply Air in this area may require cutting the factory air lines because of clearance issues.
  • The Metered Supply Air connection needs to tap into the line between the Service Brake Air Relay and the Spring Brake Quick Release Valve. On many Spartan’s that connection is not a flexible air line but threaded fittings. First off, it will be a job to un-thread the existing fittings and insert a ‘T’ for the tap (other parts will probably need disassembly in order to do this). But worse yet, the threaded port faces down and the extra length of the inserted ‘T’ will extend the whole assembly downward at least another 3-4 inches. That may not leave enough clearance and the assembly may come in contact with the drive shaft, rear end, axle or other parts when the coach is lowered during an air bag dump. Not good.
  • Again, because it is so tight in this area it is hard to find a good spot to mount the AFO Assembly where it will have enough clearance when the coach is lowered.

So instead I did my install just behind the tag axle:

  • ItIMAG2575_anno.jpg‘s much easier to crawl into this spot just forward of the engine and transmission, with relatively easy access to the tag axle Service Brake Air Relay.
  • There is a convenient chassis brace above the transmission end of the drive shaft and universal joint where the AFO Assembly can be mounted without drilling any new holes and that is immune from harm when the chassis is lowered. At least on my Entegra there was plenty of room above this brace and the bottom of the coach house floor.
  • The Service Brake Air Relay on the tag axle is for all intents identical in appearance and function to the Service Brake Air Relay on the drive axle, it just controls the brakes for the tag wheels and is synchronized with the brakes for the drive axle. The coach supply air line ends here too, so it is even easier to tap into. And best of all, the metered air can be accessed by simply removing a 3/8″ NPT plug from an unused port on the bottom of the Service Brake Air Relay and replacing it with a 90 degree push-to-connect fitting.
  • No clearance issues! No cutting of air lines! 100% reversible to factory condition!
  • UPDATE – in the 10/19/2015 version of SMI’s installation instructions for tag axle Spartan’s, they now recommend the installation on the Tag Axle Service brake Air Relay as I have described here and all parts are included.

Additional parts you will need to buy. Besides the parts included in the AFO kit you will need:

  • DOT approved 90 degree 3/8″ MPT to 1/4″ (O.D.) air line push-to-connect fitting. The part I purchased was Tectran PL1315-4C but any brand will do as long as it is DOT compliant – here is another example. UPDATE as of the 10/19/2015 SMI installation instructions a non-90 degree version of this part is now included in the SMI kit.
  • Short length of DOT approved Type B 5/8″ air line tubing. This part can be eliminated if you replace the threaded fitting on the Tag Axle Service Brake Air Relay with a threaded ‘T’ (see Note below in step 2).
  • Split loom tubing (1/4″ I.D.) to provide extra protection to the 1/4″ (O.D.) air lines you will be installing. Available here for example, or locally at most hardware stores.
  • Assorted sizes nylon cable ties. Cable ties with UV protection are preferred – these are usually black.

I was able to buy the DOT fitting and a one foot length of the air line at Pioneer Heavy Duty Parts near me in Seekonk, MA. I paid $5.90 for the fitting and $2.40 for one foot of air line. NAPA is also a possibility, but the local NAPA store I tried did not stock either of those parts and I would have had to purchase a quantity of 10 air fittings and 50′ of air line – no thanks! Your best bet may be to find a local truck part supplier, or search for an online supplier. However, very few online suppliers seem willing to sell you a short cut length of the tubing. Another possibility is a service shop that may have some scraps of new air line in the trash bin – you only need 3 or 4 inches.

IMAG2569_anno.jpg1) Mounting the AFO coach assembly: OK so here we go, lets get the AFO Assembly mounted first. As mentioned above, I mounted mine to a cross-member above the transmission end of the drive shaft. The cross member is a part of the frame and not the suspension, so the clearance between it and the underside of the coach floor remains constant whether the chassis air bags are empty or full.

IMAG2572_anno.jpgI used existing holes in the cross member and the AFO mounting plate and mounted it with two of the 3/8″ bolts supplied in the AFO kit. For orientation when looking at the photos, the AFO mounting plate is horizontal and the tank and other parts are sitting above the frame cross member. The part sticking out of the AFO Assembly that looks like a small LP gas grill regulator is facing toward the front of the coach.

UPDATE – in the 10/19/2015 version of SMI’s installation instructions for tag axle Spartan’s, they now recommend the installation on the Tag Axle Service brake Air Relay as I have described here and all parts are included with the SMI kit.

IMAG2575_anno.jpg 2) Tapping into the coach Primary Supply Air: Primary Supply Air is provided via the 5/8″ (largest diameter) tubing entering the top of the tag axle Service Brake Air Relay. On my coach the tubing was a green color. For this connection I used one of the brass ‘T’s (with  two 5/8″ push-to-connect fittings and one 1/4″ push-to connect fitting) that comes in the AFO kit, plus a short piece of purchased 5/8″ DOT brake air line. IMAG2584.jpgPop the 5/8″ factory air line out of the Service Brake Air Relay by pushing in on the release ring and re-connect it to one side of the ‘T’. Cut a short section of the purchased 5/8″ brake line (make your cut even and square) and insert into the other side of the ‘T’ – this cut piece should be as short as possible, but long enough to get a grip on it for insertion and removal, perhaps 2″ or 3″. IMAG2589.jpgNow insert the remaining end of this short section of 5/8″ airline into the Service Brake Air Relay. Because of the length of the ‘T’, the factory 5/8″ air line is now longer than it needs to be so make sure you reposition it carefully so it doesn’t kink or chafe. Was that easy or what? You now have your coach air supply tap completed – and with no cuts needed to the factory chassis air line!

Note: After visiting Spartan for Owner Training they suggested a simplified approach. Instead of inserting the “T” in the air line, instead remove the factory installed 90 degree fitting for the Primary Supply Air on the Tag Axle Service Brake Air Relay and insert the ‘T’ in its place  – the ‘T’ will have a 3/8″ NPT nipple in the middle, a 5/8″ quick connect on one end and a 1/4″ quick connect on the other end. This approach is even better as there are fewer connections and the primary air line remains at a perfect length. I believe all the parts needed for this step are included in the AFO parts kit (you will still need to separately purchase the 90 degree fitting needed for the Metered Supply Air).  Be sure to use an approved air line sealant on the threads.

IMAG2577.jpg3) Tapping into the Metered Supply Air: For this operation we just need to remove the plug in the bottom of the Tag Axle IMAG2585.jpgService Brake Air Relay. No parts from the AFO kit are needed, instead use the separately purchased 90 degree 3/8″ MPT to 1/4″ push-to-connect fitting. The threaded end of the fitting should already have thread sealant applied. Tighten until the push-to-connect fitting is pointing in a direction that will allow unobstructed routing of the 1/4″ air line to the AFO Assembly. OK, the worst is over!

IMAG2590.jpg4) Connect air lines from the taps to the AFO Assembly: Just follow the instructions in the AFO manual to connect 1/4″ air lines IMAG2597_anno.jpgbetween the tap for the Coach Primary Supply Air to the proper port on the AFO Assembly and the tap for the Metered Supply Air to the proper port on the AFO Assembly. Route them carefully to avoid any chafe and cover them with split loom for extra protection. Secure with cable ties as needed.

IMAG2604.jpg5) Run a 1/4″ Delivery Air line from the AFO Assembly to the rear of the coach: Again following the AFO manual, run 1/4″ air line (supplied in the AFO kit) from the proper port on the AFO Assembly to the rear of the coach. Just run the plastic tubing along the wiring harness (I used the passenger side) IMAG2612.jpgunder the engine avoiding any hot areas like the exhaust manifolds etc. I also wrapped this line in split loom for extra protection. Mount the bracket with the toad quick connect fitting near your hitch and connect the 1/4″ tubing to it. I used  a piece of angle aluminum and connected the bracket and fitting to my 7-pin tow vehicle electrical connector (see photo).

6) Check your installation for leaks: Now air up your coach until the purge valve pops, shut down the engine, check your blocking for integrity and then go back with some soapy water to check all your connections for leaks.

7) Testing: Assuming your towed vehicle installation is complete, from here just follow the final testing procedure in the AFO manual and you’re done!


Installing an SMI Air-Force-One Towed Vehicle Braking System On A Late Model Spartan Mountain Master Tag Axle Chassis — 6 Comments

  1. This was a fantastic post and made installing on my 2013 Entegra Anthem very easy

    Hardest part for me being 6’2 and 200 pounds was to get up above the tag to hook up the primary air hose. Other than that the install was flawless

    Thank again

    • Glad to be of help Phil! Yes, its really tight in there, but it would be much worse if trying to do the install over the drive axle!

  2. I wanted to add one more thing. There are a total of 3 air tanks on a 2013 Entegra anthem that are located just in front of the front passenger tire

    If you lay flat and crawl under the coach where the steps are. Look up and you will see 3 air tank cables with loops on the end. Tie a short rope on each loop and pull them out past the tire. Once you are out from under the coach pull each rope till all the air is out of the tanks

    Once all the air is emptied you are safe to move forward with your install

    • Thanks Phil! Yes, on the Anthem all three tank drain lanyards are in front of the right front tire. On the Aspire, two lanyards are in that spot and the third is in the passenger side rear wheel well (at least for 2013 models, if anyone finds otherwise for later years please post a comment here).

  3. Phil,
    This was a great post, I’m glad I found it. Using your instructions I just installed the Air Force One on a 2017 Aspire. All three tanks are still at the front. I did one thing different and that was to mount the AFO Assembly at the rear of the coach on the left side of the engine compartment just below the air filter. Much easier to get to. thanks again for your detailed description of everything, it made this job so much easier. I didn’t see your email but if you want I can send a picture of where I installed the AFO Assembly.

    • Rob, thanks for the info! I will be doing the install on a 2017 soon as well! Would love to see the photos. I’ll send you my email. BTW I am also a Rob, the original writer of the post. Thanks for the feedback!


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