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How to Service and Maintain
Needless to say, brakes are one of the most important components of an RV chassis. The safety of your passengers relies upon the proper performance and maintenance of this critical system. Brakes that get overheated can glaze. There are procedures to help restore brake pads that are lightly glazed but if they are seriously glazed they just won't stop as the should and the shoes will need to be replaced or relined. Freightliner publishes a procedure for burnishing lightly glazed brakes and I've posted a copy of that procedure at This Link.
In addition, the brakes need to be kept in adjustment. Air brakes operate by applying air to a diaphragm, which then pushes a rod to a link that rotates a shaft. There is a set of curved cams at the end that expand the brake shoes. As long as this mechanism is properly lubricated, it will continue to apply the brakes. Be sure to follow your chassis owner's manual for exact locations of the grease fittings that lubricate your brake components. The linkage that connects the air brake push rod to the braking cam shaft is called a "slack adjuster". This arm is adjustable and is the only real adjustment you'll ever need to make to your brakes. In the old days, you'd block the wheels and release the air brakes (if a spring loaded parking brake), then tug on the slack adjuster to pull the rod out of the brake diaphragm. You'd tighten the slack adjuster and then back it off one full turn, which would give you an inch or less of free play when you tugged on the arm. Naturally, all of the wheels needed to be the same or else you would have brakes pulling to one side instead of consistent, even braking.
Today's air brakes use self-adjusting slack adjusters. If everything is working fine, they should self-adjust automatically as you apply the brakes and shouldn't need any adjustments. However, you never know if they are working or not so it's still best to test them to verify that the slack pull is equal on all unit. The rear drive axle uses spring loaded park brakes. If there is no air pressure these brakes will lock up and won't release until air pressure builds up and you release the park brake knob. In order to test this axle you'll need to block the coach, use the jacks, or whatever to keep it from rolling. Then release the brakes to check that axle. The front axle and any tag axle brakes are not spring loaded so they can be checked or adjusted with the parking brake applied.
In order to operate your air brakes and air-ride suspension system you will need to supply it with some air. An air compressor is mounted to your diesel engine to provide plenty of air to operate your brakes and suspension. This compressor sends air out to air tanks that act as reservoirs so that there is an adequate supply during every condition, such as repeated heavy brake applications. But, this air needs to be clean. If there is moisture in the air lines in can freeze up in cold weather. The compressor is a mechanical pump so it is possible that it can consume oil during it's normal operation. Don't' worry, it's fed by your diesel engine's oil supply so you won't have to worry about having to check your compressor's oil level or change it. But, small amounts of that oil can get passed into it's air discharge and wind up in the braking system where it can damage the rubber diaphragms and components. In the old days truckers would always drain their air tanks just before a trip to remove any emulsified oil and water sludge from the tank. This was a daily task and necessary to keep the air brakes in proper working order.
Fortunately, heavy duty vehicles now are equipped with air dryers. The air dryer is located near the rear of your coach. It is connected so that all of the air compressors output passes through the air dryer prior to reaching any air tanks. The air dryer will use centrifugal force to spin the heavy contaminants down into a sediment bowl. A coalescing filter will prevent any water or oil from passing through the dryer. This moisture and sludge will also settle in the bowl. To prevent the bowl from filling up an automatic dump valve is incorporated that will dump the contents every time your air compressor reaches it's governed air pressure. This quick burst will expel the contents out through the port on the bottom. An electric heating element is also used to prevent the bowl or it's contents from freezing in cold weather.
Eventually the air dryer will fail to do it's job once the coalescing filter is all used up. Some air driers have a service schedule of 18 months while others have a 3 year cycle. You will need to verify your manual to see which one you have to see when you need to service your dryer and replace the filter element in it. Note that this service schedule is based upon expected life expectancy. If you operate your RV in dry climates you may get longer life out of your filter but if you operate in warm humid climates you may not reach the average cycle time. That's why it's still important to drain your air tanks. You don't have to do this every day but it still needs to be done. Except, this time you really are not draining the contents of the tanks as much as you are testing them. Normally, when you pull the tank drain lanyards you'll hear the spit of dry air exiting the tanks. But, one day you'll notice that there's a fair amount of moisture coming out. When this happens you'll now know that your air drier filter is no longer doing it's job and it's time for service. So be sure to make this tank drain procedure a regular part of your weekly or monthly inspection rather than wait until the published service life is reached. Once the sludge gets into your brake diaphragms and valves you will have problems begin to develop so you want to prevent that from happening.
After the air leaves the air drier it then goes to your air tanks. Air tank layouts can vary from chassis to chassis but the basics remain the same. All of the air goes first to the primary tank, also called the wet tank. Any moisture still in the air will expand and condense, settling in this tank. Hopefully no moisture or sludge will pass beyond this point. After it leaves the primary tank it goes to the secondary tanks. Generally there are two secondary tanks. One is for the front brakes while the other is for the rear brakes and air suspension. They are separated by check valves so that a failure in one system won't impact the other system. After that there may be a ping tank or whatever to help support the air bags at the rear of the coach. There are many variations to this layout but you get the general idea. Compressor first, drier second, primary tank third, and secondary tanks, etc later. Most coaches have three drain valves, one for the primary tank and one each for the secondary tanks. Generally they are operated via cable lanyards that can be found in the front wheel wells. It's best to fill the air bags to raise the coach when looking for these. Always use the awning hook to pull these lanyard rather than reach into the wheel well with your arm. If the coach were to drop you could lose your arm.
Neway IFS Suspension
There's two components to your front suspension - the air bag portion that actually suspends your coach and the mechanical steering linkage that controls the front wheels. Your air bags are controlled by a series of valves. There's a dump/fill valve that either provides air to all of the air bags to fill them up or dumps them all to allow the coach to drop down and settle. This valve is generally controlled by the hydraulic leveling jacks control panel so that it automatically dumps the air from the suspension when the jacks are deployed and fills them back up when the jacks are raised for travel. After that there is a series of height control valves, generally three but sometimes four, that control how much air pressure is sent to the bags so that they achieve the correct ride height. If a heavier load is applied to the coach in any given are it will settle and depress the arms on the appropriate height control valve, opening it up to allow more air into the bag. Once the bag has lifted the coach to it's proper height the arm will close off the valve to keep that amount of air in the bag. If the coach were to rise up in response to s reduction of load, the height control valve would move in the opposite direction and bleed some of the air out of the bag. Once the coach settled in at it's correct height the valve would then stop bleeding air and seal off the air pressure in the bag once again.
If the height proportioning valve is not properly adjusted your coach may wobble and wander all over the road. If you have a handling issue the first thing to check (after your tire pressures of course) is the adjustment of these valves. Do this prior to any front end alignment. This check should also be part of an annual service procedure. Refer to your chassis owner's manual for specifics on how to check this but for a quick example check out This Link in the RV Tech Library to help explain this better.
Outside of your air suspension system, there's lots of mechanical parts that will also need attention. The most important item is a good grease job to prevent all of the moving parts that control the suspension and steering from wearing out or seizing up. For this you'll need a good grease gun that you can operate with one hand. Be sure it has a flexible hose so that you can navigate around to the top of those hard to get at grease fittings. You'll need one hand to guide the end of the hose and the other to pump the gun. Use a good quality grease. Going cheap in this department is going to cost you later. I prefer a good moly based grease. The moly additive really helps keep the grease from squishing out under load. However, there is one place on your RV that you do want to use a cheaper non-moly grease and that is your self-adjusting slack adjusters. They need a certain amount of friction or else they won't self adjust so you may find it handy to have two grease guns and load one up with each kind of grease. There's only a couple of these fittings so you don't need a real costly grease gun for them.
Be sure to read your chassis service manual carefully and then make a note of where all of these fitting are. Put them on your checklist so that you can check them off as you do them. You don't want to miss any because it only takes one worn out part to sideline your RV. If you have a solid front axle be sure to jack it up to remove weight from the tires when greasing the kingpin bushings. If you leave the coach weight on the axle you won't get any grease around the thrust washer bearing and only half of the vertical kingpin will get greased. Actually, whenever possible it's best to have someone sit in the cab and turn the wheel as you grease. That way the grease totally surrounds everything and there are no bare spots. This helps for IFS ball joints as well as solid axle kingpins.
Stemco Front Hub Cover
Your wheel bearings are lubricated in one of two methods. The drive axle is lubricated via the gear lube from the rear axle. While initially filled during assembly it tends to keep it's own level due to the internal movement of the rear axle gear lube. It is sealed off on the outside by a gasketed rear axle flange and on the inside by a typical wheel seal. The front wheel hubs don't have any extra source of gear lube so they need to be properly filled and maintained. The front wheel hubs will have a hub cover, very similar to the one in the above image. The rubber plug in the center is easily removed with a screwdriver. It is also vented so be sure that the small pinhole in the center isn't plugged or else the pressure can build and push gear lube out past the wheel seal. You'll see a clear plastic window in the end of the cap with circular lines molded into the window. This represents the full and add levels of the gear lube in the hub. Think of it as a rotating dipstick. If the oil level is between the two lines you are fine. If it's too low, remove the rubber center plug and add some gear lube with a quart squirt bottle. Allow it to settle a bit and recheck again. One it's all set, replace the rubber plug. Be sure to refer to your chassis manual for the proper weight gear lube to use for your particular chassis. Note that tag axle wheel hubs are setup in the same manner.
Normally You have two potential places for any wheel hub to leak gear lube. Once it does leak it tends to sling the stuff everywhere and pick up dirt so it's very hard to tell exactly where the leak is coming from. In that case it's beneficial to clean the area and do short drive and then inspect the area carefully. If your leak is coming from the wheel seal it'll sling out of the backside of the hub and you'll see it on the rear of the tire. If you see an oily mess inside your hubcap and wheel rim, it's not the rear wheel seal. In this case it's most likely a gasket. A drive axle can have a leaky flange gasket or a steer or tag axle can have a bad gasket on the aluminum hub cover casting. Once these leak it's pretty much impossible to stop the leak by just tightening it. You'll need to replace the gasket. I prefer to add a good gasket sealant to the gasket as well because the thin gaskets can use the help in that area. If you have one of these leaks it's also important to check the aluminum retaining ring that holds the plastic inspection window in place. It's held in with a ring of 6 small Torx head screws. There is a small white elastic gasket that probably cannot be salvaged once the screws work loose. In this situation I've removed the retaining ring and applied clear silicone as a gasket and reassembled with good success.
Your manual probably gives you a schedule whereby the gear lube needs to be changed. To do this you'll properly need to remove the hub cover, then suck the gear lube out of the hub cavity, replace the gasket, and reassemble. If you rotate the hub cover so that the hex head plug faces down you won't be able to drain all of the gear lube from the cavity. If you do it this way be aware that you are only changing half of the gear lube.
Rockwell Rear Axle
Your rear axle is filled with gear lube to keep the axle gears and bearings all properly lubricated. An access plug is located in the axle to provide a place to add lubricant as well as check it's level. Any gear housing filled with lubricant is typically filled to one third capacity, but never over half. If the entire housing were filled up there would be no room for expansion and the foaming lube would blow the seals out. Stick your finger in the "level" hole and you should feel the gear lube come right up to the threads. Refer to your service manual for the exact location of the plug. Some are on the front by the pinion and some are in the rear cover. To drain the axle fluid a drain plug is probably located at the base of the housing. Gear lube is fairly thick so it will help if you've just run the RV for a short while to help heat up the gear lube so that it drains better. Follow the manufacturer's recommendations and add the proper amount and viscosity. When you get close, keep an eye on the sight hole to prevent overfilling.
Driveshaft and Universal Joints
The driveshaft on a diesel pusher is fairly short and hard to get to, especially if you have a tag axle. But it is critical that you grease it regularly just the same. Each universal joint will have two grease fittings, opposite each other. Grease them until you see fresh grease oozing out of the bearing caps. Be sure to wipe off any excess grease to prevent it from slinging around when the driveshaft rotates, and making a mess for you to crawl around in next time. There is also a splined slider joint that allows the driveshaft to change length as the suspension moves up and down. Give this fitting a couple of pumps of grease but that's it. If you keep pumping away you'll have grease all over the place and this joint slides slowly and isn't as heavily loaded as the needle bearings spinning around in the universal joints.
Hydraulic Fluid Reservoir
Your coach relies on hydraulic oil to operate the power steering system. This oil is stored in a reservoir at the back of the coach and generally looks like the above example. An engine driven gear pump circulates this fluid through an oil cooler and eventually the power steering gearbox up front. If you have a rear radiator the fan is belt driven and may or may not be controlled by a fan clutch. However, if you have a side mounted radiator your fan will be remotely located and not very easy to drive with a belt. Some chassis do use a jackshaft and gear reduction boxes to allow a belt driven fan drive system but the majority of side radiator coaches use a hydraulic fan motor that is driven off the same hydraulic pump, cooler, and reservoir as the power steering.
Regardless of whether your hydraulic system drives a radiator fan and power steering, or just the power steering, it will need occasional service. There is a dipstick at the top of the reservoir for checking hydraulic fluid level. Various fluids have been used on various chassis. Some use a Dexron type automatic transmission fluid, some use 15W-40 engine oil, while some use dedicated hydraulic oil, such as an AW46. Check your owner's manual for the proper fluid as well as for the service schedule for that system. The reservoir does have a mesh type steel strainer filter inside that needs to be changed, most likely every other year. In addition, a spin-on hydraulic filter is also provided. This filter typically is replaced annually. On that filter you will find a filter restriction gauge. If this gauge goes into the red zone while the engine is running your filter is plugged and should be replaced regardless of whether the service schedule says it's due or not.
Replacing these filters can get messy. There usually are no shutoff valves present so once you spin off the filter, you'll have oil pouring out of the housing. You need to be quick and then clean up or else you can drain the reservoir first to minimize the mess. If you are replacing the reservoir strainer you'll need to drain it anyway. In that case it's easier to fill the reservoir up most of the way with the cover removed until you get close. Then reinstall the cover and use the dipstick to check your oil level as you finish topping off the system. Run the engine for a bit to purge any air out of the new filter then let it sit and rest while the foam dies down before rechecking the oil level.
Tires are another important safety consideration. Tires die when they fail to hold air, either because of a puncture or blowout. Tires need to be properly matched to the load that the RV will place upon them. If the tires are properly matched, and the correct air pressure is maintained they should last a long time, barring any road hazard damage. Over the road truck tires generally are replaced when their tread is worn to an unsafe level. But RVers don't put on that many miles in a year so RV tires typically get replaced due to their age. Tires contain lubricants that keep the rubber in good condition. These lubricants are released as the rubber flexes during normal driving. If the RV is parked for long periods of time these lubricants don't get released and the rubber dries out. Sunlight also attacks tires and produces ozone, which reduces a tire's life cycle as well. While the effects of non-rotation and sunlight vary from coach to coach, depending on the environmental conditions, a typical life expectancy for most RVs is 6-7 years. After that you are running ion borrowed time and the chance for a blowout is increased.
So, how do we maximize our tire life? First of all, driving the coach helps. Sitting in one spot doesn't release those rubber lubricating compounds found in the tires. When you are parked, try to keep them out of direct sunlight. Parking in a shady area is a plus and tire covers can also help. There are various tire dressings out there but you need to be careful. Making a tire look good or shiny won't help it's life any but keeping it clean of oxidation and chalking will. Many tire dressings have chemicals, such as petroleum based products, that can actually harm the tire. Be sure you get a tire that's designed to safely feed the rubber with nutrients rather than attack it. Proper tire "nutrition" can help keep the rubber (especially the sidewalls) from drying out and causing blowouts. Dry rot is a large cause of blowouts due to sidewall failure. You can read the DOT date code on your tires to determine their actual age. Check out This Link for a breakdown on how to read those codes. Furthermore, Michelin Tires has a statement on how long RV tires can be expected to last at This Link and it's interesting to note that no matter how well they are taken care of, 10 years is the maximum.
The second major cause is underinflation. Any tire needs to have a given amount of air pressure to carry a given amount of weight. As the weight loading goes up, so must the air pressure. You could just pump the tires up to maximum inflation pressure but chances are your tires don't need that much air and it'll ride very hard. To find out what your tire pressures need to be you first need to weight your coach. A detailed account of how to do this can be found at This Link in the RV Tech Library.
SmarTire Tire Pressure Monitor System
Once you know what the pressures are you need to check them regularly. This is typically done with a quality tire pressure gauge, either a mechanical trucker's foot version or a good digital gauge. It's a daily pre-trip task although if you have a Tire Pressure Monitoring System (TPMS) you can generally skip that because the TPMS will alert you if any tire is low. A TPMS also gives you the benefit of a warning should your tire get a slow leak while driving. A slow leak may not be noticeable to the driver but the tire will lose it's ability to carry that much weight and will overheat, which generally lads to a blowout. A TPMS will give you enough advance warning that you can deal with it before it gets to the critical stage. If you use your coach's on-board air connection to fill your tires but you will need to set the cruise control to fast idle to give the compressor enough RPM to keep up with the air demand.
Generator service is similar to your engine maintenance only smaller and with a few extra changes. As always, you need to check your generator manufacturer's manual to find out filter numbers and oil viscosity and rating and coolant type used. Once you have this info be sure to change your generator's engine oil regularly. Working inside the confines of a generator enclosure can be challenging at times but the principles are the same as your RV's engine. You'll probably need to buy that small oil filter wrench to remove the oil filter and Tiffin has been known to install preliminary fuel filters in the fuel hose just prior to the genset itself. This increases the life of the Onan fuel filter inside your generator's enclosure but you do need to know that it is there. It's standard automotive filter that's available at any auto parts store and it's the first thing you should look at if your generator suddenly develops a loss of power. Air filters vary as to ease of access, depending on which model of generator you have. Check your owner's manual for the exact location. The spark arrestor needs to be occasionally removed and the soot purged. Soot will build up in this area over time so removing the plug and running the engine for a bit to burp any soot out of the system will ensure that your exhaust system remains free and breathable. Accessing the radiator cap may require removing a panel for access. Again, check your manual for specific details.
In addition to the engine, you also need to take care of the generator itself. The generator is belt driven but these belts aren't a frequent failure so I wouldn't worry about them. If they do break it requires a great deal of work to gain access to replace them so you may want to pay a Cummins/Onan service center to perform that headache when, and if, that time ever comes. The best thing you can do for your generator is run it. Sitting idle won't do it any favors. Onan recommends running it under a minimum of half load for 1-2 hours every month. In fact, on a newer generator that's come in for warranty one of the first things they'll check is the hourmeter to make sure you have been running it. Running the unit will heat up the generator windings and burn out any condensation that's formed inside the generator, which is why you need to run it at a decent load for a long enough time. It'll also flex your belt to keep it from drying out and becoming brittle.
The Kubota engine typically used in Onans isn't a wet sleeved engine so it doesn't have the cylinder liner cavitation issues that your RV's engine has. Therefore it's not as picky about coolant and most heavy duty coolants designed for diesel engines will work. My personal preference is OAT coolant because OAT coolant doesn't tend to corrode aluminum like non-OAT coolants do. There's lots of aluminum in the generator's cooling system so it's a good choice. It also means that I have standardized on a single one coolant for both the genset and RV engine, which makes carrying spare coolant simple.
Servicing your own RV isn't all that demanding as long as you don't mind a little dirt, have some time on your hands, and have a bit of mechanical aptitude. You can save some serious money, perform a better inspection, gain a better knowledge of how your RV works, and gain the satisfaction of a job well done. The cost savings can buy some diesel fuel to help you enjoy your RV the way it was meant to be, by driving to your favorite getaway place.
Of course, this tutorial merely covers the chassis end of your RV. You still have the house portion to deal with as well. For helpful tips on how to service and maintain your RV's interior, exterior, and plumbing and electrical systems check out the following links:
This tutorial written 8/5/09