Lesson 3: Chassis


Here we cover the difference between the chassis of your coach and the house portion. They are different systems and it is helpful to understand them as such. There are a handful of chassis on the market, but few enough that we can derive generalities among them to better understand our coach.

This knowledge is mostly helpful for troubleshooting and speaking with service technicians. It helps you have the right language and understand where exactly a problem might be emanating from. After this lesson, you might not see your coach the same ever again!


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All RV’s are built on a “chassis”. This is the frame that the coach sits on. It includes the actual frame, the wheels, braking system, suspension system (which includes the air compressor is equipped), and the chassis batteries. It also contains the engine and transmission.

RV diesel engines are often Cummins and often have an Allison transmission. Ford F53’s are usually all Ford components.

Although we often refer to all 3 of those components as the “chassis”, it’s important to note that each component has a different manufacturer and therefore a different warranty and a different service center for repair.

As a large RV owner, especially if you own a diesel, you will find that you are in the trucking world now!

At rest stops and gas stations, you’ll usually follow “trucks” arrows. Services can be 24 hours, but are often done at truck stops and service centers (Freightliner, Cummins, etc.)

You do not have to stop at ports of entry or weigh stations unless there are signs that specifically call out for RV’s to pull over.

Weight or axles: You will often see signs that restrict size and weight, such as “10 ton limit”. This is often to direct commercial traffic away from certain roads in order to protect them or to keep road damage limited to certain roads or lanes. These signs are often not intended for RV’s. HOWEVER, these signs also indicate weight limits for certain roads and bridges. If a bridge has a weight limit, it doesn’t care if you’re a truck or an RV – it’ll collapse just the same! So if you ignore these signs, do so with open eyes that you might be turning around if the road becomes impassible.

The components we cover in this section include: chassis support number, VIN, axles, bays, looking at the frame, house and chassis batteries, and battery cut off switches (both hard and soft switches).

Your house batteries are often made up of 6 or 12 volt deep cycle batteries. They are meant to charge fully and then be deeply discharged. They are different than your chassis batteries and should match. Batteries take a long time to charge fully. They will often ‘show’ fully charged, or a high number of volts rather quickly, but as soon as you put a load on them it will quickly discharge. This is the cause of many people thinking they are having problems with their batteries, but the truth is that they are probably not charging them all the way in the first place!

Your house batteries need to be maintained. Keep them filled with distilled water, usually about monthly. (There are systems you can buy to help with this – Flow-rite.) The house battery disconnect switch disconnects most of the power from the house batteries. But it does not disconnect all. There are phantom draws happening all the time: motherboards on the fridge, inverter, clocks, tv’s, etc. There is always a draw on the house batteries. This is why people come back to a dead coach after turning off the house batteries and leaving for a time. This disconnect switch is not a storage disconnect. It’s just a temporary ‘save my power’ switch (such as when you leave for shopping or a hike).

A full 12v battery is often actually 13V. Once it reaches 12.2/12.3 volts, it is considered 20% discharged (requiring charging). This is recommended. By the time you are at 11.8 or so you are 50% discharged and now doing damage to the batteries (shortening their life). If you have an AGS, be sure to set it to kick on gen at 12.2/12.3. If not, monitor battery status (either on panel or even better with a voltmeter) and charge them for at least 2.5 hours when it’s low.

Battery boost switch or ‘aux’ switch: this allows you to bridge the two battery banks together for use in starting the engine in case your chassis batteries die. There are rumors it can work the opposite direction for starting generators with dead house batteries.

Chassis batteries are charged when the vehicle is running. They are also charged by the system when it has a power source (generator, shore cord, solar). They are usually two 12 volt batteries, intended for deep cycle starting. But they are not the same kind of batteries as your house batteries. They are also usually maintenance free.

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