Bottom Line
It’s not dead yet
Resourceful APU owners can keep their units running - if the manufacturer goes belly-up

By Jeff Barker
contributing writer


Resourceful owners of out-of-production auxiliary power units can still improvise and keep them operating properly – even if there are no available parts and service support from the manufacturer.

There are a lot of truck owners out there who bought various brands of APUs over the years, which served them well for a long time. Unfortunately, in many cases when their units have problems and no parts for repairs are available from the possibly now-defunct manufacturers, they don’t know what to do to get them running again. With a bit of improvisation and resourcefulness it is possible to get those units to serve their intended purpose again.

Get to know your APU’s systems
Spending a little time getting to know your APU and its components will help you be able to obtain parts and do most – if not all – of your own preventive maintenance and repairs. Also, taking steps to avoid possible problems with it in the first place will help you have more peace of mind and a better night’s rest.

Engine cooling system: If you can, try to locate the manufacturer’s name and part numbers on the upper and lower radiator hoses as well as any other preformed hoses within the cooling system. Write the brands and numbers down and put them in a safe place. Also, make sure there are no coolant hoses rubbing on anything so there’s less of a chance of a premature hose failure from a hole.

Air conditioning system: If your APU uses an engine-driven air conditioner compressor, locate it and try to read the manufacturer’s name and model number. Also, locate all refrigerant lines throughout the system and make sure that none of them are rubbing on anything or will vibrate in such a way as to put stress on the fittings, which could cause them to break.

Alternator and starter: Along with the A/C compressor, try to obtain the make and part number for these items. It’s easy to take an alternator or starter in to have it rebuilt while at home. But if you need to replace one on the road, knowing exactly what you’re looking for will make it easier to locate the parts. Parts counter personnel will need those part numbers to see if that item is in their inventory.

Electrical system: Get to know where the various electrical components are located on your unit.

Relay bank: Most APUs have a relay bank that controls current flow to the A/C compressor clutch circuit, electric fuel transfer pump, engine fuel solenoid, condenser and radiator fans (if equipped), the starter motor, and the underbunk-mounted HVAC unit. Get to know what types of relays are used. The relay bank is usually mounted to the HVAC unit itself on newer APUs. But on some older Aux generators and Onan units, it is mounted near the top of the engine compartment inside the frame-mounted unit. Those are highly susceptible to corrosion and should be relocated or sealed inside a weathertight box.

Wiring harness: It helps to get to know how your unit’s wiring harness is routed and where its clamps, ties, and other restraints are located. Also, make sure the harness itself is not susceptible to rubbing through anywhere, as it would cause damage over a prolonged period. Make sure the battery cables are secure and tight inside the engine compartment as you wouldn’t want a positive battery cable to rub through and cause a fire.

Battery cables: Most APUs are set up to get their starting power from the truck’s batteries and then charge them up once its engine is running. With that in mind, it helps to keep an eye on those cables to be sure they’re not rubbing anywhere.

Engine: While a huge majority of APUs in use typically have Kubota 2-cylinder engines, units like the RigMaster have Perkins engines (which were often painted yellow and marketed in those units as Caterpillar engines). Other makes of engines are used as well. Get to know what you have and whether it’s a 2- or 3-cylinder engine. Locate a model and serial number on it and write it down in your owner’s manual. Also, locate the oil filter and write down the part number so it can be cross-referenced at an auto parts store.

Fuel system: Make sure your APU’s fuel lines are safely routed and that the rubber portions of them are not dry-rotted. Promptly replace any sections that are damaged or rotting out. Also, locate the fuel filter and try to get a part number for future reference. If you find that you can no longer obtain the fuel filter you need, it would be easy enough to do away with that filter base and locate an inline fuel/water filter with a water separator in it. I have seen these at places that sell parts for farm tractors and small diesel generators.  

Belts: Know how your APU’s belts are routed and how to replace them. Be sure to get the manufacturer name and part numbers off them. Those part numbers can be cross-referenced at an auto parts store to obtain replacements.

Got parts?
APU buyers often want to initially get their maintenance and repair parts from the people who sold them their unit. Of course, if the manufacturer of a particular brand of APUs goes out of business, that will present the owners of those units with a substantial challenge. With some knowledge about your unit and what to do in certain scenarios, you can overcome that obstacle and get your APU working properly once again.

As you become well acquainted with your APU and know who made the parts, it will become obvious that those items are also commonly used in automotive applications. For example, many APUs use General Motors and Ford components such as alternators and air conditioning compressors. Other APUs may use Sanden or Nippondenso A/C compressors that are used in many domestic and foreign applications. Regardless of what was used, you may be able to locate replacement components more easily than you think.

If you find a broken accessory bracket in or on your APU and can’t get a replacement, that will require more improvisation on your part. Fortunately, it is often easy enough to weld the bracket back together and reuse it. In other cases, it’s better to remove the bracket and have another one fabricated. If you’re handy with an acetylene cutting torch and a welder, you may be able to accomplish this if you pay close enough attention to the dimensions of the original bracket.

Do try to locate the cause of the bracket breaking apart while you’re at it and see if you can correct that problem. All too often, the initial cause of a cracked or broken bracket is bolts that work their way loose and cause excessive vibration. Use a few drops of Loc-Tite on the threads when reinstalling the bolts and be sure to torque them down properly.

Air conditioning hoses and fittings on APUs are common items that can be found at a facility that specializes in automotive air conditioning parts. If you need to have a new A/C hose made, remove the old one and have them make an exact replacement. Try to avoid reusing old aluminum fittings as they may not tighten down properly, resulting in a leak.

Engine air filters for discontinued units may be harder to locate, but removing your old filter assembly and taking a trip to a small engine parts vendor may yield a solution. Also, K&N manufactures reusable air filters for many applications that may work. You may need to fabricate a baffle enclosure with holes in it to keep the intake side of your engine somewhat quiet.

As for other engine parts, depending on your unit’s engine, a small engine repair shop or a machine shop may help you get them. You may also be able to locate the engine manufacturer’s Web site online to locate a parts vendor. The engines in many of these APUs are also commonly used in farm applications like small tractors, generators and irrigation pumps, so parts sources will be out there for years to come.

Electrical systems can be a real nightmare to deal with on some units, especially if you don’t have a wiring schematic (diagram) to refer to. You may be able to locate and download one from the Internet for your unit, and then print it out for future reference. If your wiring harness is damaged beyond repair – but its measurements and other characteristics can be recognized – an automotive electrical specialist could locate the connectors and wire colors needed to build a new harness for your APU.

Another great source of parts is used APUs that may still be in working order. These often come off of wrecked trucks or are removed from a truck before it’s sold. Also, some units out there with blown engines still have good, usable parts in them, which are often sold complete for a reasonable price.

As an example, several years ago I sold my complete Aux generator unit to a fellow OOIDA member for parts. My unit had a blown engine, but almost everything else was usable. I let him have it for $350. Also, APU manufacturers going out of business often have a “fire sale” on their parts inventory. These parts can often be had for a good discount.

If your APU needs any coolant hoses, here are a few words of caution: When buying any hoses, be sure to check the manufacturing date on them as their time on the shelf will pose a concern. As a rule of thumb, I don’t like to buy any hoses that have been sitting on a shelf for more than a year. Any longer than that generally decreases their usable service life.

If you can’t locate exact replacements, you should be able to improvise by using a flexible universal fit hose or locating a preformed hose that you could cut a section out of. I have done this by taking an old hose to an auto parts store and visually matching it up with one that would work. LL


Editor’s note: This article is for information purposes only. If you’re not sure about performing the work yourself, it’s advisable to seek the help of a competent professional.

Jeff Barker is an OOIDA member and a former certified diesel mechanic. He can be reached at