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New Day, New Way
More bang for your buck
Maintenance is essential to the optimum performance, long life and big return on your APU investment.

By Jeff Barker
contributing writer

After a hard day’s work, most anyone in his or her right mind looks forward to some time to relax in a comfortable environment just before bedtime and then get a good, quiet night’s rest.

While anti-idling laws are becoming the norm, more truck operators are embracing the benefits of auxiliary power units. Regardless of what brand or type of APU you get or already have, how well it performs and how reliable it is depends on how well the unit is maintained.

Oil and filter changes
Most APU engines out there can usually run 250 to 500 hours between oil and filter changes on non-synthetic oil, depending on the manufacturer’s recommended interval. In some situations, you may be able to schedule this to coincide with a 10,000 to 15,000-mile chassis lube interval on your truck and get it done at the same time.

While many truck operators have changed to synthetic oil in their trucks’ engines, it’s advisable to check with both the APU manufacturer and with the manufacturer of its engine to be sure this will not void the warranty.

Be aware of some truck lube shops out there, as many will charge you the high cost for servicing a reefer unit just to change the oil and filter in your APU. Ask them about this before committing to having this work done, or do it yourself if possible.

Fuel filter replacement
It makes good sense to replace the fuel filter at the same time the engine oil and filter is changed. However, if you’re extending oil drains, stick with the manufacturer’s recommendation on changing the fuel filter.

Air filter replacement
Depending on what kind of environment the APU is used in, the interval for replacing the air filter will vary. If it’s used in dusty areas, it will need to be changed more often. If you’re running the APU engine and see blue smoke from the exhaust, chances are good that the air filter is clogged and it’s beginning to suck oil from the crankcase. Replace the air filter immediately.

Belt replacement
Some APUs, depending on design, are user-friendly enough to where the belts can be easily replaced on the road with a few simple tools. Others may require a few hours of your time – not to mention a few scraped knuckles – and/or may best be replaced by a mechanic who is familiar with those particular units.

In most cases, it’s a good rule of thumb to inspect these belts closely at least once a week and replace them once a year, no matter if they appear bad or not. While the belts are removed, check the condition of any belt tensioners and idler pulleys the unit may have. Check the water pump on the engine for leaks, and check for play in the impeller shaft where the pulley is bolted to it.

When working on an APU engine, especially near the belts, it’s important to disconnect the APU battery cables to prevent the possibility of an accidental engine startup that could cause injury or – not to be dramatic, but rather very serious – death.

Coolant hoses
The coolant hoses can usually last about four years between replacement, but the hose clamps need to be snugged down at least once a year – preferably in late autumn – so the chance of a cold water leak in the approaching winter season is reduced.

A/C system
It wouldn’t hurt to get the A/C system operating pressures checked some time in the spring after the A/C has seen limited use in the winter months. The refrigerant pressures need to be at near-perfect levels so the system can keep the cab and sleeper cool when in use on hot days and nights.

During the winter months, it will help keep the A/C system lubricated if it’s run once every two weeks.

Heating system
If you run your heater for about 30 minutes during the summer months to prevent the heater control valves from becoming stuck, you will be glad you did when winter hits.

The weekly checklist
Inspecting your APU thoroughly once a week may help you catch a minor problem before it becomes something worse later on. Here’s a list of common items to check.

  • Frame mounting brackets. These can often become loose from vibration and in some cases can crack or break, especially if they’re made of aluminum.
    Battery cables and wiring harnesses. Be sure they ’re tight and secure to where they’re out of harm’s way and won’t rub on anything. Make sure all connections are clean. Spraying a coating of battery terminal protector on exposed connections once they’re cleaned will help avoid corrosion.
  • Fuel supply and return lines. Be sure there aren’t any cracks or dry-rotting in the rubber lines and that no fuel leaks exist. Be sure that they’re not rubbing on anything.
  • A/C refrigerant lines and condenser coil. The rubber A/C refrigerant lines need to be securely mounted with either rubber-lined clamps or heavy-duty nylon cable ties so they can’t rub on anything or allow vibration at the fittings. If a line comes loose it can result in a broken fitting or create another problem. If your A/C system experiences a sudden loss of refrigerant pressure, that can allow contamination to take place. If that happens, the entire system will need to be flushed before it can be recharged once the leak is repaired. If refrigeration oils are exposed to air and moisture for even a short period they become acidic and can destroy the A/C system.
  • Condenser fins. Make sure the fins in the condenser coil are clean, do not allow high-pressure water spray to get near them. This can bend the fins on the condenser coil and harm its efficiency.
  • Exhaust system. Be sure it’s securely mounted and pointed away from the underside of the sleeper, preferably toward the rear of the truck so it doesn’t go under the sleeper of your truck or others nearby.
  • Engine shutdown protection system. If your APU engine runs hot or suddenly loses oil pressure, you want it to automatically shut down before it self-destructs. This can be checked by:
    • Oil pressure: If possible, turn the APU system on without starting the APU engine for about a minute. If it shows a “warning” while disengaging the fuel solenoid on the engine, then it works.
    • Coolant temperature: Connect a jumper wire with alligator clips between the coolant temperature sensor – if it has an open, accessible connection – and a good ground point, such as an unpainted metal surface, and then start the APU engine. If it shuts down a short time later, then it’s working.

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 may be reached at