Cover Story
A longer life for your APU
Step by step, here's how to make sure you get the most out of your APU investment

By Jeff Barker
contributing writer


With all of the anti-idling regs around the U.S. and Canada, it’s become painfully obvious that if you want to stay cool in the summer and warm in the winter, key-off power is the only way to go.

If you were a holdout and are just now investing in idle-reduction technology, maintenance will be the key to making sure you’re not back at the APU dealer with needless repairs or, worse yet, needing a new unit.

Paying close attention to your newly installed APU system will help you avoid potential problems – and the rude awakenings that stem from them.

As many of us know by now, an APU is a great addition to almost any sleeper-equipped truck used in situations where a lot of overnight stays are involved. Of course, as with any truck or trailer that was just purchased, some areas should be monitored closely for a while to help catch minor problems before they potentially become more serious – and very costly. Having a manufacturer’s warranty on your APU may give you a little peace of mind, but it doesn’t change the fact that you might encounter some serious, costly downtime while waiting to get your unit repaired.

First, make sure the unit is completely turned off and disconnect one of the battery cables to the unit to prevent an accidental engine start from taking place while your hands are in there.

Know when you’re supposed to do your first oil and filter change. Some APU manufacturers recommend one after the first 100 hours of engine operation.

Take a close look at the air filter and its plumbing. Make sure all of the clamps are tight.

Look at the bottom of the unit housing and note any signs of fuel, oil or coolant residue that may be present. If it’s antifreeze, it could be a loose coolant hose clamp that needs to be retightened with a 5/16-inch nut driver. If it’s oil, check the oil filter to be sure it’s tight. If the filter is OK, you may have an oil leak elsewhere that should be examined and repaired as soon as possible before it becomes worse and could eventually cause a fire. If you changed the oil in your unit recently, you may have some oil there that leaked out when the oil filter was removed. Try to clean it up so that it doesn’t hide another problem.

Now, if your APU is like most of them out there and uses an engine-driven compressor, look at the air conditioning system and note any oily residue on the A/C lines or on the compressor itself. If any leaks are present, get them repaired before your A/C could eventually quit cooling on a 100-degree day.

Take a look at the engine wiring harness and make sure it’s securely fastened so that it doesn’t have a hole rubbed into it. Do the same for the battery cables and make sure the connections are tight at all ends.

Check the belt tension and pulley alignment. If any accessory pulleys (such as for the alternator, A/C compressor, and the generator if equipped) are out of alignment, that would indicate a mounting problem for the respective component. While the belts are still new, write down the part numbers for future reference.

Check the oil and antifreeze levels, reinstall the engine compartment covers and reconnect the APU battery cable.

Check the mounting brackets that hold the unit to the frame. Make sure there’s no cracks or loose bolts. Re-torque the bolts.

On most APUs it’s easy, but also very important, to test the engine’s emergency shutdown system. That can usually be done by taking a jumper wire and connecting one end to the low oil pressure cutoff switch (normally mounted near the oil filter base), and the other end to a good metal ground point. Start the engine and see whether or not it shuts off soon. Do the same test for the water temperature switch (usually mounted into the thermostat housing on Kubota engines). Then, while the engine is off, disconnect the wire from the oil pressure switch and check the switch itself with a continuity tester. It should show continuity to ground.

Take a little time to look at those lines that go from the frame-mounted unit to the HVAC unit under the bunk. Some coolant and A/C lines are likely in that bundle, as well as a wiring harness. Make sure those are all secure and not rubbing anywhere. Also, double-check the battery cables.

If you have an APU that has its cooling system tied into the truck’s cooling system (sharing antifreeze with the main engine), make sure the clamps where those hoses are tied in are tight and that there’s no signs of a coolant loss. If any of those clamps were not tightened down properly, the pressure within the cooling system can cause a hose to be pushed loose from the fitting, resulting in a sudden coolant loss that could leave you stuck on the side of the road.

The fuel lines that were installed for your new APU need to be rechecked periodically to prevent fuel leaks from occurring. If they are rubber lines installed with worm gear hose clamps, it would not be a bad idea to grab a 1/4-inch nut driver and snug down those clamps. Also, make sure any other fittings are tight and the fuel lines are securely mounted. Snug the fuel filter onto its base while you’re at it.

Some APU installers will remove a plate from the top of whichever fuel tank does not have the fuel-level sender unit in place and install a fuel draw unit there with the plate on top. Keep a close eye on that for any fuel seepage and retighten the plate screws if necessary. If that doesn’t cure the fuel seepage, you can have the APU installer repair it for you under warranty. Otherwise, you can get two sender unit gaskets and replace them yourself.

APUs that are not tied into the truck’s HVAC system have their own separate HVAC unit mounted under the bunk. Depending on the make of your truck, you should be able to gain access to it from inside the cab by lifting the lower bunk (on most Freightliners) or through a luggage compartment door from outside the truck. They are usually installed near the truck’s factory-installed HVAC unit.

Once you have found the HVAC unit for your APU, take some time to locate the inlet air filter screen and consult the APU manual to learn how to periodically remove and clean it. If you have any pets in your truck that shed hair or if you operate your truck in situations where a lot of dirt gets tracked into your cab, it would be advisable to clean the inlet air filter screen more often. This keeps debris from building up on the blower wheel (which could cause a loss of air flow and blower fan motor failure) as well as the heater core and evaporator core (which could cause an air flow restriction and keep your unit from heating or cooling efficiently).

Take a look at the ductwork that runs from the HVAC unit to the vents and get to know where the connections are. While the unit is turned on with the fan speed set to high, feel around the duct work with your hands and note any air leaks. If air is leaking, it might be a loose clamp or an improperly installed duct hose. Get it fixed properly and enjoy a more comfortable night’s rest. 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