GMC Western States

 Tech Center Number 4 - August 1993

Generator Basics

The most common generator problems can be prevented by a simple maintenance routine performed at proper intervals. However, just as important are proper startup, operation, and shut-down procedures. Many service headaches can be avoided or minimized by correct use of the generator.

The GMC owners manual covers proper start-up and running procedures. The manual also covers basic service items. If you don't have an owners manual, you can still get them from Cinnabar Engineering, they are worth every penny.

Maintenance intervals on cars and motorhomes are scheduled according to vehicle mileage. Stationary engines, such as the generator, u se hour meter which measures running time. If you do not have an hour meter you should have one installed. It is virtually impossible to determine service intervals without one.

You should make it a habit to inspect your generator before every trip (when was the last time you even pulled it out of its compartment?). Here is a list of some of thc things to check:

Exhaust system: Check for leaks around the manifolds, gaskets and welds. Make sure the exhaust system components do not touch any surrounding metal components in the generator compartment. Remember, the generator exhaust is fight under your bedroom window. Inspect the compartment for seams that are splitting or small holes that have developed and make sure they are sealed so exhaust gasses can't enter the living area. Most of the Onan generators had a tail pipe connected to the muffler which runs toward the back of the coach and ends between the rear bumper and body. Many of these pipes have fallen off or have been replaced by short extensions which point down instead of back. Parts are hard to find, but a replacement muffler and tail pipe is still available from Onan for about $95.00.

Fuel System: Check for fuel leaks around the carburetor and fuel pump connections while the generator is running. Be sure the fuel lines are not rubbing against anything that could cause them to wear through and leak. The Onan used in the GMC has a fuel line which runs from the fuel pump, behind the generator and off to the rear fuel tank. This line flexes every time you slide the generator in and out of its compartment.

Electrical system: Make sure the wiring is secure and there are no loose or frayed ends. Spark plug wires should be checked for cracking or stiffness due to heat. The PC board on the Onan is particularly prone to heat and dirt related problems. Pay particular attention to the wiring harness which runs to the board. The on-off switch mounted on the PC board on thc rear of the generator looks like a heavy-duty rocker switch, but is in fact a small electronic grade switch which is soldered to the board. The switch has a large plastic cover to give it the appearance of a large switch. When this switch fails, due to vibration or age, the generator could start without warning. If the switch fails, you can disable its function by disconnecting the wiring harness from the PC board, you also lose generator function. The switch can be replaced with a Radio Shack switch which has to be soldered in place.

Ventilation and cooling: Check to see that the generator compartment is clean and free of leaves or other debris which block air circulation vents on the generator. The Onan generator has a rubber gasket around the hole where the off filter passes through the sheet metal. This gasket (a rubber O-ring) is essential to proper generator air flow and cooling. A new O-ring is available from Onan.

Lubrication: Check the oil level with coach parked on level ground. Use oil as specified in your GMC owners manual, and never add or check the oil with the generator running, you'll be bathed in hot oil.

Carburetors on generators used in the GMC have provisions for adjusting the idle circuit. The carburetor's idle circuit comes into play during start-up and when the generator is running under no load. Since a generator really doesn't idle (most of us think of idle as slow engine speed), the speed is controlled by a governor that keeps the speed constant. Generators, depending on how the armature and brushes are arranged, must run at some multiple of 60 (in order to produce AC electricity at the correct 60 cycles Per second). Therefore, engine speed will be either 1800 or 3600 rpm.

One feature of the adjustable fuel/air mixture is that you can compensate for altitude changes. When altitude increases, the air pressure is lower and more fuel is added to the airstream passing through the carburetor than is necessary. Small changes in the main fuel metering screw can reduce the tendency to emit black smoke at high altitude because of an over rich condition.

To give you an idea of how much equipment your generator can power, we have included a power consumption chart, courtesy of Onan Inc.



As owners of aging GMC Motorhomes are becoming increasingly involved with the upkeep of their coaches, we will try and provide some information to help with the process.

So what is technical about dirt you ask. Nothing, but keeping dirt out of your engine, gears, and hydraulic system most definitely is. Automotive experts agree that dirt is the number one cause of engine wear. Bearing manufacturer Federal-Mogul Corporation, in analyzing the cause of engine bearing failure, indicates that 43.4% of all engine bearing distress is caused by dirt. Dirt particles are extremely abrasive, they consist of razor-like flakes of road dust and airborne grit drawn into the engine through the intake manifold. Another source of dirt is manufacturing scarf and wear metal particles generated inside the engine. Filtration is the key to preventing costly repairs caused by engine dirt.

Since the engine takes in about 9,000 gallons of air for each gallon of gas burned, intake air filtration is of primary importance. There are two types of filters available, the paper element filter which is disposable, and a two-stage foam filter which requires oil to trap the dirt. The two-stage filter can be cleaned, re-oiled and put back in service. Whichever filter type you choose, the important part is to check it regularly and keep it clean.

Some airborne dirt gets into engine oil in spite of a good clean air filter. A full-flow spin on oil filter is essential for keeping the dirt from circulating through your engine. As with air filters, there are two types of oil filters, paper, which is the most common, and fiber which is more expensive but traps smaller particles of dirt. The paper filter works best if it is changed with every oil change. All good quality filters have a by-pass feature which lets oil pass through an alternate path if the main passage clogs. If you change your oil every 3,000 to 6,000 miles, you should never have any clogging problems.

Another source of dirt entering the engine oil is the positive crankcase ventilation system (PCV). Air from the dirty side of the air filter is drawn into the left valve cover through a very coarse ball-shaped filter and then into the crankcase. This airflow, plus piston ring blow-by is drawn out of the right valve cover through a flow-limiting valve (PCV valve) into the intake manifold just below the carburetor. If the dust didn't stay inside the engine as the air passed down through the valve and tappet area and out through the right valve cover, it gets a second chance. It passes through the intake manifold, bypassing the air filter, and gets a chance to stick to the cylinder walls, a very wear intensive area. The dirt combines with dirt which managed to get through the air filter and gets scraped from the cylinder walls and pushed down into the crankcase where it gets mixed into the engine oil.

The vent for the transmission is on the top left side of the chain housing just below the cruise control and about ten inches from the exhaust manifold. In addition to a dirt entry hazard, the vent is dangerously close to the exhaust manifold. If the transmission overheats and fluid boils out of the vent, there is a good potential for fire when the fluid hits the hot manifold. The vent cap is crimped loosely over the vent tube to allow air to circulate but keep large particles out. As the transmission heats and cools, air is drawn into and expelled through the vent. Dirt gets sucked in each time the transmission cools and air is drawn in.

Darrell Winterfeldt talked about this problem at the Fiesta Roundup in San Diego. He solves the problem by running a flexible tube to the vent and extending it far away from the exhaust manifold and to a cleaner location.

Although the final drive unit has much less air space than the transmission and temperature changes are not as drastic, the unit uses a vent system identical to the transmission. The vent is lower and therefore gets blasted by water, dust and other road debris. Something to think about!



Understanding your power draw
                        Average watts       Approx
Appliance                Required            Amps
Air Conditioner         1400 to 2200        12 to 18
Blender                    600                 6
Broiler                    1350                12
Coffee Pot               550 to 700          4 to 6
Convertor/Charger        300 to 500          2 to 3
Curling Iron              20 to 50          .2 to .5
Blanket                   50 to 200         .5 to 1.5
Drill                    250 to 750          2 to 6
Frying Pan              1000 to 1350         8 to 11
Hair Dryer               350 to 1000         3 to 8
Iron                     500 to 1200         4 to 10
Light Bulbs               40 to 100 ea     .36 to .9
Microwave                700 to 1500         6 to 13
Refrigerator             600 to 1000         5 to 8
Space Heater            1000 to 1500         8 to 13
Television               200 to 600        1.5 to 4
Toaster                  750 to 1200       6.5 to 10
Vacuum                   200 to 500        1.5 to 4
VCR                      150 to 200          1 to 1.5
Water Heater            1000 to 1500         8 to 13
Water Pump               500 to 600          4 to 5



 Tech Info