Tech Tip

Regular Oil Checks the Key
to Transmission Maintenance

By Peter Cannon

Following a manufacturer's maintenance manual for school bus parts, most would agree, is vital. Perhaps none more crucial, both in terms of safety and money, than transmissions. You are, as one maintenance director said, simply "pouring money into a hole" if you disregard suggested maintenance practices and the transmission keeps slipping.

So how is the correct preventative maintenance carried out on a transmission?

Allison Transmission, the leading supplier of school bus transmissions across North America, recommends periodic inspection at regular intervals, like the maintenance needed for most automotive products, is the most effective way to prevent transmission failure.

"The most important thing you can do from a preventative maintenance perspective is to check the oil on a regular basis," said Larry Miles, product service manager at Allison. "We advise the school bus people to check the oil every day. It may not always need it but it shows you're giving the transmission attention and it let's you keep on top of what's going on in there."

The specific operating demands of a school bus determine the frequency of both cleaning and inspecting the exterior of the transmission. A general guide for school bus transmissions, most of which are either Allison's AT500 or MT600 series', is fluid change intervals for those with a brass filter every 50,000 miles or 24 months, whichever comes first. Internal sump and governor paper filters in a shallow pan (four inches), deep pan (5.3 inches) and brass screen filters in a shallow pan (four inches) or deep pan (5.3 inches) should be changed every 25,000 miles and 50,000 miles, respectively. The external auxiliary filter, Allison recommends, should be changed after the first 5,000 miles and at 25,000 miles or 12 months thereafter, whichever comes first.

The company's recommendations for correct preventative maintenance procedures described in this article are specifically for the AT500 products and most are applicable to the MT600 series as well, Miles said. Technicians servicing Allison's MD electronic medium-duty transmission and transmissions manufactured by other companies should refer to owner manuals for relevant maintenance practices.

What to Look For

The transmission should be inspected for loose bolts (transmission and mounting components), transmission fluid leaks, shift linkage freely positioned by transmission detent, full movement of mechanical modulator linkage, and vacuum or air line and modulator for leaks. Also, check for damaged or lose hydraulic lines, worn or frayed electrical connections, driveline U-joints and slip fittings, and PTO linkage and driveline.

Operator manuals, Allison notes, specify how often the transmission hydraulic level should be checked.

In addition to timely inspection of the exterior, maintaining proper transmission fluid levels is pivotal in the continued smooth operation of the product.

Allison says it is important the fluid level be maintained at all times since if it is too low, the converter and clutches will not receive an adequate supply of liquid. Conversely, if the fluid level is too high, the fluid will become aerated, the transmission will overheat, and the fluid may be expelled thought the breather or dipstick tube.

The foaming and aeration of the fluid also affects transmission performance. Fluid levels either too high or too low will result in aeration. If the fluid level is too low, both air and fluid are drawn in by the input pump and directed to the clutches and converter. This will cause converter cavitational noises and irregular shifting, Allison notes.

What is normal fluid level? It will show as being slightly below the planetary gear units. If the transmission is overfilled, aeration will result as the planetary units will run in the fluid.

Checking Fluid Levels

The transmission fluid level should always be checked twice. If inconsistent readings occur with regularity, the transmission breather and vent hole in the dipstick fill tube should be checked to ensure they are clean and free of debris. Any abnormal fluid levels should be recorded in a shop's maintenance records.

Allison recommends cold checks of fluid levels be carried out when the bus is parked on a level surface, the parking brake has been applied, and the wheels are blocked. Then, the engine should be run at 1000-1500 rpm for one minute to purge air from the system. Let the engine idle and follow that by shifting to Drive and then Reverse to allow the hydraulic circuits to be filled with fluid. Next, shift to Neutral or Park. At that point, the sump temperature should be 60F-120F.

Wipe the dipstick clean and check the fluid level. If it is within the REF FILL (Cold run) band, the level is satisfactory for operating the transmission until the fluid is hot enough to perform a Hot Check. If the fluid is not within the parameters, however, it should be raised or lowered to bring the fluid level within the band.

A hot check should be performed immediately after the normal operating temperature is reached, i.e. 160F-200F sump temperature and 180F-220F converter-out temperature. Shift the vehicle to Drive and then to Reverse to fill the hydraulic circuits with fluid. Follow that by shifting to Park or Neutral and apply the parking brake. Wipe the dipstick clean and check the fluid level. A safe operating fluid level, Allison notes, is anywhere between the Full and Add lines within the hot run band or below the Hot Overfull mark and above the cold run band. Raise or lower the fluid level to meet those parameters.

Transmission Fluid Recommendations

Allison recommends using DEXRON III hydraulic fluids for standard, duty, highway applications. C-4 fluids should be used for severe duty and off-highway applications. When using hydraulic fluids on an Allison transmission, the company warns users to check for a DEXRON III or C-4 fluid license or approval number on the container. Otherwise, the hydraulic fluids may not be suitable for use in those transmissions and other parts can be damaged.

"We tell all of our mechanics the single-most important thing is the fluid and if you see it's not that dark, cherry red color and it looks black or has that 'burnt' smell to it, you need to get it cleaned out immediately," said Harlan "Ike" Pinkston, Jefferson County Schools' director of vehicle maintenance in Louisville, Ky.

"And the wrong transmission fluids can deteriorate rubber components and seals."

A number of factors, including geographic location, preheat capabilities, and duty cycle must all be considered when choosing the optimum viscosity of fluid to use. A school bus should be run for a minimum of 20 minutes in neutral before attempting range operation.

Box: Operating Temperature Requirements for Transmission Fluid

Viscosity Grade Temperature (F)

SAE 0W-20 -31

DEXRON-III -17

SAE 10W -4

SAE 15W-40 5

SAE 30 32

SAE 40 50

Transmission Fluid Contamination

Allison recommends examining the drained transmission fluid for evidence of dirt or engine coolant (water). If there is evidence of coolant, check the heat exchanger for leakage between the coolant and fluid areas. Fluid in the coolant side of the cooler is another sign of leakage, Allison notes.

Metal particles in the fluid generally indicate damage has occurred in the transmission. When metal particles are found in the sump, the transmission will need to be disassembled and inspected to determine the source of the particles. Also, all internal and external transmission circuits, the cooler, and all other areas where metal particles could lodge must be cleaned.

Similarly, if the engine coolant leaks into the transmission hydraulic system, it must be completely disassembled, inspected, and cleaned in order to prevent malfunction and damage. All traces of the coolant and varnish deposits must be removed and clutch plates contaminated with ethylene glycol must be replaced.

For hydraulic systems with a retarder, Allison advises replacing the main cooler while models without a retarder require the installation of an auxiliary filter. This requirement applies whether the transmission is overhauled or replaced by a new or rebuilt unit.

Following transmission manufacturer recommendations when it comes to preventative maintenance are valuable both in preserving the life of a transmission and in saving money.

Pinkston said his technicians rebuild approximately 25-30 transmissions each year in his current 849 bus fleet. Transmission overhauls can run anywhere from a few hundred dollars to as much as $3,500 apiece and he attributes the low number of rebuilding jobs to sticking with the recommended preventative maintenance procedures.

"Allison recommends we change the fluids and filters every 25,000 miles but we do it every 24,000 because it fits into our regular maintenance schedule," Pinkston said.