Auto Tranny Checkup

The No. 1 routine maintenance item for both automated and fully automatic transmissions is the fluid level check.

On mechanical-type transmissions, fluid level should be checked at the most frequent maintenance interval your fleet employs – typical of traditional, conservative oil change intervals. ArvinMeritor recommends 10,000 to 15,000 miles, while Transmission Technology Corp. recommends 25,000. If you are running extended oil change intervals (beyond 25,000 miles), check the level at least twice between each oil change.

On automatics, the level should be checked weekly, says Nick Bond, vice president and general manager of Russ Moore Transmissions of Fort Wayne, Ind., a major Allison transmission remanufacturer. Allison’s Dan Murphy, service manager 1000-2000 Series, prefers a daily check just in case the unit should spring a leak.

Manuals must be checked with the vehicle on a level surface so the fluid level at the plug will be accurate. The transmission should have sat for at least 15 minutes, so the oil will have drained off gears and bearings and back into the sump. Allow a hot unit to cool enough so you won’t burn yourself. Put a pan under the level plug (typically located in the center of one side of the unit), clean off around the plug with a clean rag, and then remove the plug. The fluid level should be right at the level of the plug so a little fluid drains out upon removal. Don’t go in with your finger and reach downward for the level, because anything below the bottom of the hole is too low. Top up the level with a funnel just to make sure it’s right where it belongs. Make sure to use the approved engine oil, transmission oil or synthetic transmission fluid, not an EP GL-5 axle oil or multi-viscosity engine oil. Don’t mix different types, grades or brands.

The best place to find specifics on fluid specs and viscosities is the owner’s manual or the component manufacturer’s website.

Keeping the level all the way at the top is so important because most transmissions are lubed by splash alone. When the level drops even a little, the total amount of lube on the working parts also drops. John Mater, a product service manager at Eaton truck components operations, points out that a mere 1-inch drop in fluid level means you’ve lost a gallon of vital lube. After making sure the levels are OK, replace the plug.

On the AMT-7 from Transmission Technology Corp., there is an additional sump at the front of the transmission that supplies the hydraulic actuator for the dry clutch. Check this sump in the same way. If no fluid leaks out when you pull the plug, remove the filler cap and pour slowly just until fluid starts to seep out of the hole in the side. Use Dexron III automatic transmission fluid rather than the SAE 50- or 30-weight synthetic fluid used in the main gearbox. Then, replace both the cap and plug.

After checking the fluid levels and replacing the plug or plugs, torque them to the spec given in the owner’s manual.

Low fluid level in an automatic quickly leads to major trouble, so it’s important to check the level and refill on time. When fluid gets low, the pump pickup starts to draw in some air along with the fluid. With air in the mix, the main and lubrication pressures start to drop (the main fluid pressure operates the clutches). This results in rising temperatures because there is less fluid flow through the oil cooler. Eventually, the clutches will lose their torque-carrying capacity and begin to slip, especially under heavy loads. Air in the fluid may reach the point where it causes the fluid to cavitate in the converter.

Also avoid overfilling automatics, because this, too, will cause air to be entrained and affect clutch operating pressures and shift quality.

Once the fluids are up to snuff, make a careful inspection of the transmission case for leaks. Bill Nash, service manager and manager of field service and warranty at ZF Meritor, and John Mater, listed the leak locations to check. Look over the case joints (where individual sections are bolted together and have a gasket), the breather, the top cover, PTO cover, and both input and output shaft seals. Mater says that if there is a leak at the input shaft seal, it might be an air leak pressurizing the transmission. Check also for leaks along the cooler lines and where they connect to the cooler and transmission, as well as within the cooler itself. Tighten clamps or replace lines, if necessary, to stop leakage. Nash says that if the unit has a transmission-to-engine coolant-type heat exchanger, and it develops an internal leak, you’ll have antifreeze in the transmission lube because the cooling system operates under pressure and the transmission does not. Look for a high fluid level in the transmission and a milky appearance.

Both Nash and Mater recommend a check for loose or missing bolts at every maintenance interval.

And look at connectors to make sure none have pulled loose. If they have, fill with di-electric grease to protect them from moisture, and replug them securely.

Check the range and splitter (on Autoshift 18-speeds) cylinders and air lines and connections for leaks. This should be done with brake system air pressure at 90 psi or higher. You should hear no sound of escaping air except for a brief period during shifting. Shift the range and splitter to high range/high split positions and listen again for air leaks. Sometimes the leaks are internal, leading to slow shifting or causing air to pressurize the case. In those cases, range or splitter shift components will have to be rebuilt and new seals installed.

Mater pointed out that transmission shifting problems can often be traced to poor air system maintenance. So, always follow OEM recommendations. Drain air tanks daily and rebuild the air compressor at the first sign of engine oil in the system. Maintain air dryer desiccant and filters. Clean air enables the transmission shift components to function with little trouble.

The AMT-7 is a fully automated 7-speed using a synchronized gearbox and standard dry clutch. But, both shifting and clutching are fully automated. The transmission mechanicals are completely conventional except for a hydraulic clutch actuation cylinder that uses automatic transmission fluid.

While checking things over, look at the clutch. Most late-model trucks have self-adjusting clutches with an indicator that will show wear. This includes the dry clutch on the AMT-7. Look at the indicator to make sure there is still clutch lining left, and if the clutch is almost worn out, have it replaced.

If the clutch is not adjusting, the driver will notice a reduction in pedal free play. This means the clutch grabs with the pedal too high off the cab floor. The driver may also notice a lack of grabbing power when he releases the clutch. If this occurs, have the self-adjusting mechanism repaired or, in the case of standard clutches, adjust free play as necessary.

The FreedomLine transmission has a display that tells the operator when the clutch is worn. A tech can read the percentage of lining left using a Pro-Link electronic tool or TransSoft software that operates with a PC. You can also read fault codes off the dash display – see the truck owner’s manual for how to do it. If the clutch is not adjusting properly, you’ll get a fault code.

Since keeping the clutch in adjustment is a key to long life, monitoring the operation of the self-adjusting mechanism, or adjusting manual clutches periodically, will lower costs.

With torque-converter automatics, both experts recommended that technicians ask the driver about performance, and investigate any change in shift character.

Fluid changes
The service literature from Eaton Fuller says, “Where transmissions are concerned, lubrication is probably the most important part of keeping a vehicle operating.” Synthetic lubes protect the transmission from wear much better. Mineral oil-based fluids usually oxidize much faster and are also less slippery than synthetics. The manufacturers extend both change intervals and warranty by big margins for synthetics users. While the synthetic fluids cost a lot more, total cost of fluid alone, not to mention downtime or repairs, is lower because of the longer intervals, so it’s hard to justify using mineral lubes.

Recommended synthetics are usually straight 50-weight lubes, with 30-weight oils that might perform a little better in very cold conditions also approved by some manufacturers. The manufacturer will provide either an exact spec or a list of approved lubes. The critical characteristic of an approved lube is that both the base lubricant and additive package will remain chemically stable long enough to allow the extended change interval.

For mechanical transmissions, the change intervals are as follows:

Eaton Fuller Autoshift
With approved synthetic lubes, the change intervals are 250,000 miles for over-the-road highway service, with a 500,000-mile interval for the original factory fill with synthetic. On-highway severe service requires a change at 1,000 hours; off-highway usage requires a change every 2,000 hours.

For approved mineral lubes, the change interval in over-the-road highway service is 60,000 miles. When running in on-highway severe service, the interval is 500 hours; off-highway usage requires a change every 1,000 hours.

The ZF Meritor FreedomLine has a conventional dry clutch, but both the clutch and transmission are operated and actuated automatically. A microprocessor constantly monitors road conditions and throttle position to operate the clutch and transmission with no driver intervention.

ZF Meritor
For its SureShift and FreedomLine transmissions, with approved synthetic lubes, the change interval is 500,000 miles. With approved mineral lubes, it is 50,000 miles.

Transmission Technology Corp.
TTC recommends the use of an SAE 50- or SAE 30-weight synthetic lube in its AMT-7. It must meet the specs shown in the truck owner’s manual. The change interval is five years or 500,000 miles in normal over-the-road service, and two years or 250,000 miles in severe service. The Mercon/Dexron III fluid operating the clutch actuation mechanism need not be changed, only topped off as necessary.

Changing the lube involves driving the vehicle until the transmission is warm, parking it on a level spot, and then removing the drain plug and draining the lube into a suitable drain pan until the flow ceases. If the unit has an external filter, remove that and allow fluid to drain from the opening while the fluid drains from the main transmission.

You may also want to pull a cooler line at the lowest visible connection, and make sure the fluid has drained from the cooler. Reconnect this line securely. Replace any external filter according to its manufacturer’s directions.

Reinstall the plug and torque it to manufacturer’s specs.

Allison Transmission
Murphy reports that the standard mileage for fluid replacement on an Allison is 25,000 miles. With synthetic transmission fluid, it’s 100,000. The vehicle manufacturer will often provide additional intervals by vocation in each truck’s owner’s manual, so refer to that if you are running in a demanding application.

A smart alternative is to do fluid analysis. Consult the manufacturer for the specific levels of impurities and other standards your analysis lab should look for. Then, change the fluid according to what the analysis report says. In cases where synthetic fluids are used in less demanding applications, this may lengthen your change intervals considerably.

The filter in an Allison is always changed or cleaned with the lube change. However, with some transmissions, use of synthetic fluid may carry the interval beyond the safe filter change interval. Paper filters sometimes deteriorate at a rate that’s independent of fluid quality or even the quantity of material they can hold. Consult the manual for the specific information relating to your particular transmission model. Both time and mileage intervals often apply, and the change should be made as soon as either interval has lapsed.

Changing the filter does not mean discarding all the fluid in the gearbox, so a separate filter change that’s more frequent than the fluid change is practical. You can work out an interval for the fluid change that’s a multiple of the filter change interval.

Another advantage of analysis is that it will tell you if the interval is too long, and needs to be tightened to preserve the transmission.

Bond recommends change intervals of 100,000 miles or one year for over-the-road trucks, and 25,000 miles or one year for trucks in pick-up and delivery service. Be sure to observe the one-year limit and not just miles. That catches the trucks where there is very high wear per mile.

Bond’s company’s experience with lubes and mostly earlier Allison designs causes them to prefer the use of mineral ATF because the transmissions were originally designed for it. Synthetic is thinner and may slightly affect shift character.

Allison fluid changes occur with the transmission warm and involve removing the drain plug and filters. Russ Moore’s people remove the pan and clean it in a parts washer, a smart procedure after a number of changes in any application. The company often installs an auxiliary, in-line filter between the transmission and cooler, which helps keep coolers from clogging.

Remember the key is to keep fluids clean and in plentiful supply. This will practically guarantee long, trouble-free transmission life.


For more information, contact the following:

Allison Transmission
Tel. (800) 252-5ATD
Fax (317) 232-3626
www.allisontransmission.com

Eaton Corp.
Tel. (800) 826-4357
Fax (616) 342-3312
www.truck.eaton.com/na/

Russ Moore Transmissions
Tel. (219) 482-9414
Fax (219) 482-5006
www.russmoore.com

Transmission Technologies Corp.
Tel. (800) 401-9866 or (419) 470-8200
Fax (248) 471-2520
www.ttcautomotive.com

ZF Meritor
Tel. (248) 435-1000
Fax (248) 435-1393
www.arvinmeritor.com

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