Diagnosing specific issues in your truck’s rear drive axles can certainly be a challenge. It takes a significant level of know how to not only know what has gone wrong, but also how to fix it.
Cummins-Meritor’s diagnostic course aims to take you through how to address rear drive axles with high lubricant temperature, which can result in differentiation between your axles, excessive torsion or improper driveline angles.
Incorrect temperatures can quickly become a serious issue. Drive axles should not be operating in excess of 121 degrees Celsius for extended periods, and the lubricant temperature of both axles in a tandem set should not differ more than negative one degree Celsius.
Rear drive axles require lubricants containing G L five, a level of extreme pressure, or EP, additives. The EP additives protect heavily-loaded parts, such as the ring and pinion gears, to help prevent surface fatigue, scoring, galling and welding of the ring and pinion gears.
These EP additives can easily degrade when a drive axle overheats. For example, the EP additive in drive axle lubricant begins to degrade when the lubricant temperature is consistently above 121 degrees Celsius. The higher the temperature, the faster the additive degrades.
High lubricant temperature could be caused by a number of factors, including mismatched tyres on the drive axles causing excessive main and inter-axle differentiation or mismatched axle ratios on tandem axles causing excessive inter-axle differentiation.
Low or high lubricant levels, incorrect lubricant type or viscosity for your axles, excessive drivetrain torsional activity or excessive torque input to the drive axle caused from incorrect drivetrain application can also cause high lubricant temperatures.
Your first step in identifying any temperature related issues should be to gather as much information as possible. Make sure you have all available information on the truck, including reported condition, service condition and service history. This will allow for a quicker and more efficient diagnostic process.
The next step should be to visually inspect the underside of the truck. Look for oil residue or other signs of lubricant leaks on all driveline components, and for any obvious damage with the driveline, suspension, and axle components.
Next should be the drive axle lubricant check. Remove the fill plug for the suspected axle and check the lubricant level, which should be even with the bottom of the fill hole.
If the lubricant level is low, fill to the specified level with an approved lubricant and proceed to the next step – a road test.
If no obvious signs of leaks or damage are found, the truck needs to be taken onto the road, preferably under the same conditions which initially caused the high lubricant temperature. Listen and feel for any unusual noises or vibrations.
During the road test process, you must also get an accurate drive axle temperature measurement. If your truck has a dash temperature gauge, it may not necessarily be completely accurate, so use an infrared digital thermometer or a calibrated temperature gauge and stop to check the temperature periodically.
If the measurements come back above the maximum recommend 121 degrees Celsius, check for obvious items first. If the lubricant level is low, contaminated, contains a burnt odour, or black in appearance, take a lubricant sample and have a lube analysis completed. This should tell you whether you need to change your lubricant.
Also check the wear conditions of your tyres and look for signs of overloading, tread depth differences, air pressure differences, and tyre size differences.
You may need to find the rolling radius and ratio mismatch between your tyres in this step. To find the rolling radius of your tyres, place a straight edge in the axle shaft center hole level with the ground. Make certain the straight edge goes beyond the tire sidewall. Measure from the straight edge to the ground.
The issue could also be in your ratio mismatch, should not be greater than one per cent between your rear-rear and the forward-rear drive axles. You can check this by either checking the model number ID tags, the pinion ratio numbers, or by doing a tyre rotational check.
Locate the ID tags on the forward and rear axle differential carriers, and then compare the axle ratios on both tags.
Finally, verify the engine and transmission is within the torque design limits of the drive axle. Check the engine ratings, torque output, gross vehicle weight and gross combined vehicle weight.
If you can’t find any abnormal conditions after this process, drain the lubricant from each drive axle take a lubricant sample and complete a lubricant analysis to ensure the correct type and viscosity lubricant is being used, then refill to the proper level with the correct lubricant before performing another road test.
If the lubricant temperatures are still excessive, contact the OEM or component manufacturer for assistance.
