Troubleshooter: Troubleshooting automatic transmissions

It used to be the case that a bad driver could find many ingenious ways to damage a transmission. Fortunately, modern automatic transmission technology has strengthened the products’ control system capabilities and thrown up walls of defense against poor driving and other operator-induced issues. Service technicians can let out a collective sigh of relief.

“Allison automatic transmissions are torque-converter equipped, planetary-geared transmissions,” says Keith Duner, manager of service technology, Allison Transmission. “The torque converter provides a lot of shielding from driver abuse. We no longer have to worry about things like harsh clutch engagement.”

Although it’s true that modern automatic transmissions are more robust than their predecessors, that’s not to say that transmissions don’t ever fail, or that preventive maintenance practices can be put on the back burner. When an automatic transmission is speced and installed correctly, treated with timely fluid and filter changes and subjected to routine maintenance checks, it should have a long and productive life. If it’s largely ignored or improperly installed, however, an automatic transmission can cause trouble.

“If there are some areas of concern about how an automatic transmission functions, they probably stem from modifications that occurred to the vehicle after its manufacture,” says Duner. “Were the wiring harnesses addressed correctly? Were the people who made the modifications knowledgeable about the product, or do the changes have the potential to induce a problem?”

These are unknowns that a technician must consider when diagnosing a transmission problem. For example, over-torquing filter cover bolts can lead to stripped bolt hole threads and a transmission removal for main case replacement, according to the Allison website. Work performed improperly can result in expensive re-repairs and extended vehicle downtime – both of which make for unhappy customers.

The Inspection
When a customer complains that his vehicle shifts late or harshly, or has a vaguely defined shift complaint, a technician should be prepared to do a complete examination of the transmission.

“If a shift complaint is the start point of the service event, typically what a technician should do is extract any diagnostic codes that are embedded in the control systems,” says Duner. “All Allison transmissions are electronically controlled, which means they have the ability to store and maintain diagnostic codes.”

The control system constantly goes through a cycle of internal and external review to understand its condition and environment. Once a problem is noted, a diagnostic trouble code (DTC) is stored. These codes are logged in the electronic control unit’s memory by order of severity or recentness of occurrence. A maximum of five codes can register in the memory at one time.

“Some DTCs will be relatively benign and will not cause operator enunciation. They’ll only be visible to a technician using a diagnostic tool,” says Duner. “The more severe DTCs will turn on a check transmission light, either within the shift selector or on the instrument cluster that advises the operator to take his vehicle to a service outlet as soon as possible.”

When addressing an automatic transmission issue, always start with a precursory check of components. Look for things like loose bolts, mounting equipment condition, loose or damaged hoses and chaffing, binding or frays on the wiring harness. Because automatic transmissions are electronically controlled, the condition of wires and connectors are especially important.

Also, since transmissions are heat generators, their proper functioning is partially dependent on the vehicle’s cooling system. Make sure these two systems are cooperating so that the transmission is not unduly stressed.

“There is an interface to the cooling system that is represented through hoses that connect the transmission to the radiator. The condition of those hoses is something that must be inspected,” says Duner.

Another small, but important, design feature of the Allison transmission that frequently is overlooked is the breather: an external vent that allows for expansion and contraction of fluid based on temperature differentials to protect seals from damage. The vent on most of the on-highway transmission models is mounted on the top of the main case.

“The vent should be clear and unobstructed. It shouldn’t be in a location that is a repository of water and road spray. Also, no transmission fluid should be seen at that breather. If you see fluid, then that’s typically an indication of an over-fill fluid condition,” advises Duner.

Fluid and Filter Changes
Allison automatic transmissions are designed to run dry, which means any sign of a transmission fluid leak, whether on the transmission or under the vehicle, warrants further investigation. Automatic transmission fluid is a multi-purpose substance that helps transmissions function well; proper fluid and filter service might be the singularly most important aspect of transmission maintenance.

“Automatic transmission fluid does much more than just lubricate,” says Duner. “The fluid actually is a component of the torque transmission between the engine and the transmission. It’s a lubricant. It’s used to pressurize and apply clutch packs within the transmission. It’s used to carry heat away from sources. It provides a variety of support to the transmission.”

Something as simple as changing transmission fluid and filters according to manufacturer guidelines saves your customers from future headaches and breakdowns. But one drain interval doesn’t fit all. Allison provides a matrix of mileage and service hour change intervals based on the vehicle and its vocational use. These guidelines are heavily influenced by severity of duty, so it’s important to both consult manufacturer guidelines by model number, and to talk to the customer about how he’s using his vehicle. If he’s operating in extremely harsh or abnormal conditions, then the service interval duration needs to be accelerated.

Allison also provides guidelines for selecting the proper filter and for its change interval. Filters that aren’t changed in a timely manner will impact the movement of fluid through the product, causing shift complaints. “All mechanical components generate wear material,” says Duner. “That’s a byproduct of being a mechanical component. But when filter performance starts to fail, the control system will have an increasing amount of difficulty dealing with the degradation of fluid flow as filters begin to plug up. In extreme cases, a technician will remove a filter from a transmission and find that it has either imploded or exploded, depending on which filter it is.”

The Benefits of Fluid Analysis
Fluid and filter change intervals are determined largely by a truck’s vocational use. But because no vocational truck, for example, operates in the exact same environment as another, a customer may want more detailed information about the state of his transmission fluid. If you have a customer whose truck operates in an extreme or unusual condition that makes fluid and filter change intervals unpredictable, his truck or fleet might be a good candidate for a transmission fluid analysis program.

The value of a fluid analysis lies in the establishment of trends. A single test is less important than conducting a series of tests to define normalcy for a vehicle. Once you establish a pattern, abnormalities in the results can pinpoint transmission issues.

“The goal of analysis is to find early indicators that represent proactive opportunities to get in front of failure,” says Duner. “You look for things like wear metal and glycol contamination.”

“Transmissions do not like antifreeze. A semi-common failure mode is a vehicle radiator segment that has failed, allowing the crossover of transmission fluid to engine coolant. An early indication of this is the appearance of glycol in a fluid analysis.”

Ethylene glycol coolant mixtures deteriorate non-metallic components like rubber and gasket material. It also harms steel parts, such as bearings and gears, because of reduced lubricity. The frictional capacity of the drive clutch plates can be lessened as a result of surface film or impregnation and the presence of glycol will deteriorate clutch plate material, according to the Allison web site.

When extracting a transmission fluid sample to submit for analysis, make sure to take the sample from a vehicle at operating temperature and from a clean source. If you don’t, you run the risk of influencing the analysis results and opening the door to unnecessary or incorrect repairs or fluid and filter change intervals.

Modern automatic transmissions are designed to be durable and long running, but if maintenance rules are not followed, they will be neither. Keep an eye on the state of the transmission fluid and the filters and watch all those wires; make sure they are in dry, clean positions and that they’re not frayed. A little regular maintenance work should, in most cases, be enough to guard against a major breakdown.

Also, although modern automatic transmissions are all electronically controlled, don’t let that trip you up. “Just because it’s electronically controlled doesn’t mean the problem is electronic,” says Duner. “There are a lot of vehicle basics that should be investigated. Don’t forget the basics.”

Get on the Vehicle Data Bus
Technicians are flooded with codes and classifications. It’s easy to get lost in the vast webs of numbers and letters. But it’s going to become increasingly important to know the SAE J1939 set of standards and what it means to diagnostic work.

SAE J1939 opens pathways of communication between components on a truck. It is a type of vehicle bus, which is an electronic communication system that connects components on a vehicle and helps with vehicle diagnostics.

“It’s important that technicians start looking at vehicles holistically,” says Keith Duner, manager of service technology, Allison Transmissions. “They need to understand the intricacies of these vehicle data buses.”

Each of the control systems in a vehicle is on an electronic data bus and is communicating with the other components so as to let everyone know what’s going on at any given time.

“This becomes increasingly relevant in diagnosing transmission performance issues going forward,” says Duner. “And while all component suppliers and OEMs go to great lengths to build sophisticated diagnostic tools, a basic understanding of J1939 is very relevant to a technician’s ability to execute repairs in a timely fashion.”

Know When to Ask for Help
Troubleshooting in today’s diesel shop is an exercise in content management for a technician. There are myriad parts numbers, diagnostic codes, engineering parameters and model numbers to juggle; there is no way everything is going to be remembered.

Fortunately, we have the Internet. Allison Transmission, for example, has a detailed public-facing web site,, which is filled with troubleshooting tips and service manuals by model number. Technicians also can find guidelines there for fluid and filter maintenance, as well as resources for additional help with more specific questions. There’s no need to play service guessing games with a wealth of information at your fingertips.

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