Driving about 45 MPH there is a sudden shudder in the vehicle. It almost feels like running over a rough spot in the road or a cattle guard. As quick as it appeared, it is gone, until the next time it occurs.
Torque converter shudder and diagnosis
Many people describe this feeling as running over a rough spot in the road. Very often the problem is a torque converter shudder. A torque converter shudder is a brief shake or shudder in the vehicle when internal clutches apply inside the torque converter. They refer to this application of clutches as "lock-up." Often this problem is mis-diagnosed and sometimes they needlessly rebuild transmissions as a result.
A quick test
On many vehicles, one way to identify a torque converter shudder is lightly touching the brake pedal. While carefully maintaining vehicle speed with one foot, delicately apply the brakes with the other. If the shudder immediately stops, with brake application, the problem is likely the torque converter clutch. When the operating system sees brake application, most vehicles release the torque converter clutches. A trained transmission-service technician can also test for torque converter lockup using a specific vehicle scan tool. These tests are important as a slight engine misfire very often causes the same sensation and is may be mistaken for a torque converter shudder.
How a torque converter lockup works
The torque converter is a coupling between the engine and transmission of the vehicle. The purpose is to allow the vehicle to come to a stop, with the engine running. To do this something has to slip, and that is the torque converter. From this perspective, it acts much like an automatic clutch.
A torque converter works by transferring power, through fluid motion. An easier way to understand might be to consider two desk fans. We connect one fan to a source of power and turn it on. We do not turn on the second fan, but place it facing the first. Motion of air, coming from the running fan, can make the blades in the second fan turn. A torque converter works similarly, but uses fluid rather than air. Fluid is much more viscous than air and transmits much more power.
They attach the torque converter housing to the engine flywheel. When the engine runs, blades inside the converter rotate through the transmission fluid. The motion of this fluid causes other blades, attached to the transmission, to turn. This drives the vehicle and at idle, allows the engine to run with the vehicle remaining still.
Slipping is necessary in order for the vehicle to sit still with the engine running. Because some slippage continues when driving, fuel mileage suffers. Modern vehicles use a clutch inside the torque converter to prevent slipping, when we no longer need it. This clutch "locks-up" so the transmission torque converter rotates the same speed as the engine. This transfers more of the available power and helps fuel mileage.
When the internal clutch applies, they call this torque converter lock-up. This occurs after the vehicle is rolling and ceases when it comes to a stop. This clutch requires very specific lubricants in the transmission fluid to work properly.
What causes torque converter clutch problems?
Torque converter clutch lubrication is one reason for so many automatic transmission fluids (ATF) on the market. Without the additives in these fluids the clutch may chatter when it engages, causing the shudder sensation. If allowed to chatter, we may damage the clutch. Material from the clutch will circulate with the fluid and can ruin the automatic transmission. Not replacing the automatic transmission fluid often enough may create a torque converter shudder.
Using improper fluid, as when flushing a transmission, can cause a torque converter shudder and many other problems. This is a particular problem with Honda and Ford's vehicles. Most automatic transmission fluid contains friction modifiers to help prevent shudder. Time, heat and mileage cause the additives in automatic transmission fluid to become depleted. Without these additives, clutches may chatter and wear. Depleted fluid can also cause gasket and seal shrinkage, resulting in leaks. Ford rear-wheel drive vehicles have had a good deal of trouble with this.
Preventing problems with a torque converter shudder
Prevention and even correction is often as simple as a proper transmission service. Most transmissions have a pan and a removable filter. On these vehicles, we drain the fluid and we remove the transmission pan for a proper service. This allows the technician to inspect the transmission for wear and broken parts. At this step, small problems can be easily corrected, preventing major breakdowns.
While the pan is off, we tighten the bolts on the transmission valve body to the specified torque. In time the valve body gasket shrinks, from heat and the bolts loosen. A loose valve body can develop cross-leaks among the passages. This can cause transmission failure. Properly tightening these bolts can help prevent this problem.
After inspection and checking the valve body bolts, we replace the transmission filter and seal. Next we install the pan and we fill the transmission with the proper fluid. Replacing the filter is key on transmissions with replaceable filters. Restricted transmission filters lower internal pressure and can destroy a transmission very quickly.
On transmissions that do not have a pan or replaceable filter, a double drain and fill is the proper service. First the technician drives the vehicle and then drains the fluid. The proper fluid is added and they drive the vehicle again, to remove any remaining debris. Finally they drain the fluid a second time and refill with the proper fluid.
When the service is complete, we drive the vehicle until it reaches full temperature. The fluid level is checked multiple times and adjusted if needed. A final inspection for leaks completes the proper service.
A proper transmission service is very different from a transmission flush. With a transmission flush, they circulate new fluid through part of the transmission. They do not replace the filter nor do they inspect internal components or tighten valve body bolts.
Bigger automatic transmission problems
If we find excessive debris in the transmission pan or if the shudder persists after a proper service, we may have to replace the torque converter. In other cases, we might need a transmission-rebuild. Continuing to drive with the shudder can result in automatic transmission failure. When this occurs the transmission will need to be repaired, replaced or rebuilt. Proper automatic transmission service, at regular intervals are the best prevention. Let the experts at AGCO properly service your transmission. AGCO, it’s the way to go.