In our workshop, we see a number of transmission issues, on a number of different cars, from BMW, Audi, Volvo, and Honda. Almost always on vehicles not serviced by us, but rather referred to workshop by the dealer, or another repair facility. Over the years we’ve compiled a very effective set of diagnostic principles to make the determination of the failure mode, and corrective action, very straightforward.
The old bromide that “An ounce of prevention is worth a pound of cure” never rang truer than in the case of the automatic transmission. Proper servicing, which is all too often overlooked on today’s newer cars with their extended service intervals, will go a long way to preventing problems in the first place. We strongly advise to all of our customers, to consider changing the transmission fluid and filter at the 60,000 mile mark. We see all kinds of crazy schedules from the manufacturers, from never changing transmission fluid, to changing at 90,000, or 120,000 miles. This is a recipe for failure, setting the customer up for a very expensive, and very unexpected transmission failure down the road. Nothing is less costly, or can do more to prolong the life of your car, than changing the fluid and filter (if equipped) on a regular basis, BEFORE failure.
What Is Really The Cause?
Torque converter problems are sometimes misinterpreted as symptoms of a failing transmission. Unfortunately, this can lead people to think that they need to spend thousands of dollars to rebuild or replace their automatic transmission when the cost to replace a malfunctioning torque converter is considerably cheaper. However, diagnosing the cause of a transmission issue isn’t easy. In many cases, the torque converter will not actually be the source of the problem (you might just have a fluid leak!). The purpose of this guide is to simply help you narrow down the possibilities and educate yourself before you get your transmission checked out.
What Does A Torque Convertor Really Do?
In a nutshell, a torque converter is a fluid coupling that transfers torque from the engine to the transmission. It is mounted between the engine and transmission, bolted directly to a ‘flex plate’ which is spun by the crankshaft.
Internal combustion engines create power by burning fuel that forces the pistons to turn the crankshaft located at the bottom of the engine. This rotational force is transferred to the transmission by the fluid pressure inside the torque converter.
Inside of the torque converter cover lives a series of propeller-like blades called the pump. This assembly spins in unison with the engine crankshaft, forcing transmission fluid onto another blade assembly called the impeller. This second set of blades is connected to the transmission input shaft. The amount of hydraulic pressure that it creates inside the transmission dictates the gear and ultimately, the speed of the vehicle.
The impeller’s speed is regulated by the engine side of this hydrodynamic circuit (ie. speed of the pump blades). When the vehicle is stationary, or the driver applies the brakes, the impeller will slow considerably, while the pump continues to spin. This allows the torque converter to act like the clutch in a manual transmission – it allows the engine to continue running while the vehicle is at a complete stop.
Once the transmission fluid has been hurled onto the impeller blades, it has to return to the pump in order to keep the cycle going. Since the fluid is now flowing in a different direction than the pump, it has to be reversed to avoid slowing down (and stalling) the engine.
To do this, a third finned wheel called the stator is located between the two turbines on the transmission pump shaft. Its blades are precisely angled so that when the transmission fluid hits them, it reverses direction and gets channeled back to the pump. When the vehicle stops, its built-in one-way clutch causes it to stop spinning, breaking the hydrodynamic circuit.
How Does It Work?
Once the vehicle starts to accelerate from a stop, the stator is once again free to spin. In the split second that the transmission fluid hits the back of the now-released stator, it starts to spin the transmission pump, and briefly multiplies the torque coming from the engine side of the circuit. This causes the transmission pump to force more fluid in the transmission, resulting in movement.
Once the vehicle is in motion, the stator’s one-way clutch allows it to start spinning in the same direction as the other turbines, reversing the fluid flow and completing the hydrodynamic circuit.
After all of the transmission gears have been shifted through and the vehicle has reached cruising speed, the lockup clutch engages, connecting the front cover of the torque converter (aka the pump) to the impeller. This causes all of the turbines to work together in a direct drive/overdrive scenario.
6 Signs of a Torque Converter Problem
It isn’t easy to isolate and diagnose a torque converter issue without taking the transmission/drivetrain apart, but there are several symptoms to look for. A few of the signs of a malfunctioning torque converter include: shuddering, contaminated fluid, gears change at high RPMs and strange sounds such as clicking or whirring.
Since a torque converter is responsible for translating engine torque into the hydraulic pressure needed to shift gears inside the transmission, a damaged fin or bearing can cause the transmission to delay a shift, or slip out of gear. Slipping can also be caused by there being not enough or too much fluid in the transmission, or excessively worn or dirty fluid. You may also experience a loss of acceleration and a noticeable reduction in your car’s fuel economy.
If the temperature gauge indicates that your car is overheating, it could be a sign that there has been a drop in fluid pressure and there is a problem with your torque converter. If a converter is overheating, it won’t be able to transfer power from the engine to the transmission. This results in poor throttle response, and excessive wear and tear on the internal workings of the transmission. Low fluid levels or a malfunctioning convertor clutch solenoid can also cause a transmission to overheat.
If the lockup clutch inside the torque converter is starting to malfunction, you may experience shuddering at around 30-45 mph. The sensation is very noticeable and typically feels like you’re driving over a rough road with many small bumps, or even like an engine misfire. As the converter switches over to direct drive, a worn lockup clutch can make the transition difficult, resulting in this sensation. The feeling may start and stop abruptly and may not last long, but if you’ve experienced it several times, it’s time to get your transmission checked.
Contaminated Transmission Fluid
A torque converter is filled with automatic transmission fluid (ATF). If the fluid is contaminated, it can do damage the parts inside. This can result in worn bearings on the stator, or damaged fins on one of the turbines.
If you notice a significant amount of black sludge/grime/debris in the fluid it could mean that the converter or transmission itself is damaged. In this case, change the fluid and drive around for a while before checking the fluid again. If the problem persists, get your car checked by a professional.
Higher Stall Speed/Gear Engagement RPM
The ‘stall speed’ is the point at which the engine RPMs are high enough for the torque converter to transfer power from the engine to the transmission. In other words, it is the RPM at which the converter will stop the engine speed from increasing if transmission output is prohibited. If the torque converter is broken, it won’t be able to transfer the engine’s rotational force into hydraulic pressure correctly. This will result in the transmission taking longer to engage the engine, causing the stall speed to increase. Here is how to do a stall speed test. You’ll have to find out what your vehicles stall speed is beforehand (typically 2000 to 2500 RPM).
It’s not uncommon for the torque converter to emit strange noises as it begins to fail. Some of the sounds you might hear include a ‘whirring’ sound coming from bad bearings, or ‘clinking’ sound coming from a broken turbine fin.
Your Advocate, On Your Side
At AMC, because we are independent and locally owned, we are YOUR advocate, and our team makes a point of checking recalls and service campaigns from the manufacturer on every visit, and keeping our customers up to date.
Not a simple repair, yes, but one, if done correctly, will last several years. That’s our goal with BMW, and other autos here, fix it right the first time, and prevent problems from happening in the first place. 30 years of service experience have well taught us that “an ounce of prevention is worth a pound of cure.” Be cured, once and for all, and give us a call, we’re happy to answer any service questions you might have on your BMW, Audi, Mercedes, Lexus, Volvo or Volkswagen.
Earning your trust, every time you turn the car…that’s what we do…every day…for the last 30 years. Click here see what our happy customers have to say about us and our service at AMC Customer Reviews.
If you have questions, or if we can be of further assistance, just call us at (207) 882-969, we’d love to meet you, and your car!
Bruce and the AMC Service Team