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What Is a Torque Converter?

John is a fervent writer, gamer, and guitar lover. He is a former automatic-transmission repairer, welder, and hobbyist game developer.

If you work with vehicles, or you’ve ever owned a car with an automatic transmission that’s needed repair work, you may have heard the term “torque converter” before. But what is a torque converter?

A torque converter from a GM5L40E transmission. This particular model is notorious for failing torque converters.

A torque converter from a GM5L40E transmission. This particular model is notorious for failing torque converters.

In the context of an automatic transmission, a torque converter acts as a kind of interface between the gearbox and the engine. It essentially acts in the same role as a regular manual transmission clutch, allowing the engine and transmission to turn at different speeds when necessary. This is necessary to prevent the engine from stalling at lower speeds.

But that’s an incredibly basic explanation. Let’s really get into what a torque converter is, what it does, how it works, and what vehicles you might find it in.

Why Are Torque Converters Needed?

Now that we kind of know what it is—why do we need it?

Your regular internal combustion engine (that is, the type of engine in most cars since the dawn of car kind) cannot immediately produce sufficient torque to get a big heavy vehicle moving from standstill. Engines have a minimum speed that they need to be turning at in order to keep running. If they drop below this speed, the engine stalls and will need to be restarted.

The solution to this is to put a coupling between the engine and the driving load, which in the case of an automatic transmission, is a torque converter. This allows for the transmission rotation to be independent of the engine rotation when needed. Another way of putting it is that the engine can carry on turning even when the transmission is not.

Most modern torque converters also provide a lockup function, which essentially locks the engine and transmission rotation together when the speeds of each are roughly the same.

Do All Cars Have Torque Converters?

And do all cars need a converter?

In a word, no. In a few more words, manual transmission cars don't have them at all. These cars have a basic clutch arrangement which is controlled by the driver, who will cause the clutch to engage, disengage, or slip as needed. The exact same principle applies to semi-automatic transmissions (though now it is a computer controlling the clutch).

You will also not find a torque converter in certain types of electric vehicles. The reason for this is that the electric vehicles I’m referring to are able to produce full torque instantly. They don’t need an interface to cushion the drivetrain while the engine torque builds up, because the torque is already there. Things like transmissions and torque converters are essential for a lot of cars, but they do make the drivetrain less efficient, so designers won’t use them if they’re not necessary.

Finally, certain types of automatic transmission also do not need a torque converter. Specifically CVT (continuously variable transmission), and DCT (dual-clutch transmission), because the transmission itself is capable of acting as that interface.

A GM5L40E transmission with torque converter seated in the bellhousing. This model of transmission is fitted to a number of different models of vehicle.

A GM5L40E transmission with torque converter seated in the bellhousing. This model of transmission is fitted to a number of different models of vehicle.

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How Do Torque Converters Work?

The simplest way to explain how a converter works is to imagine two regular cooling fans facing each other. If you turn on one fan, it will cause the inactive fan to spin. However, if you lock the inactive fan so that it can’t spin, the active fan will carry on as normal, unaffected. A torque converter works a little like that.

Of course, the “fans” are actually turbines, and the medium used is not air, but transmission fluid. The interior of the torque converter is cleverly designed so that centrifugal forces generated by the spinning casing (which is directly driven by the engine) force the oil where it needs to be.

A torque converter from a ZF4HP24, cut open to reveal its inner workings.

A torque converter from a ZF4HP24, cut open to reveal its inner workings.

A crucial aspect of this setup is what happens to the oil once it has completed it’s task. If it was allowed to just slosh around, it would quickly cause the torque converter to become very inefficient. Instead, the oil is redirected by the stator, which sits at the centre of the converter. On converters that have a lockup feature, this is also where the feeds for that will be situated. In these cases, the lockup will usually be controlled by a solenoid in the transmission itself which will direct oil into the converter when needed.

What Happens When Torque Converters Go Bad?

A variety of symptoms can result from a faulty torque converter, but for the sake of this article, I’m going to focus on three particular symptoms that I feel cover the main examples of converter faults you’re likely to face. I repeat: this is not a comprehensive list of all converter faults.

1. Fault Mode

Fault mode (or limp mode, or failsafe mode) is a fail state of modern transmissions. When a fault is detected by the transmission’s control module, it may put the gearbox into fault mode, locking it into a single gear to limit damage and keep the occupants of the car safe. When this happens, a scan of the vehicle's systems may point to a solenoid, but it is often the physical converter which is causing the problem. In some cases it can be both, such as in the GM5L40E (more on that shortly) where the torque converter lockup solenoid can go faulty, taking the converter itself with it.

2. Juddering

Possibly the most common fault you’ll experience from a torque converter that is having issues is a juddering sensation when driving a lockup speeds, particularly when going uphill. This juddering can feel a bit like driving over a rumble strip, or cattle grid, and is caused by the lockup plate being worn enough to not quite apply properly. Sometimes this juddering sensation is mild, but other times it can be very harsh. In situations like this, the fault will almost always get worse as the converter components continue to deteriorate. One of the worst culprits for this is the ZF6HP26 transmission, especially when fitted in a Land Rover Discovery III.

3. Loss of Drive

If a torque converter has been allowed to degrade to an extreme, there is a strong chance your vehicle will lose drive altogether. This can happen as the result of one of two things. The first involves the lockup plate wearing down, either over time with wear and tear or due to a fault. When that happens, the lockup plate itself can wear down to the point where it no longer applies properly, causing a loss of drive, but the other thing that can happen is debris from that worn plate can also clog up the transmission filter.

Automatic transmissions (especially those with torque converters) need hydraulic pressure to operate. If the filter is blocked, no transmission fluid can be pulled into the system, so no hydraulic pressure is achieved. This cause of drive loss would still be the fault of the converter, even if the converter itself hasn’t yet failed completely.

One of the more egregious perpetrators of this fault is GM, with its 5L40E transmission, particularly in L322 model Range Rovers.

Now You Know

And that’s what a torque converter is, where you might find it, what it does, and what can go wrong with it. If the subject should ever come up, you will be well prepared to sound like you know what you’re talking about!

© 2017 John Bullock

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