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Transmissions: Replacement, Rebuild or Repair

Manual Transmissions

Transmission DSC07161 200x153Repair 32x32First let’s mention that manual transmissions from this century rarely fail. Occasionally a manual transmission will die from abuse, sometimes because the clutch or shifter failed and the pilot forced shifting until the unit failed. The moral of this story is: Don’t shift fast unless really you need to, and never push really hard on your shifter. We have seen manual transmissions go a quarter million miles without a fluid change. The fluid was not pretty, but they made it.

If you change your manual transmission fluid by 80,000 miles, you probably won’t need another transmission ever. All manual transmissions in these modern cars run synthetic fluid rated GL4. If a manual transmission dies, rebuilding is much less expensive than an automatic. Replacement with used units is an option, just don’t expect perfection every time.

DSG and CVT Automatic Transmissions

DSG automatic transmissions are a recent breed since 2005.5. DSG means Direct Shift Gearbox. They are built like a manual transmission with 2 clutches instead of a torque converter. The Transmission Control Unit (TCU) is located inside the transmission. Early versions had a small rate of TCU failures, as well as broken dual mass flywheels and clutch pack issues. These problems were resolved within a few years, and modern units are very hardy.

CVT Automatic means Continuously Variable Transmission. Based on a steel link belt driven between two pulleys that vary in diameter to effectively change gear ratios infinitely, only a few of our fleet every had them, just some two wheel drive Audis since 2004. Early versions had a high rate of TCU failures, which are located inside the transmission. Inherent weaknesses of the steel band vs pulleys, clutch assemblies and planetary gear sets made them inappropriate for high power applications.

The Automatic Transmissions

transmission cogs 67x80It must be said that there are a couple of discouraging Scoreboards concerning automatic transmissions, their lifespans and failure rates, and your choices to remedy the problem. The factory removed the transmission service from their list of maintenance requirements in 1996. They felt that the new synthetic fluid was so good, you never had to change it. Really it was because said service added to the cost of maintenance and thus made their JD Powers ratings fall. They were afraid that additional cost would result in selling less cars.

Since then, we have found Eurovans and many V8 cars tend to have 30% or more of their transmissions fail before 100,000 miles. None of them ever make it to 150,000. Unless someone services their transmission, and regularly. Volkswagen 4 cylinder 4 speed automatic cars tend to start breaking transmissions around 130,000. The 5 speed automatics have a better track record. The hardiest units seem to be in the Audi 6 cylinder cars. Since 2004, the failure rate is fairly low, except for a few sporadic units like the Asian sourced Aisin brand transmissions found in the 2007 Jettas. Modern 6 to 8 speed automatic transmissions are gaining a good track record, but lack of maintenance will always take a toll.

The other bad Scoreboard is rebuilding. First, let us mention that we at Karmakanix have a policy of not dissing on other shops, and this statement is NOT aimed at ANY repair facility. Of all the 4 speed automatic transmissions for cars from this century that got rebuilt at an independent transmission rebuilder, it goes like this: 15% go back under the knife at least 3 times, yet never end up even drivable for more than a few weeks or months. About 50% may get multiple repairs, then work flaky for the rest of their numbered days. A whopping 80% never work, shift and drive like the car did new. They have something fluffy and/or iffy about the way they work. From our experience, these Scoreboard numbers are genuinely conservative.

The biggest factor in rebuild reliability is the valve body. In the valve body, electric solenoids and complicated piston-like valves and springs control the fluid supply to the different clutches and bands to achieve shifting. Since most of these units never had the fluid changed early in life, the valve bodies are often quite worn and cannot accurately control fluid transfer. The reason that a valve body causes transmission failure is simple in theory and very complicated in reality.

All the clutches and bands in an automatic transmission are controlled by diaphragms that are designed to leak at a specified rate. A clutch or band is actuated by controlling the volume and pressure of the flow of fluid to it. To disengage that clutch or band, one cannot reverse the flow of the fluid, one can only slow up and stop that flow, and the predetermined leakage rate allows the clutch or band to disengage. Consider that any gear change involves more than one clutch and/or band. Many different components may be engaging and disengaging simultaneously.

Testing the leakage rate of each component is called a leakdown test. That requires a special adapter that goes in place of the valve body, and has fittings to accept a leakdown tester, the same tool used to check for engine compression loss. A leakdown tester uses air pressure to feed the circuit, and shows the pressure applied versus the pressure maintained, giving a leakdown specification as a percentage of the pressure difference. Normal leakdown is given as a range and varies with each component, such as 35 – 45% or 60 – 75%. Performing leakdown tests can take between 2 and 4 hours, depending on the model.

A valve body is an aluminum housing with many dozens of passageways filled with steel pistons, springs, balls and electronic solenoid control valves. Old fluid with more filth than a sewer plant is really going to wear the aluminum badly and cause a loss of the fluid control necessary to work all those clutches and bands in smooth unison. The transmissions can beat themselves to death or slip like bananas on ice. This can cause transmission failure no matter how well rebuilt the rest of the transmission may be. Valve bodies vary in price, generally between $1000 to $2000, which is why rebuilders generally do not replace them.

At Karmakanix, we use mostly factory transmissions which virtually all come with new valve bodies, hence the price. For transmissions which are not available from the dealer, we give our local rebuilder free reign to replace the valve body with a new unit. With some cars such as Eurovans, we require a new valve body. With older cars with non-electronic transmissions, such as Vanagons, where new valve bodies are not available, we depend on our rebuilder to evaluate when a valve body has too much wear and must be replaced with a used unit.

Replacing an automatic transmission with a used unit is often worse than trying to have an aftermarket shop rebuild it. You are quite likely going to end up getting a unit very close to dying like yours did. If you could get a used transmission with 40,000 mile or less on it, it would probably be OK. What are the odds of finding one of those for a ten-plus year old car?

Ultimately, understand that transmission failure in general is not about age and miles, it is all about servicing the automatic transmission fluids. Never changing your transmission fluid will never result in long term reliability. And wear to internal parts may yield a transmission that cannot be successfully rebuilt for less than the cost of a factory rebuilt transmission.

The Facts About a Factory Transmission

When it comes to automatic transmission failure time, Karmakanix will recommend replacement with a factory unit. Factory rebuilds work perfect right out of the box, and if properly serviced, will remain that way for as long as you own it. We have only ever had one issue, and the dealer replaced it for us at no charge. While it is true, factory units cost way more than any other option, they are generally the only option that makes sense. What is the cost of being without your car for weeks while someone tries to get it right? Months? Keep going.

A failed automatic transmission represents much more of an issue than most because of the cost and the possible risks. Before considering any course of action, the question must be asked: Is it worth it for this car and for this owner? An accurate inspection of the rest of the vehicle is in order. Ask your Service Advisor to check on your vehicle’s service records for any transmission fluid change.

Torque Converter Faults

A problem from the first decade of this century, torque converter faults can be quite variable and require accurate evaluation. First a little history and description. Inside a torque converter is a fluid coupling which transfers rotating power between the engine and the transmission. Inside are 2 sets of vanes, the impeller and the turbine, which transfer power through the fluid by spinning very close to each other. This allows the engine to idle while at a stop when the transmission is not turning. The vanes let the engine run at a higher rpm than the transmission, which allows the engine to produce and transfer more torque, which is the twisting force. The increase in torque is created by a third set of vanes called the stator, which has a smaller diameter and is located between the two main vanes. Rather than trying to explain, we include a short 4 minute video which helps conceptualize the basic functions of a torque converter.

Operation of a Torque Converter

Torque Converter Basics

A YouTube video animation by Green Translations which shows the basic parts and explains how a torque converter works.

It can be seen that the torque converter is going to waste some amount of energy proportional to the force and the amount of slippage, which is going to cause a loss of fuel mileage as well as heat up the fluid. So at high speeds when the vehicle is cruising and torque multiplication is not needed, a lockup clutch inside the torque converter directly connects the engine to the transmission. The first versions in our fleet in the early 90’s simply locked solid all at once, often with a bit of a clunk. Within a few years, the lockup clutch engagement became progressive and smoother.

Rarely, a torque converter fails in such a manner that the lockup clutch will not disengage, a dampener spring breaks, or a vane gets badly damaged. Then the engine gets stalled when the vehicle comes to a halt. Very occasionally, the engine will run, but the severely increased drag or damage may cause the vehicle to shudder when slowing down and may require the driver to keep the brakes pressed hard to keep the vehicle stopped.

Most torque converter failures come with a code from the Transmission Control Unit (TCU) which names the torque converter lockup clutch. Many times the customer has not noticed anything unusual. That is because most faults are set when the lockup clutch engages too slowly, but eventually works. Sometimes the clutch may slip each time the driver lightly presses the accelerator, and uncommonly the clutch may always slip or not engage at all.

The system that engages the lockup clutch is a set of passageways that feed transmission fluid to the clutch assembly and control the buildup of fluid pressure with a solenoid on the valve body. The failure modes vary with age and model. A seal inside the torque converter can leak, causing the pressure to be slow to build up. The lockup clutch itself inside the torque converter may be damaged and slipping. The lockup clutch solenoid or wear in the valve body can be the interfere with proper fluid pressure control. Low fluid line pressure from the main pump circuit might be responsible, but would generally show other shifting symptoms as well.

A technician with training and a computer can usually identify and quantify torque converter lockup clutch issues. A measuring block generated by the TCU will give values as to whether the lockup clutch should be engaged and the amount of slippage at any given moment. Slipping over 15 to 30 rpm under light acceleration when the clutch should be locked up is about the limit. Any slippage while cruising is wrong. Some vehicles give us more information, like the voltage, duty cycle, or current to the lockup solenoid.

Determining the difference between a slippage problem inside the torque converter versus a sticking valve body may well require extensive diagnosis including a leakdown check of the lockup clutch hydraulic system.

An electrical issue with the solenoid, wiring or circuit board will have its own code to that effect in most vehicles. Solenoids can be tested for resistance values and the current mapped and checked. The circuit boards are the plastic PC board inside the transmission, which can get brittle and damaged by old transmission fluid. Rarely, the Transmission Control Unit (TCU) may be damaged, possibly by hooking up jump cables backwards, or grounding out some large unfused positive wire.

Most of the problems with codes indicating slippage faults are due to failed torque converters. Now the issue really is whether or not it is worth it or absolutely necessary to replace the torque converter. If the code occurs due to slight slippage, but the torque converter is working most of the time, we may advise not replacing it. If the transmission fluid is full of metal flake or large chunks of friction material, then the transmission may be close to failure, and should just be replaced or rebuilt. If the torque converter slips constantly or consistently, there are no other shifting problems at all, and the fluid does not show evidence of internal transmission damage, then the torque converter may be worth replacing. One needs to juxtapose the cost of torque converter replacement against the value and future of the vehicle.

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