Leading Causes of Manual Transmission Failure - Transmission Digest

Leading Causes of Manual Transmission Failure

The most common reasons a manual transmission failed are lack of lubrication, incorrect lubrication and improper or no clutch release.

Up To Standards

  • Author: Mike Weinberg, Manual Powertrain Editor
  • Subject matter: Manual units: major problems
  • Issues: Lubrication, clutch release

The most common reasons a manual transmission failed are lack of lubrication, incorrect lubrication and improper or no clutch release.

Complexity of lubrication

Lubrication should be simple, but in the modern world NOTHING is simple. There was a time when virtually all stick transmissions ran on 90w gear oil. The synchronizer rings were brass or bronze, the cars and truck shifted around 3,000 rpm, and we had 3- and 4-speed transmissions. 90 weight did not like cold weather and generated multiple complaints to the manufacturer due to cold shift problems. You needed two men and a boy to shift one of the units on a cold day until the fluid heated up.

Chrysler engineers, tired of the constant never-ending cold-shift complaints, specified ATF (automatic transmission fluid) for the manual transmissions, and cold shift problems declined due to the very thin viscosity of the lube fill. There were some changes in the gear alloy used to make the gears as ATF had a much lower film strength than 90 weight gear oil. ATF was also better at finding any place it could leak, but better gaskets compensated for that.

At this point in history the horsepower wars began and 5-speed and 6-speed transmissions entered the market. Fuel economy and higher horsepower led to higher shift points, and the old synchro rings were not up to the task. The new designs were made of clutch paper like automatic clutch plates, Kevlar, carbon fiber and sintered metals, and each of these demanded a very specific fluid for lubrication. Single synchronizers morphed into dual and triple ring designs to shift at much higher revs and handle much more horsepower. The transmission installer now had to stock a variety of manual transmission lubes as well as a growing number of lube fills for automatic transmissions that were undergoing the same development.

At this time we started to sell fluid as self-defense against installers who would note the lube fill tag on the unit, but if unable to provide it, they would put whatever was handy into the unit, which invariably generated a number of calls on Monday morning. Putting the wrong fluid in a unit with porous synchro compounds would cause notchy grinding shifts, and after draining the unit of the wrong oil, might take two weeks of driving to get it right, with loads of happy customers. Use the oil that is specified for the unit, even if it means you cannot deliver on time. Anything else is financial suicide and the death of 1,000 cuts.

Clutch release & slave cylinder

Clutch release is something that seems never to have been taught in shop class. It is the single greatest cause of problems and damage in manual transmissions. Let us take a walk-through history to study the changes. Back in the day clutches were actuated by direct linkage from the clutch pedal to the side of the bell housing through a series of levers, rods and springs. It was a simple system that failed as bushings, springs, and discs wore. Springs and levers gave away to flexible cables, and then to early hydraulics where the slave cylinder was mounted close to the bell housing.

Things began to get more complicated. The clutch can be adjusted to fully release the input of the transmission from the torque of the motor, which permits clean wear free shifts. The clutch can be adjusted where it never completely locks up and the disc will slip, hurting the clutch pressure plate disc, and flywheel, but causing no transmission problems. This shows up as slippage, with engine rpm walking away under acceleration, and the forward speed of the vehicle falling off. Burning up the clutch set does not hurt the transmission. Now we have incomplete clutch release that destroys the trans in short order. What happens here is the driver steps on the clutch pedal and the slave cylinder (Figure 1) cannot move far enough to cause the pressure plate to completely release the clutch and create a 50/1,000 air gap between the disc and the pressure plate and flywheel. This means that the torque of the engine is still turning the transmission input shaft, and as the driver tries to complete the shift the engine power as well as the vehicle weight and momentum gets applied to the synchro ring and damage starts immediately.

This situation worsened dramatically with the introduction of the concentric slave cylinder (Figure 2). This slave cylinder is hollow and mounts to the front of the transmission. Adjusted properly and properly bled of air in the hydraulics, it works fine. However one needs to measure the travel of the slave to the fingers of the pressure plate and determine if the slave cylinder needs to be shimmed forward toward the engine block. I have never seen any directions to do this on any packaging, but it is necessary to determine how much travel is required. This is not something you can see or measure when the trans is installed, but it is very easy to do before you bolt things together. To put this problem to bed make a few measurements (Figure 3).

1) Lay a straight edge across the fingers of the pressure plate.

2) Measure the distance from the fingers of the pressure plate to the engine block surface that the bell housing will be flush to when bolted up. Example: 310/1,000

3) Make sure that the concentric slave is completely contracted toward the transmission face. Using a straight edge, measure from the contact section that will press on the pressure plate flange of the release bearing to the front flange of the bell housing that will bolt up to the engine block. Example: 780/1,000.

4) Subtract the finger height measurement from the block surface from the bell housing to release bearing 0.780-0.310=0.470. We are looking for an air gap from the slave to the fingers of 190/1,000. Subtracting 0.190 from 0.470 we arrive at a 280/1,000 shim that must be placed behind the slave cylinder before it is bolted on to the trans to move the slave forward toward the pressure plate fingers and create a perfect clutch release. This is a very simple deal. In many cases you will not need to make any shim changes when you are replacing stock parts with stock parts. Changing brands, doing custom-unit swaps, dual and triple disc from single disc, etc., always measure shim size and install the correct shim. The number 190/1000 has worked for us forever and you will save a tremendous amount of time and transmission damage by making a few simple measurements. Back in the day when we were racing Cadillac CTS-Vs for GM and using Exedy performance clutches, we actually had to manufacture a shim plate that was 500/1000 or half an inch.

Bleeding the clutch

Another issue that needs discussion is bleeding the clutch. Looking at most of the modern concentric slave cylinders, they are very difficult to bleed. They do not leave you adequate room to do this right and easily. The answer here is to extend the bleeder hose before installation. On the CTS-V Cadillacs we made them 3 feet long and tied them up out of the way. Bleed the clutch from below. Use a power bleeder and the bubbles from below will naturally rise up to the master cylinder. There are units that have the master cylinder mounted to the firewall at an angle (Ford Ranger). Jack up the rear of the truck until the master cylinder is level with the ground and bleed from below if you want to deliver this car sometime this month. Back in the ancient times there was no way to get a good pedal on a SAAB without a power bleeder.

Help those who want help

Occasionally I get calls on the tech line regarding the problems caused by lack of proper clutch release. We explain the procedures to rectify the situation and most callers seem to be grateful for the help and advice.

Lately, with some callers, there has been a sense of superiority, often due to information gleaned from the internet; a change in attitude. Some will tell me I don’t know what I am talking about, and my answer is “why are you calling me?” I am an old-school guy who came up the hard way and was grateful for anything I was taught by experienced people. We are not impressed with today’s uninformed people who claim to have 2,000 friends on Facebook. I probably know about 6,000 people worldwide and they are acquaintances. I have two friends and they will come out at 3 a.m. in the rain to help me move a body with no questions.

Be kind to those who are helping you, give back whenever you can, learn something new more than once a day, be proud of your profession and the people who made it what it is. Be a man, which means keep your word, and be courteous to all. Respect you have to earn.

You May Also Like

Tires Vastly Improved, but Check the Specs

The advancement of technology in the automotive field is rapid and unrelenting. Forces that shape the marketplace, state and federal regulations, the need to attract new customers, and the need to be different and at the same time profitable are driving the car makers to develop technology at a pace never seen before.

Up To Standards

Author: Mike WeinbergSubject Matter: Tire technologyIssue: Check tires before working on vehicle

Technical Training

The advancement of technology in the automotive field is rapid and unrelenting. Forces that shape the marketplace, state and federal regulations, the need to attract new customers, and the need to be different and at the same time profitable are driving the car makers to develop technology at a pace never seen before. This also has an effect on prices, with every aspect of the components of the car becoming more expensive and more important to the operation of the vehicle. Simple wear items such as a car battery have improved in technology and longevity with a steep rise in price.

Are We Speaking the Same Language?

If you are repairing transmissions for a living, you will invariably spend some time on the phone ordering parts and speaking with technical hotlines to assist in your diagnosis of problems. Having been on both ends of a tech line for over half a century and an equal amount of time buying parts, I have learned a whole new language. To be successful communicating with those entities, one must understand the language and be speaking about the same correct topic with whoever is on the other end of the conversation.

Electronics In Dual Clutch Units

This article is the final segment of our exploration of the dual-clutch transmission, or direct shift gear box (VW and Audi) or DualTronic in BorgWarner’s brand. We have included several schematics from a VW Touran model, as VW has the largest amount of these transmissions in use. VW using its VAS 5051 diagnostic system provides for, as they say, “guided fault finding,” which means that a test schedule is available for the unit and provides testing for sensors, actuators and the mechatronic (computerized) valve body.

Hydraulic & Fluid Controls in Dual-Clutch Units

To recap, we have looked at how the dual-clutch transmission functions, essentially two gear boxes in one with the input shafts driven by hydraulically applied clutch packs that drive three concentric shafts that are one inside the other. The innermost shaft drives a gerotor-type of fluid pump that provides pressurized fluid to actuate the dual clutches, lubricate and cool the components, and shift the transmission into the selected gear. The next two shafts are driven by the two clutch packs with one shaft shifting the even-number gears and one shifting the odd-numbered gears.

Controls Make Shifts Happen in Milliseconds

If you have been following the previous chapters of this series of articles, you are starting to understand the function of dual-clutch transmissions. We have used illustrations from the VW Direct Shift Gear Box (DSG) as VW has about 2 million of these units on the road at present.

Other Posts

The Beauty of Having Two Separate Gear Sets

In last month’s article, we began to study the dual-clutch transmission, looking at its potential to eventually replace the common torque converter-planetary automatic transmission. In this chapter, we will look at the mechanical theory of operations on how these gear boxes work. We will be using illustrations from VW and Audi who were the first to mass market this design in 2003, in the Audi TT and VW Golf models.

Dual Clutch Transmissions: Are They the End of the Torque Converter?

We have enjoyed two major designs of transmissions for many years. The manual transmission where the driver disconnected the power flow from the engine to the transmission by stepping on the clutch pedal and manually moving the shift lever from the present gear to the next gear to be selected.

The New Year: It Will Be As Good As We Make It

Note: Mike Weinberg wrote this new-year outlook piece 10 years ago. Looking back to that time is instructive as it shows how business owners have endured tough times and how they must persevere in the same fashion going forward.

Litigation: It’s a Lawyer’s Nation

In business, it is very difficult and costly to operate in this regulatory minefield. It is also necessary to school yourself in operating a business properly to avoid litigation whenever possible. You are working on cars and trucks. The driver’s and passengers’ lives and well-being are in your hands. Any mistakes in repair procedures or defective parts can come back to haunt you and can become extremely expensive and time consuming to settle. You hear every day about automotive recalls that cost the manufacturers billions of dollars, such as defective airbags and faulty ignition switches that result in death and injury. The following are some tales from the past that may give you a better perspective on how to fine-tune your operations to avoid such disasters.