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How to Diagnose Manual Transmission Problems
Copyright AA1Car
It seems that every few years the vehicle manufacturers add more gears. Simply put, more gears mean less rpm drop between shifts - and that allows the driver to keep the engine within its optimum power range as the vehicle's speed changes.
Five-speed transmissions were the hot performance setup in the 1980s and 1990s, then came six speeds in such vehicles as the Acura Legend, Acura NSX, Mitsubishi 3000 GT, Toyota Supra Turbo, Porsche 911, and BMW 540i, M3 and 6-Series. Now we have transmissions with 8 gears.
A state-of-the-art production 6-speed transmission is BMW's Sequential Manual Gear box (SMG). The SMG tranny, which is about a $1,425 option on BMW 6-Series, M3 and Z4 models, does not have a manual shifter in the classic sense because gear changes are handled electrohydraulically. There is also no clutch pedal. The driver chooses an automatic or manual shift mode and the computer handles the rest. The manual mode allows the driver to shift gears at will, either by tipping the shift lever on the center console, or by pressing shift paddles on the steering wheel (similar to Formula 1 race cars). The computer controls the throttle open and operation of the clutch to flawlessly change gears - and faster than any human could make the maneuver. We are talking 80-millisecond shifts at full throttle! The computer is programmed to shift more aggressively depending on how the vehicle is being driven: Soft and smooth for normal, everyday driving and really quick and hard when the situation demands it.
Ferrari uses a similar automated manual 6-speed gear box called Selespeed in its F355 F1 car and the 360 Modena, and a 5-speed version that shifts a little slower and softer in the Alfa Romeo 156.
Another high-tech manual transmission that has appeared in recent years is Borg Warner's DualTronic, which is used in the Audi TT 3.2L and known as a Direct Shift Gear box (DSG). Like the BMW and Ferrari automated manual gear boxes, this transmission has no clutch pedal and is electrohydraulically controlled. It can operate in a semi-automatic mode in which the driver changes gears using buttons or the shift lever handle. There is also a fully automatic mode, where the computer decides which gear is selected.
Unlike other manual transmissions, the Audi TT DSG transmission has two multi-plate clutches. One connects to the 1st, 3rd and 5th gear shaft and the second connects to the 2nd, 4th and 6th gear shaft. This allows smoother and faster shifts than a conventional manual gear box. By simultaneously disengaging one clutch and engaging the other, the transmission shifts seamlessly from one gear to the next without pausing. Upshifts take only eight milliseconds (10 times faster than BMW's SMG transmission), making it the fastest shifting manual that is currently available.
The automated manual gear boxes are still fairly new, so it may be awhile before you see them. But sooner or later, late-model vehicles equipped with these state-of-the-art electronic manual gear boxes will be out of warranty and in your shops for repairs. In the meantime, there are plenty of conventional 5-speed and 6-speed manual transmissions to keep you busy.
MANUAL TRANSMISSION DIAGNOSIS
People buy vehicles with manual transmissions for several reasons, one being that a manual transmission often comes standard and costs less than an automatic. Manuals are also more durable than automatics and make sense for drivers who put a lot of miles on a vehicle or plan to keep it a long time. And they are just fun to drive: Manuals give the driver more control over the drivetrain and the engine's power output to the wheels. But they can also be very tiring to drive in heavy stop-and-go traffic.
Consequently, most manual transmission problems fall into one of three areas: Clutch related (worn or slipping clutch); clutch or shift linkage problems (leaky slave or master hydraulic cylinder, broken or misadjusted cables, worn release bearing, etc.); or the gear box itself (bad synchronizers, noisy, worn or broken gears, worn bearings, bent or broken shift forks, pops out of gear, and so on).
If the transmission seems to be making noise, don't assume the problem is a bad transmission. A lot of things can produce noise that may sound like it is coming from the transmission. This includes worn CV joints, loose or broken motor mounts, and flywheel or clutch problems. A test drive will confirm the complaint and help you diagnose what might be causing the noise.
- If the noise is only noticeable at idle, does it change when the clutch pedal is depressed? A change would probably indicate a problem with the release bearing or pilot bearing. To find out what is causing the noise, set the parking brake, place the vehicle in neutral and start the engine.
- If you hear growling or grinding noises when the clutch is engaged, the cause is the transmission input shaft bearing.
- A squealing sound that occurs when the clutch pedal is depressed and held is usually caused by a bad pilot bearing or bushing.
- A chirping noise that intensifies when the pedal is slowly depressed would indicate a bad release bearing.
- If you hear chirping while idling in neutral and the noise goes away when the pedal is slowly depressed, the fork/pivot ball contact point is making the noise.
Tip: On some BMW M3 models, what sounds like transmission noise at idle is often caused by a bad dual mass flywheel. Replacing the flywheel usually cures this problem.
- Noise that occurs only while driving or in a certain gear usually indicates a gear or bearing problem inside the transmission.
- If a particular gear grinds when shifting, that gear's synchronizer is probably damaged and needs to be replaced.
- If all the gears grind or clash when shifting, the clutch may be dragging or not fully releasing when the clutch pedal is depressed.
- Gear noise in all gears at all speeds may be the result of worn shaft bearings, thrust washers or gears, or a low lubricant level in the gear box (check the level).
- A transmission that is stuck in gear can be caused by a bent shift linkage, broken shift cable, a shift rail or interlock failure, or a broken gear inside the transmission.
- A transmission that jumps out of gear may be due to bent, binding or loose shift linkage, a loose transmission or clutch housing, worn or damaged input shaft bearings, broken motor mount, worn tapered gear teeth or synchronizer hub splines, loose shifter fork or rails, interlock hardware that is broken or missing, or excessive end play or runout in the output shaft or countergear.
- If nothing happens when the transmission is put into any gear, the clutch may not be engaging, a CV joint or U-joint may be broken, or the main drive gear or countergear is stripped.
TRANSMISSION LUBRICATION
The type of lubricant that is used in a manual transmission can have a significant impact on noise as well as how the transmission shifts and feels - especially during cold weather. A lubricant that is too heavy for cold weather can make a transmission sluggish and hard to shift. A lubricant that is too thin for hot weather may increase noise and wear. The safest recommendation is to always use the type of lubricant specified by the vehicle manufacturer.
BMW, for example, uses automatic transmission fluid as the factory fill lubricant in many of its manual gear boxes. Saab uses 10W-40 motor oil. But some people replace the original lubricant with a synthetic gear oil. Synthetics are great lubricants because they flow well at low temperatures and maintain their viscosity at high temperatures. Synthetics are often touted as being lifetime lubricants, but one transmission rebuilder says synthetics may cause shift problems in a newly rebuilt transmission. They recommend using a mineral oil lubricant for the initial break-in period, then changing to a synthetic after 2,000 to 3,000 miles of driving.
The oil level inside a transmission is critical to keep the gears and shaft bearings lubricated. A low fluid level can ruin a manual transmission in a few thousand miles or less, so always check the oil level when doing other preventive maintenance on the vehicle. If a transmission is making noise, checking the level won't do much good because the damage will have already been done. Adding oil may reduce the noise a bit, but sooner or later the transmission will probably have to be overhauled or replaced.
Transmission oil leaks should not be ignored. Replacing a leaky seal now can reduce the risk of a premature transmission failure caused by loss of lubricant. A leaky input shaft seal may also allow oil to contaminate the clutch, causing additional problems and requiring the clutch to be replaced, too.
SPECIFIC TRANSMISSION PROBLEMS
The following vehicle-specific tips were provided by Nat Wentworth at Eriksson Industries, a rebuilder of import transmissions:
- On ZF 5-speeds in certain BMW applications, shift problems can be caused by worn detent balls and springs. Typically, the transmission may not want to go into 5th gear when it is cold. In this case, the transmission does not have to be rebuilt or replaced because a shift detent repair kit can be installed without having to remove the transmission. The BMW repair kit to ask for is P/N 1053298008.
- Another BMW transmission problem that sometimes occurs is popping out of 1st and 2nd gear when the vehicle is under load. The cause is a worn guide sleeve inside the transmission, which does require replacing or rebuilding the transmission.
- Some Porsche 6-speed transmissions reportedly have a pinion shaft bearing that can go bad.
- On certain Saab models, a plastic speedometer gear inside the transmission can sometimes break. When this happens, it causes additional driveability problems because the engine computer needs a speed signal for spark timing and other engine control functions. The gear is not accessible from the outside, so the transmission must be removed and opened up to replace the gear.
OVERHAUL OR REPLACE YOUR MANUAL TRANSMISSION?
Overhauling a manual transmission or transaxle is not as difficult as overhauling an automatic, but it does require a certain amount of know-how, experience and special tools, such as pullers and drivers. On Audi and BMW transmissions, for example, it takes several tons of pressure to press-fit the overdrive gear on the cluster gear.
Overhaul kits are available for many manual gear box applications and typically include bearings, gaskets and seals. Synchronizer rings and other parts are not included and should be replaced on an as-needed basis.
Those who rebuild transaxles say several components should always be replaced: The input shaft bearing, the mainshaft side bearing and the two differential bearings.
Setting up the proper amount of crush fit on the differential bearings is extremely important and must be done correctly, otherwise the bearings won't last. The crush fit is determined by using shims of various thickness until the amount of rotating resistance is correct (which may require splitting the case several times). Reusing the old shims may or may not work because the tolerances change when the bearings are replaced. Better to let a specialist who knows what he is doing handle the overhaul than to risk a comeback.
Rebuilt manual transmissions and transaxles are sometimes sold on an exchange basis and sometimes they are sold outright. On hard-to-find applications, you may have to send the transmission to the rebuilder to have it overhauled.
When choosing a transmission rebuilder, look for someone who has previous experience with the model of transmission you need to be fixed. A shop that rebuilds automatic transmissions may or may not have the expertise to set up a particular manual gear box correctly. Also, look for the longest possible warranty. Six months to a year should be the minimum.
The other option is to install a new or used transmission. New transmissions are very expensive (say $5,000 versus $1,400 to $2,000 for a rebuilt). Used transmissions are the least expensive option but are always risky. Considering the labor involved to replace a transmission or transaxle, you had better make sure the transmission is a good one before you put it in - and that is hard to do because you don't know what condition the synchronizers, bearings and gears are in until the vehicle is driven.
Other items that should be examined when replacing a transmission include the motor mounts, clutch, release bearing, clutch linkage, flywheel, CV joints and boots.
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CVT TRANSMISSIONS
Continuously Variable Transmissions (CVTs) have been around since the 1980s and have long been used in snowmobiles. In recent years they have been appearing in more and more cars. Subaru offered a CVT transmission in the Justy, as did Ford in the European version of the Fiesta. Newer vehicles that have CVTs include certain Honda Civic models, Nissan Maxima and Altima, MINI cooper and others.
CVTs have several important advantages compared to automatic transmissions. They are smaller, lighter and cheaper to build. Compared to a manual transmission, a CVT offers an infinite range of gear ratios and no shifting is required. But a CVT does not work like a manual or an automatic. Consequently, it creates a rubber band effect where the engine revs up when you step on the gas as the transmission gear ratio changes. It creates the sensation of a clutch that is slipping or an automatic with an overly sensitive passing gear.
Durability limits how much horsepower a CVT can safely handle. A steel belt that rotates between a pair of pulleys must be strong enough to handle the engine's power output. For small displacement engines (3,500 cc or less), current CVTs are up to the task. But for larger displacement high-output high torque engines, a manual or conventional automatic transmission is still required.
Audi's Multitronic CVT, which is offered in the A6 model, uses a chain instead of a belt to carry the power between the pulleys, and a special torque sensor to control the force with which the pulleys grip the chain. The steel chain has 1,025 link plates, 75 pairs of pins and can handle up to 221 ft.-lbs. of torque. Audi also programs its CVT computer to reduce the rubber band effect so the driver feels less of a disconnect between engine rpm and vehicle speed. The Multitronic also has a manual shift mode that allows the CVT to be shifted like a manual 6-speed gear box, including downshifts to use engine braking to slow the vehicle.
Another twist on CVT technology is Nissan's Extroid transmission. Instead of using a belt or a chain, the Extroid transmission uses a pair of rotating rollers between an input disc and an output disc. A special oil is used to provide both lubrication and friction between the rollers and discs. Varying the angle of the rollers changes the gear ratio of the transmission. Introduced on the Nissan Cedric/Gloria and Skyline 350GT-8 in Japan, this design reportedly can handle higher torque loads (up to 286 ft.-lbs.) and comes with a manual mode that simulates a manual 6-speed transmission.
More Transmission & Clutch Articles:
Common Clutch Problems
Clutch Service
Diagnosing & Replacing a Faulty Clutch
Flywheel Resurfacing
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