MANUAL TRANSMISSION REPAIR AND DIAGNOSIS

After studying this chapter, you will be able to:

  • Remove a standard transmission from a car.
  • Disassemble and inspect a manual transmission.
  • Assemble a manual transmission.
  • Install a manual transmission.
  • Adjust manual transmission linkage.
  • Diagnose common manual transmission problems.

Since manual transmission construction varies with the manufacturer, a service manual should be used for exact specifications and procedures. This chapter discusses typical methods and rules that apply to most makes and models. As a result, you will be much better prepared to use a vehicle service manual and perform actual transmission repairs.

Transmission Identification

When repairing a manual transmission, you must be able to identify the exact type of transmission.

Usually, there will be an ID TAG (identification label) or stamped set of numbers on the trans- mission. These numbers, together with vehicle make, year, and model particulars, can be given to the parts counterperson when ordering new parts.

Manual Transmission Service

Many problems, which seem to be caused by the transmission are caused by clutch, linkage, or drive line problems. Keep this in mind before removing and disassembling a transmission.

Manual transmission removal

Fig. 18-1. Preparing for transmission removal. A – Drain gear oil. B – Remove drive shaft. C – If you do not or cannot drain transmission, install plastic cap to prevent oil leakage during removal. (Subaru)

 

To remove a manual transmission, first secure the car on a hoist or set of jack stands. A hoist is better because it allows you to stand up while working.

Unscrew the transmission drain plug and drain the oil into a catch pan, Fig. 18-IA. Remove the drive shaft, Fig. 18-IB. Install a plastic cap over the end of the transmission shaft, Fig. 18-IC. This will help keep oil from dripping out.

Disconnect the transmission linkage at the trans- mission. Unbolt and pull the speedometer cable out of the extension housing. Also, remove all electrical wires going to switches on the transmission.

Often, the cross member (transmission support bolted to frame) must be removed before the transmission. Support the transmission with a jack and use another jack under the rear of the engine. Operate the jack on the engine to take the weight off the transmission. Be careful not to crush the oil pan. Never allow the engine to hang- suspended by only the front engine mounts. ‘

Depending upon what is recommended in the service manual, remove either the transmission-to- clutch cover bolts or the bolts going into the engine from the clutch cover.

Figure 20 Fig. 18-2. Transmission jack should be used to avoid back injuries when transmissions are removed or installed by hand. Note arms and chains for securing transmission to jack saddle

 

Fig. 18-3. Follow service manual directions during transmission disassembly. A – A soft mallet may be needed to free rear extension housing from case. B – Dummy shaft is used to drive out countershaft and reverse idler shaft. C – A puller like the one shown may be needed if the countergear is supported by ball or roller bearings. D – After removing snap rings, front bearing cover and other parts, lift output shaft and other components out of case. (Nissan and Mitsubishi)

 

CAUTION! A manual transmission is very heavy and clumsy. If you are NOT using a transmission jack, Fig. 18-2, ask another mechanic to help you lift the transmission out of the car.

Slide the transmission straight back, holding it in “alignment with the engine. You may have to wiggle the transmission slightly to free it from the engine. Clean the outside of the transmission and take it to your workbench.

Manual transmission disassembly

Disassembly procedures will vary from one trans- mission to another. Always consult a service manual. If improper disassembly methods are used, major part damage could result.

Basically, remove the shift fork assembly and cover. With a shift rail type, remove the shift lever assembly.

If the transmission has an inspection cover, observe transmission action with the cover removed. Shift the transmission into each gear by moving the small levers on the shift forks. At the same time, rotate the input shaft while inspecting the condition of the gears and synchronizers.

Unbolt the rear extension housing. As in Fig. I8-3A, tap the extension housing OFF with a brass hammer.

Going to the front of the transmission, remove the front bearing retainer and any snap rings. Carefully, pry the input shaft and gear forward far enough to free the main output shaft.

Next, use a dummy shaft (shaft type driving tool) to push the reverse idler shaft and/or countershaft out of the case. See Fig. 18-3B.

Note! On some transmissions the countergear is supported by ball or roller bearings at both ends. These types of bearings are commonly withdrawn with a puller Fig. I8-3C. However,’ always follow recommended removal procedures as outlined in a service manual.

Now you can remove the input shaft and the output shaft assemblies. Slide the output shaft and gears out of the back or top of the transmission as a unit, Fig. I8-3D. Be careful not to nick the gears on the case.

Cleaning and inspecting parts

With all of the parts removed from the case, inspect everything closely. First check the inside of the case for metal shavings. If brasses colored particles are found, one or more of the synchronizers or thrust washers are damaged. These are normally the only parts in the transmission made of this material. If iron chips are found, main drive gears are probably damaged. After checking the case, clean the inside with solvent. Then, blow it dry with compressed air while wearing eye protection. Also, clean the transmission bearings and blow them dry.

DANGER! When blowing bearings dry with, compressed air do NOT allow the bearing to spin. Air pressure can make the bearing whirl at tremendously high rpm. The bearing can explode and fly apart with lethal force.

Next, inspect all of the main drive gears, Fig. 18-4. Look for wear patterns or chips on the gear teeth. The gears are usually casehardened. If wear is more than a few thousandths of an inch, the casehardening will be worn through and the gear must be replaced.

Figure 21 Fig. 18-4. Clean and inspect parts closely after removal. Check shaft snap ring grooves for wear and damage. Inspect gear teeth closely for signs of wear, chipped teeth or other troubles. Also check synchronizer

 

If gear tooth wear is uneven, check the shaft bearings and shafts. They may be worn or bent. A dial indicator can be used to check the transmission shafts for straightness. Look at Fig. 18-5. Refer to specifications for the amount of allowable run out.

Figure 22 Fig. 18-5. If gear wear is irregular, check shafts for run out. Lathe type support or V-blocks will hold shaft. Use dial indicator to measure run out. Reading indicates a worn or bent shaft. (Honda)

 

Inspect the synchronizer assemblies, especially if the transmission has gear shifting related problems. Check the teeth, splines, and grooves on the synchronizers, Fig. 18-6. If worn, replace parts as needed.

When removing the gears from the output shaft, keep everything organized on your workbench. All snap rings, spacers, and other parts should be installed exactly as removed. If the synchronizers are to be reused, scribe alignment marks on the sleeve and hub. This will let you realign the same splines during reassembly.

Fig. 18-7 shows an exploded view of one type of transmission. Since it is typical, study how all of the parts are positioned and held on their shafts.

Figure 23 Fig. 18-6. Check shift forks and synchronizers for wear. Install feeler gauge between fork and groove. If more than specifications replace parts as needed

 

Fig 18-7 Check every part closely for wear and damage. A Service Manual will normally provide an illustration like this one for exact transmission being repaired

 

Replace worn or damaged parts

Any worn or damaged part in the transmission must be replaced. This is why your inspection is very important. If any trouble is NOT corrected, the transmission overhaul may fail. You would have to complete the job a second time.

Always renew all gaskets and seals in the trans- mission. Even though a seal or gasket might not leak before disassembly it could start to leak after assembly. Fig. 18-8 shows a common way of replacing a rear seal.

When replacing a gear on the output shaft, you should also replace the MATCHING GEAR on the countershaft. If a new gear is meshed with an old worn gear, gear noise can result.

Frequently, you will need to replace input shaft bearings, output shaft bearings, and sometimes countershaft bearings. These bearings are prone to wear ‘because they support a great amount of load. Fig. 18-9.

Most transmissions now use metric fasteners. If a new bolt or nut is needed, make sure it is the correct thread type and length. Mixing threads will cause part damage.

Fig. 18-8. All seals should be renewed during a transmission rebuild. Rear seal can be removed and installed with transmission in car. A – Removing oil seal. 8 – Driving in new seal. Coat outside diameter of new seal with non-hardening sealer before installing (Ford)

 

Fig. 18-9. If bearings show signs of wear or feel rough when turned by hand, renew them. A – Special puller may be needed on some bearings. B – Use driver to install new bearings. Do not hammer on the inner portion of bearing or damage will result. (Mitsubishi)

 

Transmission reassembly

After obtaining new parts to replace the old worn ones, you are ready for transmission assembly. Generally the transmission is assembled in the reverse order of disassembly. Again, refer to a service manual for exact directions.

The service manual will usually have an exploded view of the transmission, Fig. 18-7. It will show how each part is located in relation to the others. Step-by- step instructions will accompany the illustrations.

To hold the needle bearings into the coutergear or other component, coat the bearings with HEAVY GREASE. This is illustrated in Fig. 18-10. Then, install each bearing into position. The grease will hold the bearings as you slide the countershaft into the gear.

Also, following manufacturer instructions, measure the endplay or clearance of the gears and synchronizers as needed. Look at Fig. 18-11.

The endplay between the countergear and case should be checked. If excessive, thicker thrust washers are required.

Fig. 18-10. Heavy wheel bearing grease is commonly used to hold small needle roller bearings during reassembly; Grease will hold bearings in countershaft gear as shaft is

 

After the transmission shafts and gears are in place, pour the recommended quantity of oil into the case.

Assemble the shift fork mechanism. Look at Fig. 18-12. Then, with the synchronizers and shift forks in neutral, fit the shift fork assembly on or in the case. Check the action of the shift forks.

Make sure the transmission shifts properly before installing it. This could save you from having to remove the transmission if there are still problems.

Note! With the transmission out of the car, it is wise to inspect the condition of the clutch.

Transmission installation

Fig. 18-11. After installing gears and synchronizers on output shaft, check clearance as described in service manual. This mechanic is checking clearance between snap ring and synchronizer hub. If greater than specifications snap ring or other parts are worn.

Fig. 18-12. Make sure nothing in the shift mechanism is worn or damaged. Check shift rail, fork-to-synchronizer contact points, shift gates and other components. Use approved method of sealing shift cover on case. Refer to a service manual for details

 

Before transmission installation, place a small amount of grease in the pilot bearing and on the throw-out bearing inner surface. Do NOT, however, place lubricant on the end of the clutch shaft, input shaft splines, or pressure plate release levers. Grease in these locations can spray onto the clutch friction disc, causing clutch slippage and failure.

Place the transmission on the transmission jack. Position it behind the engine. Double-check that the throw-out bearing is in place on the clutch fork. Carefully align the transmission with the engine.

The input and output shaft must line up perfectly with the centerline of the engine crankshaft. If the transmission is tilted, even slightly, it will NOT fit into place.

With the transmission in high gear to hamper input shaft rotation, slowly push the transmission into the clutch housing. You may need to raise or lower the transmission slightly to keep it in alignment. When the transmission is almost in place, wiggle the extension housing in a circular pattern while pushing toward the engine. This should help start the input shaft in the crankshaft pilot bearing.

WARNING! DO NOT use the transmission bolts to pull the transmission into the clutch housing. The transmission input shaft could be smashed into the crankshaft pilot bearing. Serious part damage may result. If the clutch and pilot bearing are installed correctly, the transmission should slide fully into place BY HAND.

With the transmission bolted to the clutch cover, install the rear cross member and engine mount. Reinstall the clutch linkage, transmission linkage, and other parts. Adjust the clutch linkage.

 

Adjusting Transmission Linkage

To adjust many types of transmission linkage, place the gearshift lever and transmission levers in neutral. Then, as in Fig. 18-13, insert a steel pin in the hole in the shifter levers.

If the pin will NOT fit through the hole, lengthen or shorten the linkage rods. Adjust the rods so that the alignment pin fits easily through the hole in the shifter assembly.

This basic procedure will vary with other types of gearshift mechanisms. When in doubt, refer to the specific directions in a manual. 4

After adjustment, lower the car to the ground. Road test the car and check for leaks.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.