Manual Transmission
Specifications of transmission we inspected are listed below

Toyota 5 speed manual transaxle

Model C50, C51, C51 and C150

The disassembling procedure is as follow

1.       The rear recover is removed to expose fifth gear.

1.       Shift nut is removed while input shaft is held constant.

2.       Fifth gear synchronizer, fork and gears are removed. The gears need special puller to remove and c clips need to be removed before pull gears. Special care need to be taken not to damage the gears teeth while pulling them off.

3.       Before removing the transaxle casing, detents has to be removed with care. Detent balls and springs are very easy to be lost.

4.       Transaxle case is removed. Shafts, gears, Shift mechanisms are all exposed

1.       Shift mechanisms are removed.

1.       Input, output and idler shafts are removed.

2.       Final drive ring gear and differential are lift out the transmission casing. Dissembling completed

The reassembling procedure is exactly the reverse of disassembling. However two things are very important to be remembered:

1.       Before putting transmission casing back, make sure the shift rails are correctly installed so the detents could be fitted in.

2.       When putting back the fifth gear special pressing tool will be needed. Don’t forget to put circlips back.

The basic structure of this transmission is shown in the diagram below

The torque from the engine is not directly transmitted to the drive line to drive the vehicle instead torque from the engine is first transmitted into the transmission. The transmission will then vary the torque according to the driver’s decision. The torque may be increased or decrease by the transmission before goes into the drive line. Torque is varied by means of gearing. 

The basic operation of C50 is described below with our testing results:

The torque from the engine flows into the transmission through the input shaft. Then flow out the differential which is connected to the output shaft into the drive line.

There are six gears on the input shaft as shown in the diagram. These gears are 1 – 5 and the sixth gear is the reverse gear. These gears are called driver gears because the torque flows from these gears will drive their mating gear on the output shaft. On the output shaft there are also six gears which are driven gears and they are constantly in mesh with their driver gears apart from the reverse gear. Transmission with this type of gear arrangement is called SYNCHROMESH TRANSMISSION. The advantage of this type of arrangement is easier gearshift and minimizes gear wearing. Gears 1, 2, 5 and reverse on the input shaft are integral to the shaft. Gears 3 and 4 are freely spinning on the input shaft. The mating gears on the out put shaft are exactly the opposite apart from the reverse gear. Gear 3, 4 and reverse is integral to the output shaft. 1, 2 and 5 are freely spinning. Reverse gears don’t touch each other. They are connected by an idler gear which only connects them when selected.

Both input and output shaft is placed on bearings on both ends – cage roller in the front and ball race at the back to handle rotation. The final drive is using tapered bearings which are much stronger and provide support on both horizontal and vertical movements. All constant but removable gears on the shaft are fitted with needle bearing which are also aimed to handle end thrusts as well. All shafts, gears and bearing are visually inspected and are free of scores, wears or any other physical damages. All bearings are also rotating freely.

Power will only flow through the driver and driven gear when the free spinning gear is holding constant by the synchronizer. The following diagram show the power flow when first gear is selected.

Synchronizer A is shifted to the right to select gear 1 on the output shaft. This will hold the driven gear 1 to the output shaft. Therefore transfer power from the engine to driver gear 1 which is part of the input shaft, then from the driver gear 1 to driven gear 1 to rotate the output shaft.

All other gears won’t be able to provide passage for the power flow. Driver gear 2 is rotating as it is part of input shaft but the driven gear 2 is freely spinning. Driver gear 3 and 4 are free spinning on the input shaft therefore they cannot drive the driven gears. Driver gear 5 is connected to the input shaft but its driven gear is freely spinning on the output shaft. The other four forward gears work the same. Each synchronizer controls 2 gears. A controls gear 1 and 2, B controls 3 and 4, C controls 5 and reverse.

The reverse gear is somehow different. It is not a synchromesh type gear, instead it is a sliding engaged gear. It has a third gear – idler gear on a separate shaft. Driver reverse gear on the input shaft is part of the shaft. Driven reverse gear is actually the synchronizer A as shown in the first diagram. These gears are not in contact with each other. When the reverse is selected, the idler gear moves up to engage both reverse gears on the input and output shaft. The function of this third gear also reverse the direction of the rotation therefore reverse the vehicle.

Notice when synchronizer C is moved to the left to select reverse the idler gear is moved up connected driver and driven reverse gear. The power from the engine is flowing through the driver reverse gear to the idler gear. From the idler gear is then flowing to the driven reverse gear therefore rotate the output shaft in opposite direction.

An important thing should be noticed is that reverse gears are not helical gears as all other forward driving gears. They are using spur gears with straight teeth. Spur gear is not as strong or durable as the helical gear. However it is quite suitable for reverse operation. As a sliding engaged gear it made the design much easier than if using helical gears. However due to the fact it has much less contact area when engaged, reverse operation shouldn’t be done over long period of time or at high speed as the spur gear might get damaged.

As we looked at the gears on the input and output shafts we can see that driver gear and its mating gear are normally in different sizes. This is how the transmission varies the torque coming from the engine with gear ratio. Gear ratio is calculated as driven gear teeth number over driven gear teeth number. Therefore with higher gear ratio the output torque will be increased and speed will be decreased. As the gear ratio decreases the opposite will happen.

The following table shows the number of teeth of each driver vs driven gear and their gear ratio

Gear	Driver teeth	Driven teeth	Gear ratio
First	12	38	3.2
Second	21	40	1.9
Third	24	38	1.6
Fourth	33	33	1
Fifth	31	38	1.2
Reverse	12	39	3.3
Final Drive	17	69	4.1

First gear has the highest gear ratio amount all forward driving gears so it output largest torque with least speed. Fourth and fifth gear have much lower gear ratio therefore they output much less amount of torque but have higher speed therefore could lower the engine rpm when cruising.  

A striped down synchronizer is shown below

The inserts are fitted onto the slots on the hub and hold by springs. The hub has splines in the center which hold the hub onto the shaft. The sleeve holds onto the hub. The sleeve has splines inside which can slide on the hub. When sleeve is pushed by shifting fork, it slides towards the selected gear. As the sleeve moves it takes the inserts with it. Inserts pushes baulk ring forward. The baulk ring has a corn inside which will contact with the corn on the selected gear. Baulk ring will try to either decrease or increase the rotating speed of the selected gear. This action will synchronize the speed of selected gear and the synchronizer. Before the speed is synchronized the baulk ring will also block the sleeve to prevent the sleeve trying to engage with the selected gear therefore damage the gear or synchronizer itself. As the baulk is synchronizing the speed, it will be dragged by the selected gear half a teeth outline with the sleeve. Maximum amount of dragging is limited by the insert inside the recess of the baulk ring. Only when the sleeve and gear are synchronized, sleeve will be able to slide over the baulk ring and slide onto the gear dog teeth. Selected gear will then be connected to the shaft by the synchronizer and moves with the shaft as one.

The gear shifting mechanism for this transmission is very similar to the diagram above. Gear shifting contain two actions

Select gear rail

Selector lever will allow driver to select the shift shaft – each shaft controls one synchronizer therefore two gears.

Shift the gear

Once synchronizer is selected then shift lever is used to select one of two gears which is controlled by that synchronizer.

To prevent more than one gear is selected at same time ball type INTERLOCK mechanisms are used inside the housing of shift shafts. As one shift rail is selected the other two rails are blocked. This transmission also uses a interlock plate as shown which only allow control finger to act on one shift rail at a time, as the other two are blocked by the interlock plate.

This transmission also used detent mechanism to prevent jump out gear and also hold gears accurately in their correct position. Detent balls are pushed into the grooves on the shift shafts to hold shafts in position.