Maintenance and repair of the YaMZ-238A MAZ gearbox

Gearbox care consists of checking the oil level and changing it in the crankcase. The oil level in the crankcase must match the control hole

It is necessary to drain the oil hot through all drain holes.

After draining the oil, you need to remove the cover at the bottom of the crankcase, in which the oil receiver of the oil pump with a magnet is placed, rinse them thoroughly and reinstall them.

In this case, you should pay attention not to block the oil line with a cover or its gasket.

Remote drive of the YaMZ-238A main box

To flush the gearbox, it is recommended to use 2.5 - 3 liters of industrial oil I-12A or I-20A in accordance with GOST 20799-75.

When the gearbox control lever is in neutral position, the engine is started for 7 - 8 minutes, then it is stopped, the flushing oil is drained and the gearbox is filled with oil provided for by the lubrication map.

It is unacceptable to flush the gearbox with kerosene or diesel fuel.

During the operation of the gearbox drive, adjustments are possible:

  • - position of the lever 3 (see Fig. 1) shifting gears in the longitudinal direction;
  • - the position of the gear lever in the transverse direction;
  • - blocking device for telescopic elements of the longitudinal rod.

To adjust the angle of inclination of the lever 3 in the longitudinal direction, it is necessary to loosen the nuts of the bolts 6 and by moving the rod 4 in the axial direction, set the angle of inclination of the lever to approximately 85 ° (see Fig.1) with the neutral position in the gearbox.

Adjustment of the position of the lever in the transverse direction is carried out by changing the length of the transverse link 17, for which it is necessary to disconnect one of the tips 16 and, after unscrewing the nuts, adjust the length of the link so that the box control lever is in the neutral position against the inclusion of 6 - 2 and 5 - 1 gears, had an angle of approximately 90˚ to the horizontal plane of the cab (in the transverse plane of the car).

Adjust the locking device of the gearshift drive should be done as follows:

  • -raise cab;
  • - unpin the pin 23 and disconnect the rod 4 from the fork 22;
  • - clean the earring 25 and the inner rod from old grease and dirt;
  • - push the inner rod until the locking bush 15 “clicks”;
  • - unlock the earring nut 25 and, inserting a screwdriver into the slot of the inner link shank, unscrew it until the angular play of the earring disappears;
  • - holding the shank 24 from turning, tighten the locknut;
  • - check the quality of the adjustment. When the blocking sleeve 21 is displaced towards the spring 19, the inner rod must be pulled out without jamming to its full length, and when the rod is pushed inward until it stops into the grooves, the blocking sleeve must move clearly with a “click” until the sleeve stops against the lower protrusion of the earring.

When adjusting the drive, the following requirements must be observed:

  • - adjust with the cab up and the engine off;
  • - prevent bending and curvature of the outer and inner movable rods;
  • - in order to avoid breakage, connect the rod 4 to the plug 22 so that the hole in the earring for the pin 23 is located above the longitudinal axis of the rod 4;
  • - check the neutral position of the gearbox with the cab raised by free movement of the lever 18 of the gear change mechanism in the transverse direction (relative to the longitudinal axis of the vehicle).

The roller 12 in the neutral position of the box has an axial movement of 30 - 35 mm, while the compression of the spring is felt.

The gearbox drive adjustments described above should be made when removing and installing the engine and cab.

Possible gearbox failures



Difficult to engage gears in main gearbox. The inclusion of first gear and reverse occurs with a rattle

- Incomplete clutch disengagement (clutch leads)

Adjust clutch pedal freeplay or replace damaged parts

Difficulty shifting into 2nd, 3rd, 4th and 5th gears. Turning on occurs with a rattle

- Wear of conical synchronizer rings

Replace Synchronizer

Gears are not enabled in the main box

- Wear of the gear rims of the synchronizer carriage, gear couplings installed on the gears of the 2nd, 3rd, 4th and 5th gears, the gear clutch of the first gear and rear its stroke or gear rims of the gears of the first gear and reverse gear

Replace defective parts

- Wear of parts or misalignment of the remote gearshift drive

Replace defective parts; adjust remote shifter

Shifting gears in the demultiplier comes with a bang and a rattle

- Increased pressure in the pneumatic control system of the demultiplier

Adjust pressure reducing valve

  • - Wear of the cone rings in the synchronizer
  • - Wear of the blocking chamfers of the fingers and synchronizer carriage

Replace Synchronizer

Non-disengagement or delayed engagement of gears in the demultiplier when the main gearbox shift lever is in neutral position.

The indicator lamp does not go out for a long time when the fast range is turned on and does not light up for a long time when the slow range is turned on.

Air exits through air distributor saloon

- Inlet valve rubber wear

Replace valve

- Valve stem O-ring wear

Replace O-ring

- Uneven fit of the inlet valve to the shoulder of the body

Replace O-ring

- Inlet valve sticking in the depressed position

Replace defective parts

- O-ring hardening inlet valve body ring

Rinse, blow out and lubricate the valve stems and cap bore. Polish the stem if necessary

- Wear or hardening of the O-rings of the power cylinder piston rod

Replace O-rings

- Air exits through the gearbox breather when the slow gear is engaged in the demultiplier

Replace O-ring

- Wear or hardening of the O-ring of the air distributor piston

Replace O-rings

- Wear or hardening of the O-rings of the air distributor spool

Replace cuff

- Low pressure in the pneumatic system of the gearbox

Replace cuff

- Wear or hardening of the cuff of the piston of the power cylinder

Replace Aperture

- Air distributor inlet valve diaphragm rupture

Replace Aperture

- Air comes out through the gearbox breather.

Clean or replace breather

- Rupture of the diaphragm of the locking device of the first gear and reverse

Replace defective parts

- Air exits through the gearbox breather when the fast gear is engaged in the demultiplier

Clean or replace breather

Self disengagement of gears while driving

- Incomplete gear engagement due to wear of the fork crackers, due to misalignment of the remote control drive

Tighten fasteners, replace worn parts, adjust control drive.

- Wear of gear couplings of gears and synchronizer carriages due to incomplete engagement

Replace worn parts

Increased gearbox noise

- Loosening the bolts securing the clutch housing to the flywheel housing

Tighten bolts

- Gear teeth wear

Replace gears

- Wear of shaft or gear bearings

Replace bearings

- Not enough oil in the gearbox

Add oil up to the upper mark on the oil level indicator

Gearbox oil leak

- Wear or loss of elastic cuffs

Replace cuffs

- Increased crankcase pressure

Rinse breather

- Violation of tightness on sealing surfaces

Tighten fasteners

Enable slow range at vehicle speeds above recommended

- Opening the contacts of the electrical circuit

Restore contact

- Broken wires of the ASBP system

Replace Wire

- Faulty ASBP blocking relay

Replace relay 6312.3747

- Faulty speed sensor

Replace speed sensor 1101.3843

- Faulty blocking valve

Replace defective parts

Gearbox repair

Removing the gearbox.

Required for removal:

  • - roll over the cab;
  • - drain oil from gearbox;
  • - remove landing gear;
  • - loosen the clamps, remove the clean air pipe of the air cleaning system;
  • - unpin, unscrew the nut and disconnect the gearbox control rod from the gearshift lever;
  • - unscrew the bolts and remove the second cross member of the frame;
  • - disconnect the exhaust pipe of the muffler;
  • - unpin, unscrew the nuts and remove the rear engine mount;
  • - unscrew the bolts of the bracket with fuel pipes from the gearbox cover;
  • - unscrew the bolts and remove the brackets of the rear engine mount;
  • - disconnect the clutch release shaft lever from the valve stem fork;
  • - disconnect the electrical wires from the sensor of the signal lamp of the gear shift in the additional box;
  • - unscrew the bolts securing the cardan shaft to the gearbox flange, disconnect the cardan shaft and hang it to the left side member of the frame;
  • - disconnect the air supply pipeline to the pressure reducing valve;
  • - unscrew the bolts and remove the muffler pipe;
  • - bring a pit lift with a device for removing and installing the gearbox or another lifting vehicle under the gearbox;
  • - unscrew the bolts of the clutch housing from the flywheel housing, remove the gearbox and disconnect it from the engine;
  • - lower the gearbox on the lift, raise the rear of the car and roll out the lift with the gearbox from under the car, then lower the car.

The gearbox is installed in the reverse order.

Disassembling the gearbox

The gearbox should be disassembled in the following sequence:

  • - disconnect air ducts from the air distributor;
  • - remove the top cover of the additional box with the control mechanism as an assembly;
  • - unpin and unscrew the three locking bolts on the holder of the small synchronizer of the additional box;
  • - remove the small synchronizer from the secondary shaft of the main box;
  • - unscrew the nuts securing the crankcase of the additional gearbox to the crankcase of the main gearbox;
  • - disconnect the additional gearbox from the main gearbox;
  • - remove the driveshaft mounting flange, secondary shaft cover, speedometer drive worm, rear support ball bearing, front support roller bearing, synchronizer carriage and remove the shaft together with the rear support roller bearing.

Further disassembly of the additional gearbox is not difficult. The shafts from the crankcase must be removed as an assembly.

To disassemble the main gearbox, you must first remove the mainshaft bearing circlip, then use the extractor bolts to remove the mainshaft bearing centering ring.

The shafts from the crankcase of the main gearbox must be removed as an assembly. After disassembling the box assembly, the parts must be washed in kerosene or diesel fuel and blown with compressed air.

External inspection reveals cracks, breaks, thread breaks, chipping and breaks of gear teeth, and other defects. Gears must be replaced in case of cracks or broken teeth, as well as increased tooth wear.

The most quickly worn part of the gearbox is the synchronizer, which may have such defects as loose fit of the clutch pins on the carriage, wear of the cone rings, teeth of the carriages and splines.

When the clutch on the synchronizer carriage is loosened, the unusable pins are drilled out and replaced with new ones with brass welding. The place of welding is cleaned.

After replacing the failed parts, the gearbox is assembled in the reverse order.

When installing the gearbox on the car, it is recommended to use lifts or special stands to avoid breakage or bending of the clutch discs. After installing the gearbox, its drive is adjusted.