Isuzu Cars and Trucks 1981-1991

4-Speed 2-Wheel Drive Overhaul

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BEFORE DISASSEMBLY

Cleanliness is an important factor in the overhaul of the manual transmission. Before opening up this unit, the entire outside of the transmission assembly should be cleaned, preferably with a high pressure washer such as a car wash spray unit. Dirt entering the transmission internal parts will negate all the time and effort spent on the overhaul. During inspection and reassembly all parts should be thoroughly cleaned with solvent then dried with compressed air.

Wheel bearing grease, long used to hold thrust washers and lube parts, should not be used. Lube seals with clean SAE 30 weight engine oil and use ordinary unmedicated petroleum jelly to hold the thrust washers, needle bearings and to ease the assembly of seals, since it will not leave a harmful residue as grease often will. Do not use solvent on neoprene seals, friction plates if they are to be reused, or thrust washers.

Before installing bolts into aluminum parts, always dip the threads into clean transmission fluid. Anti-seize compound can also be used to prevent bolts from galling the aluminum and seizing. Always use a torque wrench to keep from stripping the threads. Take care when installing new O-rings, especially the smaller O-rings. The internal snaprings should be expanded and the external rings should be compressed, if they are to be reused. This will help insure proper seating when installed.

MAJOR COMPONENT DISASSEMBLY
  1. Remove the shift quadrant box, shift block and withdraw the lever.
  2.  
  3. Remove the front cover retainer, oil seal and Belleville spring.
  4.  
  5. Remove the front bearing snapring.
  6.  
  7. Remove the speedometer driven gear and backup light switch.
  8.  
  9. Remove the rear housing and oil seal.
  10.  
  11. Remove the mainshaft, cluster gear and top gear shaft assemblies.
  12.  



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Fig. Major component disassembly 4-speed

MAINSHAFT, CLUSTER GEAR AND TOP GEAR SHAFT DISASSEMBLY
  1. Remove the reverse idler gear thrust washers and idler gear.
  2.  
  3. Remove the reverse idler shaft retaining bolt, lock plate and shaft.
  4.  
  5. Remove the detent plate, spring and ball.
  6.  
  7. Remove the reverse shift rod and arm.
  8.  
  9. Remove the top/3rd and low/2nd shift rod by driving out the pins.
  10.  
  11. Remove the top/3rd and low/2nd shift arms.
  12.  
  13. Remove the interlock pin.
  14.  
  15. Remove the reverse gear outer snapring, speedometer drive gear, key, inner snapring and reverse gear.
  16.  
  17. Remove the mainshaft locknut and washer.
  18.  
  19. Remove the cluster gear locknut and washer.
  20.  
  21. Slide the intermediate plate off the shaft assemblies.
  22.  
  23. Remove the cluster gear bearing snapring.
  24.  
  25. Remove the mainshaft bearing snapring.
  26.  
  27. From the cluster gear assembly, remove the counter reverse gear, collar, cluster rear bearing, cluster gear and needle bearings.
  28.  
  29. Remove the top gear shaft, press off the bearing and remove the needle bearings.
  30.  
  31. Remove the top/3rd blocker ring, mainshaft snapring and top/3rd synchronizer.
  32.  
  33. Remove the 3rd blocker ring, 3rd gear and needle bearings.
  34.  
  35. Remove the mainshaft bearing, thrust washer, low gear, needle bearings and collar.
  36.  
  37. Remove the outer low/2nd blocker ring, low/2nd synchronizer, inner low/2nd blocker ring, 2nd gear, needle bearings and mainshaft.
  38.  



Click image to see an enlarged view

Fig. Mainshaft, cluster gear and top gear assembly 4-speed

INSPECTION
  1. Using a micrometer, check the shift arm thickness where it rides on the synchronizer. Low/2nd = 7.8 and 197.9mm (0.307 and 190.311 in.), 3rd/4th = 7.1 and 197.2mm (0.280 and 190.283 in.) and reverse = 6.86.9mm (0.268-0.272 in.).
  2.  
  3. Using a inside caliper, check the free length of the detent springs. Low/2nd and top/3rd = 27.5 and 1930.5mm (1.084 and 191.201 in.). Reverse = 24.7 and 1927.7mm (0.973 and 191.091 in.).
  4.  
  5. Using a spring tension tester, check the detent spring tension. Low/2nd and top/3rd = 8.8 and 199.3 lbs. (4.0 and 194.2 kg). Reverse = 13.9 and 1914.8 lbs. (6.3 and 196.7 kg).
  6.  
  7. Using a feeler gauge, check the clearance between the blocker ring and gear. Standard clearance 1.5 and 190.8mm (0.059 and 190.031 in.).
  8.  
  9. Using a feeler gauge, check the clearance between the blocker rings and inserts. Standard clearance 3.51 and 194.00mm (0.138 and 190.157 in.).
  10.  
  11. Using a feeler gauge, check the clearance between the clutch hub and inserts. Standard clearance 0.01 and 190.3mm (0.0004 and 190.012 in.).
  12.  
  13. Using a dial indicator and holding fixture, check the mainshaft run-out. Limit 0.03mm (0.001 in.).
  14.  
  15. Using an inside T-gauge, check the gear inside diameter. Low/rev gear 45.0 and 1945.1mm (1.772 and 191.776 in.). 2nd/3rd gear 41.0 and 1941.1mm (1.614 and 191.618 in.).
  16.  
  17. Using a micrometer, check the clearance between the bushing and idler gear shaft. Standard clearance 0.041 and 190.15mm (0.0016 and 190.006 in.).
  18.  
  19. Using a dial indicator, check the spline play. Top/3rd and low/2nd 0.0 and 190.20mm (0 and 190.0079 in.).
  20.  
  21. Replace components that do not meet specifications.
  22.  



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Fig. Shift arm thickness check 4-speed



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Fig. Detent spring check 4-speed



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Fig. Blocker ring-to-gear clearance check 4-speed

ASSEMBLY

NOTE
The mainshaft front and rear needle bearings are interchangeable. Pack the needle bearings with grease to keep them in place during installation.

  1. Install the needle bearings onto the mainshaft. Install the 2nd gear with the taper cone facing rearward.
  2.  
  3. Install the inner low/2nd blocker ring, low/2nd synchronizer assembly, outer low/2nd blocker ring and collar.
  4.  
  5. Install the needle bearing onto the mainshaft and low gear with the taper cone facing forwards.
  6.  
  7. Install the thrust washer with the oil groove facing forward. Press on the mainshaft bearing.
  8.  
  9. Install the needle bearings, 3rd gear, top/3rd blocker ring and top/3rd synchronizer assembly.
  10.  
  11. Install the mainshaft snapring and measure clearance with a feeler gauge. Selective snaprings can be purchased to adjust clearance. Standard clearance is 0.0 and 190.05mm (0.0 and 190.0019 in.).
  12.  
  13. Install the top/3rd blocker ring and needle bearings.
  14.  
  15. Using a press install the top shaft bearing so that the snapring groove faces forward. Install the cluster gear rear bearing so the ring groove is facing rearward.
  16.  
  17. Install the needle bearings and intermediate plate.
  18.  
  19. Install the mainshaft bearing snapring.
  20.  
  21. Install the cluster gear bearing snapring.
  22.  
  23. Install the cluster gear collar, reverse counter gear, washer and locknut. Torque the locknut to 72 and 1987 ft. lbs. (100 and 19120 Nm).
  24.  
  25. Install the mainshaft lock washer and nut. Torque to 87 and 19101 ft. lbs. (120 and 19140 Nm).
  26.  
  27. Install the reverse gear, snapring, key, speedometer drive gear and snapring.
  28.  
  29. Install the reverse idler shaft, lock plate and bolt. Torque to 30 ft. lbs. (34 Nm).
  30.  
  31. Install the interlock pin, top/3rd shift arm, low/2nd shift arm and bolt. Torque the bolt to 16 ft. lbs. (22 Nm).
  32.  
  33. Install the reverse shift arm, reverse shift rod, detent ball, detent spring and spring plate.
  34.  
  35. Install the reverse idler gear thrust washer with the flange is fitted to the stopper in the intermediate plate. Also, install the idler gear so that the undercut teeth are turned outward.
  36.  
  37. Install the reverse idler gear and outer thrust washer.
  38.  



Click image to see an enlarged view

Fig. 2nd gear positioning 4-speed



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Fig. Low/2nd synchronizer assembly 4-speed



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Fig. 4-speed synchronizer assembly 1. key, 2. spring, 3. clutch



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Fig. Top/3rd synchronizer assembly 4-speed

MAIN COMPONENTS ASSEMBLY
  1. Install the rear housing oil seal using a seal installer J26508.
  2.  
  3. Install the mainshaft, cluster gear and top gear assemblies to the main housing. Do not force the mainshaft into the housing.
  4.  
  5. Install the rear housing and torque the bolts to 30 ft. lbs. (42 Nm).
  6.  
  7. Install the backup light switch, speedometer driven gear and mainshaft front bearing snapring.
  8.  
  9. Install the Belleville spring, front cover oil seal, front cover and torque the bolts to 16 ft. lbs. (22 Nm).
  10.  
  11. Install the clutch fork, shift block and quadrant box assemblies. Make sure the shift lever is fitted into the support properly.
  12.  

 
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