DIAGNOSIS INFORMATION FROM EXCHANGE PROGRAM
TRANSMISSION APPLICATIONS: SUBJECT:
1988 HM-290 MANUAL DIAGNOSIS INFORMATION FROM EXCHANGE TRANSMISSION (RPO MCO, MG5) PROGRAM
TRANSMISSION MODELS: VEHICLE APPLICATIONS:
ALL MODELS 1988 C/K TRUCKS
BULLETIN COVERS: (Figure 1) ---------------- Diagnosis information On 1988 C/K Truck applications equipped with a HM-290 4 or 5-speed manual transmission. This information was gained from the now expired exchange program disassembly analysis of returned transmissions. Before repairs are attempted, refer to the following diagnostic chart as a guide.
SERVICE ACTION:
Preliminary Diagnosis Procedure.
- Insure correct fluid level and proper type.
- Insure that the clutch is working properly (spin-down time and lost motion).
- Insure that all engine and transmission mounts are tight.
Diagnosis Information
1988 HM-290 DIAGNOSTIC CHART
NOTE: FOR ALL FIGURE REFERENCES, REFER TO THE HM-290 UNIT REPAIR SECTION 7B1 OF THE 1988 SERVICE MANUAL
CONDITION System INSPECT COMPONENT FOR CAUSE (General Description) ----------------- --------- ---------------------- SHIFTING Excessive Shift Proper Type Of Fluid Improper fluid, USE ONLY GM P/N 1052931 (automatic Effort Required transmission fluid or engine oil do not work well with the synchronizers which results in increased shift effort).
Refer to Clutch Diagnosis
Output Shaft Bearing Damaged/worn parts or (33) improper shim selection affecting speed-down rate Input Shaft Bearing of gears. (214)
Shim Selection (212)
Shim Selection (252)
Snap Ring(s) (6), Too much end play (allowing the hub(s) to move "chuck" (15) & (29) back and forth while trying to shift).
Input Shaft Snap Ring Missing or too thin of (210) and Input Shaft selective snap ring(allowing Spacer (211) input shaft to move "chuck" back and forth while trying to shift)
Threaded Thrust Rings Low or no torque (allowing (Inner and Outer) output shaft to move "chuck" (39) & (41) for back and forth while trying Proper Torque to shift).
(Clutch Spin-down Time Shift Rail Front Binding shaft/rails. OK, Lubricant Type Housing Bearing(s) And Level OK) (228)
Rear Shift Rail Bearing(s)(308)
Shift Shaft (116)
Gritty Shift Feel(Into Shift Shaft Bearing "Peened" or damaged parts. Or Out Of Gear) (309)
Shift Shaft (117)
Shift Shaft Lever (230)
Shift Shaft Rollers (118)
Roller Pins (119)
No Shift (Shift Lever Shift Finger (121) Broken or damaged parts. May Move Into Gear But No Engagement)
Hard Shift Into 3rd And 3-4 Shift Fork (108) 3-4 shift fork (108) 4th machined with the 3-4 rail hole off center.
CONDITION system INSPECT COMPONENT FOR CAUSE (General Description) ----------------- --------- ----------------------- SHIFTING (Cont.) No 4th Gear Or Very 3-4 Synchronizer Synchronizer key (11) Hard Shift Into 4th Assembly (7) cocking in 3-4 synchronizer assembly. Refer to: GMC Bulletin 88-T-75 Chevrolet Bulletin 88-143-7B
Hard Shift Into 1-2 Bias Load Torsional Spring is too long, rubbing Shift Gate Spring (234) and Front on front housing by 5th and Housing (200) reverse detent cam support (237).
5th and Reverse Detent Loose 5th and reverse Support (237) and Bolts detent cam support(237). (204) & (236)
Hard Shift Into 1st Finger (121) Non-conforming finger Gear hitting 3-4 rail shift slot while shifting. (Look for shinny/wear spot on the bottom edge of the finger that faces the rear housing.)
NOISE First Gear Only Gears (22),(53) or Damaged or worn first speed (54)/ Bearing (17) gear set/bearing.
Synchronizer Parts Damaged or worn 1-2 (2) and (19) synchronizer hub/ sleeve assembly.
Second Gear Only Gears (18) and (53) or Damaged or worn second (54)/Bearing(17) speed gear set/ bearing assembly.
Synchronizer Parts Damaged or worn 1-2 (2)and (19) synchronizer hub/sleeve.
Noise/Rattle in Shift Shaft Socket Roll pin rubbing. -2nd And 4th (123) Roll Pin (122) Refer to: -2nd Only Rubbing On Reverse GMC Bulletin 88-T-77 - Reverse Gear (25) Chevrolet Bulletin - 1st/2nd/3rd And 88-148-7B Only 1st, 2nd And (Take Off Shift Lever GM Canada 887050-103 3rd Gears, Cannot Housing (135) or (136) Shift into 2nd, Roll Pin (122) MUST Be 4th Or Reverse Flush With Bottom of Shift (Noise Goes Away Or Socket (123) Lessens When Shift Lever Is Pulled Toward Drivers Side)
Third Gear Only Gears (13) and (53) or Damaged or worn third speed (54)/Bearing (14) gear set/bearing.
Synchronizer Parts (2) Damaged or worn 3-4 and (7) synchronizer hub/sleeve assembly.
Fourth Gear Only Synchronizer Parts (2), Damaged or worn 3-4 (7) or Input Shaft (1) synchronizer hub/sleeve assembly or input shaft.
CONDITION System INSPECT COMPONENT FOR CAUSE (General Description) ----------------- --------- ---------------------- NOISE (Cont.) Fifth Gear Only Gears (53) or (31)/ Damaged fifth speed gear Bearing (30) set/bearing.
Synchronizer Parts Damaged or worn (2)and(26) synchronizer hub/sleeve assembly.
Reverse Gear Only Gears/Bearing (17) and Damaged reverse speed gear Countershaft (53)or(54) set/bearings. Synchronizer Parts (28) and (8) Damaged or worn 5-R synchronizer hub/sleeve assembly.
Reverse Idler Gear (49) Damaged or worn. NOTE: and Countershaft (54) DO NOT REPLACE PARTS if Mating Gear Teeth the top edges of the gear teeth or the bottom pocket of the gear teeth have a wavy machining pattern, these are not gear teeth contact points so they wil not cause gear noise.
All Gears Exhaust System Grounding out on chassis (Varying Noise Level or body making noise. For Each Different Motor/Transmission Gear) Mounts and Accessories
(If The Noise Gets Countershaft Bearing or Damaged or worn counter- Louder In 5th Than Shaft (201),(53)or(54) shaft gears/ bearings. In 3rd) for the Rear Housing
(If The Noise Gets Countershaft Bearing or Damaged or worn counter- Louder In 3rd Than Shaft (201),(53)or (54) shaft gears/bearings. In 5th) for the Front Housing
Noisy All Gears Fluid Level Insufficient fluid level.
Countershaft Bearings or Worn countershaft gears Shaft (201),(53)or(54) /bearings.
Gears (1),(53) or (54) Worn or damaged input gear and countershaft input gear.
(Noise Will Be Lower Or Pilot Bearing (3) Damaged or worn bearing. Not At All In 4th, Input/Output Shafts Turning Same Speed So The Pilot Bearing Will Not Be Turning)
All Speed Gears and Damaged gear/bearing. Their Bearings (13), (14),(17), (18), (22), (25),(30) and (31)
Input Shaft (1) Damaged or worn shaft /gears.
Noisy All Gears (cont.) Check Shim (212) or Too much end play (252) and Snap Ring(s) (allowing the shaft(s) Selection (210),(6), /hub(s) to "chuck" back (15) and (29) and forth).
Reverse Gear (49) Improperly finished (Cannot Tell Visually) reverse gear teeth (49).
Rattle/Noise All Gears Idler Shaft (51) On Hick on edge of idler Units Built Before shaft that contacts December 1,1987 reverse gear (49).
Noise All Gears Reverse Speed Gear(25) High spot on geartooth (a shinny spot in the (Starts Out As A middle of the gear teeth Whine Then Turns Into about 1/8 inch in from A Growl) the cone part).
Noise All Gears Reverse Idler Gear (49) Reverse idler gear and 1st Speed Gear (22) rubbing on back of Input Shaft (1) and 1st speed gear. Countershaft (54) Input shaft clutching teeth edge rubbing on side of countershaft input mating gear.
Growl Noise All Gears Bearing Races(1), (23), "Peening" or damage. (24),(33),(202) & (214) (Look at races at an angle in bright light.)
Shift Lever Rattle/Buzz Shift Lever Housing Non-conforming shift Or A Ticking Noise Assembly (135)or(136) lever housing assembly (135)or(136). (Too much clearance.)
Can Be In All Gears (Remove boot (130) and (If shift lever housing And Neutral measure clearances be- seal is not available (Noise May Lesson if tween yoke and housing use gasket maker GM You Hold The Shift and yoke and shift P/N 1052943.) Lever) lever,the axial (No service for shift (The Noise Usually Gets clearances should be lever housing assembly, 0.03 to 0.1 mm and ball except boot-replace as Worse At Higher Engine socket that mates into an assembly.) RPM's) transmission should have no up-and-down move- ment when pulled and pushed on.)
Neutral Rattle Loose Vacuum Line On Improper running engine (Clutch Engaged) Engine transferring irregular pulses to transmission Disconnected Spark Plug gear set,causing rattle. (Will Be Louder When The Wire Accessories Are On, As When The Air Conditioner Out Of Tune Engine Compressor Is Cycling) Clutch Disc Not Anti- Clutch disc transferring Rattle Version engine pulses to trans- mission gear set causing rattle. Refer to: GMC Bulletin 88-T-89 Chevrolet Bulletin 88-166-7C GM Canada 887050-107
Refer To Clutch Diagnosis Input Shaft Bearing (214) Damaged or worn bearing.
Pilot Bearing (3) Damaged or worn bearing.
Countershaft Bearing/ Worn countershaft bearings/gears or any Gears (201),(53) or (54) speed gear bearing. Speed Gear Bearing(s) (14),(17) or(30)
GEAR GRINDS/CLASH Any Forward Gear Proper Fluid Level Improper/low fluid. USE Grind Proper Type of Fluid ONLY GM P/N 1052931. (Automatic transmission fluid or engine oil do not work well with the synchronizers which results in gear clash.)
Refer To Clutch Diagnosis
Interlock Ball(s) (323) Two gears trying to engage at the same time. Missing/damaged inter- lock parts. 3-4 Shift Interlock Pin (110)
Synchronizer Ring(s) (2) Synchronizer ring (2) bronze transfer to Speed Gear(s) (1), (13), speed gear clutching (18), (22) and (31) cone.
1st And 2nd Gear Grinds 1-2 Synchronizer Sleeve 1-2 Synchronizer sleeve (20) installed backwards (Especially When Cold) (20) (clutching teeth on sleeve have different angles to aid in cold shifts). Worn or damaged clutching teeth.
1-2 Synchronizer Sleeve Damaged or worn sleeve (20) and Speed Gear(s) (20) teeth or teeth of (18) and (22) speed gear(s)(18)or(22).
Reverse Reverse Synchronizer Damaged or worn 5-R (Spin-Down Time Is OK Parts (28) and (8) synchronizer hub/sleeve With Engine At Hot Idle assembly. And Vehicle At A (A slight grind is (Have customer hold down Complete Stop) design intent because clutch pedal for a reverse is not little longer period of synchronized.) time before shifting into reverse.) HOP-OUT Slips Out Of Gear On Acceleration Or Deceleration (Transmission Under Load)
1 st And/Or 2nd 1-2 Synchronizer Sleeve Damaged or worn sleeve/ (20) or Hub (21) hub splinelock.
Slips Out Of Gear On Acceleration Or Deceleration (Transmission Under Load)
3rd And/Or 4th Roll Pin (107) Height Roll pin (107) not at 12.5mm proper depth, rubbing on front housing (200).12.5mm is NOT MEASURED FROM TO OF FORK, REFER TO SERVICE MANU FOR PROCEDURE.
3-4 Synchronizer Sleeve Spline lock omitted or (8) (Cannot Tell only partial lock on Visually) sleeve.
3-4 Synchronizer Sleeve Damaged or worn sleeve/ (8) or Hub (12) hub spline lock.
5th And/Or Reverse 5-R Synchronizer Sleeve Damaged or worn sleeve/ (8) or Hub (28) hub spline lock.
(Coast ie. Not Accelera- Shift Plunger Assembly Damaged, worn or missing tion Or Deceleration) (242),(243),(244)or parts. (Any Gear) (245)
Shift Rail(s) Detent Assembly(112), (109), (101),(324),(323),(325) or (236)
LEAKS (From Clutch Area)
Slave Cylinder Refer To Slave Cylinder Leak Diagnosis
Input Bearing Retainer Input Bearing Retainer Oil drain back hole not (205) Oil Drain Back lined up with hole Hole Location in front housing.
Seal (209) Damaged or worn.
Input Shaft (1) Damaged or worn.
Sealant Lack of proper sealant.
Input Bearing Retainer Cracked or damaged. (206) & (207)
Flatness of Front Housing (200) and Input Bearing Retainer (206)
Shift Shaft/Rails Plug(s) Plug(s) (221) Cocked or lack of sealant. Countershaft Bearing Countershaft Bearing Cocked or fack of Plug Plug (250) sealant. Damaged bore.
Engine Rear Main Seal Refer To Engine Leak Diagnosis
(Leak From Front Housing) Oil Fill Plug (222) Lack of sealant. Oil Drain Plug (225) Damaged threads. Loose.
Breather Hose Adapter Damaged or missing. (224)
Breather Assembly (223) Damaged or missing.
Bolt (226) Loose.
Bolt (236) Loose. Lack of sealant.
Hex Head Plug (249) Loose. Lack of sealant.
Plug (245) Loose. Lack of sealant.
Front Housing (200) Loose bolts. (Bolts (312) Lack of sealant. Mating surfaces not flat or damaged. Cracked housing.
Rear Housing Rear Housing (300) Loose bolts. or(301) Lack of sealant. [Bolts (312)] Mating surfaces not flat or damaged. Cracked housing.
Plug (326) Cocked Lack of sealant. Damaged hole. Detent Spring Cover (325) Loose. [Bolts (326)] Lack of sealant. Mating surfaces not flat or damaged.
Seal Yoke Oil Seal Damaged or worn. (319) (2WD)
Rear Extension (2WD) Damaged or worn Bearing (318) (allowing slip yoke to move more than seal.
Bolts (2WD) (304) Loose bolts. and (305) Lack of sealant.
Rear Housing (4WD) Loose bolts to (301) transfer case. Lack of sealant. Mating surfaces not flat or damaged.
Output Shaft Oil Seal Worn or damaged. (4WD) (320)
Rear Housing (cont.) Back Up Lamp Switch Lack of sealant. (313) Damaged threads. Loose. Leaking assembly.
Machining Holes Ball Plug Improper staking.
Reverse idler Shaft Damaged, worn or O-Ring (52) missing.
Reverse Idler Bolt (311) Lack of sealant. Loose. Boot (130) Cracked. Torn.
O-Rings (129) & (131) Missing or worn.
(Only On Some Models) Shift Lever Housing Inside seal torn (135) & (136) or cracked. [Bolts (204)] Loose bolts (204). Lack of sealant. Mating surfaces not flat or damaged.
Seal (315) Worn or torn. Bore damaged. Debris in seal groove of sensor.
Electronic Speed Sensor Internal leak. (316)
MAGNETIC PARTICLES ON SHAFTS/GEARS Oil Drain (225) and/or Improper staking to Oil Fill (222) Plug(s) hold magnet in place. Magnet Missing (Proper Magnet Retention Should Be Checked Every Time Transmission Is Serviced)
General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.
