DIAGNOSIS INFORMATION FROM EXCHANGE PROGRAM. GMC Truck GMC K Pickup

Model and Year: 1988 C/K TRUCKS EQUIPPED WITH HM-290 4 OR 5 SPEED MANUAL TRANSMISSIONS (OPTION MCO, MG5) BULLETIN COVERS: (Figure No.1) --------------- Diagnosis information on 1988 C/K Truck applications equipped with a HM-290 4 or 5-speed manual transmission. The 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). (Reference 1988 c/k Manual X-8731, Section 7B page 1 and 7C page 1) - Insure that all engine and transmission mounts are tight.

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 - Improper fluid, USE ONLY GM Effort Required | Of Fluid P/N 1052931 (automatic | transmission fluid or engine | oil do not work well with the | synchronizers which results in | increased shift effort). |--------------------------------------------------------- | * Refer to Clutch | Diagnosis |--------------------------------------------------------- | * Output Shaft - Damaged/worn parts or improper | Bearing (33) shim selection affecting | speed-down rate of gears | * Input Shaft | Bearing (214) | | * Shim Selection (212) | | * Shim Selection (252) |--------------------------------------------------------- | * Snap Ring(s) (6), - Too much end play (allowing | (15) & (29) the hub(s) to move "chuck" | back and forth while | trying to shift). | | * Input Shaft Snap - Missing or too thin of | Ring (210) and selective snap ring (allowing | Input Shaft Spacer input shaft to move "chuck" | (211) back and forth while trying | to shift). | | * Threaded Thrust - Low or no torque (allowing | Rings (Inner and output shaft to move "chuck" | Outer) (39) & (41) back and forth while trying | for Proper Torque to shift). |--------------------------------------------------------- | (Clutch Spin-down | * Shift Rail Front - Binding shaft/rails. Time OK, Lubricant | Housing Bearing(s) | (228) | Type And Level OK) | * Rear Shift Rail | Bearing(s) (308) | | * Shift Shaft (116) |--------------------------------------------------------- Gritty Shift Feel | * Shift Shaft - "Peened" or damaged parts. (Into Or Out Of | Bearing (309) Gear) | | * Shift Shaft (117) | | * Shift Shaft Lever (230) | | * Shift Shaft Rollers (118) | | * Roller Pins (119) |--------------------------------------------------------- No Shift (Shift | * Shift Finger (121) - Broken or damaged parts. Lever May Move | Into Gear But No | Engagement) | |--------------------------------------------------------- Hard Shift Into | * 3-4 Shift Fork - 3-4 shift fork (108) machined 3rd And 4th | (108) with the 3-4 rail hole off | center. |--------------------------------------------------------- | SHIFTING (Cont.) | No 4th Gear Or | * 3-4 Synchronizer - Synchronizer key (11) cocking Very Hard Shift | Assembly (7) in 3-4 synchronizer assembly. Into 4th | Refer to: | GMC Bulletin 88-T-75 | Chevrolet Bulletin 88-143-7B | GM Canada 887050-12 |--------------------------------------------------------- | Hard Shift | * Bias Load Torsional - Spring is too long, rubbing on Into 1-2 | Spring (234) and front housing by 5th and Shift Gate | Front Housing (200) reverse detent cam support | (237). | | * 5th and Reverse - Loose 5th and reverse detent | Detent Support (237) cam support (237). | and Bolts (204) & | (236) |--------------------------------------------------------- Hard Shift Into | * Finger (121) - Non-conforming finger hitting 1st Gear | 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 - Damaged or worn second speed | (53) or (54)/ gear set/bearing assembly. | Bearing (17) | | * 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 88-148-7B - 1st/2nd/3rd And | GM Canada 887050-103 Only 1st, 2nd | (Take Off Shift Lever And 3rd Gears, | Housing (135) or (136) Cannot Shift | Roll Pin (122) MUST Be Into 2nd,4th Or | Flush With Bottom of Shift Reverse | Socket (123) (Noise Goes Away | Or Lessens When | Shift Lever Is | Pulled Toward | Drivers Side) | |--------------------------------------------------------- Third Gear Only | | * Gears (13) and (53) - Damaged or worn third speed | or (54)/ Bearing (14) gear set/ bearing. | | * Synchronizer Parts - Damaged or worn 3-4 | (2) and (7) synchronizer hub/ | sleeve assembly. |--------------------------------------------------------- Fourth Gear Only | * Synchronizer Parts - Damaged or worn 3-4 | (2), (7) or Input synchronizer hub/ | Shaft (1) sleeve assembly or | input shaft. |--------------------------------------------------------- Fifth Gear Only | * Gears (53) or (31)/ - Damaged fifth speed gear | Bearing (30) set/bearing. | | * Synchronizer Parts - Damaged or worn synchronizer | (2) and (26) hub/ sleeve assembly. |--------------------------------------------------------- Reverse Gear Only | * Gears/Bearing (17) - Damaged reverse speed gear | and Countershaft set/bearings. | (53) or (54) | | * Synchronizer Parts - Damaged or worn 5-R | (28) and (8) synchronizer hub/ | sleeve assembly. |--------------------------------------------------------- | * Reverse Idler Gear - Damaged or worn. NOTE: DO | (49) and Counter- NOT REPLACE PARTS if the | shaft (54) Mating top edges gear teeth or the | Gear Teeth bottom pocket of the of the | gear teeth have a wavy | machining pattern,these are | not gear teeth contact | points so they will not | cause gear noise. |--------------------------------------------------------- | All Gears | * Exhaust System - Grounding out on chassis or (Varying Noise | body making noise. Level For Each | * Motor/Transmission Different Gear) | Mounts and Accessories | |--------------------------------------------------------- (If The Noise Gets | * Countershaft Bearing - Damaged or worn countershaft Louder in 5th Than | or Shaft (201), (53) gears/bearings. In 3rd) | or (54) for the Rear | Housing |--------------------------------------------------------- (If The Noise Gets | * Countershaft Bearing - Damaged or worn countershaft Louder In 3rd Than | or Shaft (201), (53) gears/ bearings. In 5th) | or (54) for the | Front Housing |--------------------------------------------------------- Noisy All Gears | * Fluid Level - Insufficient fluid level. | | * Countershaft - Worn countershaft | Bearings or Shaft gears/bearings. | (201), (53) or (54) | | * Gears (1), (53) - Worn or damaged input gear | or (54) and countershaft input gear. | |--------------------------------------------------------- (Noise Will Be | * Pilot Bearing (3) - Damaged or worn bearing. Lower Or 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 | * Check Shim (212) or - Too much end play (allowing Gears | (252) and Snap the shaft(s)/hub(s) to | Ring(s) Selection "chuck" back and forth). | (210), (6), (15) | and (29) | | * Reverse Gear (49) - Improperly finished reverse | (Cannot Tell Visually) gear teeth (49). |--------------------------------------------------------- Rattle/Noise | * Idler Shaft (51) On - Hick on edge of idler shaft All Gears | Units Built Before that contacts reverse gear | December 1,1987 (49). |--------------------------------------------------------- Noise All Gears | * Reverse Speed Gear - High spot on gear tooth (a (Starts Out As A | (25) shinny spot in the middle of Whine Then Turns | the gear teeth about 1/8 Into A Growl) | inch in from the cone part). |--------------------------------------------------------- Noise All Gears | * Reverse Idler Gear - Reverse idler gear rubbing | (49) and 1st Speed on back of 1st speed gear. | Gear (22) | | * Input Shaft (1) and - Input shaft clutching teeth | Countershaft (54) edge rubbing on side of | countershaft input mating | gear. |--------------------------------------------------------- Growl Noise | * Bearing Races (1), - "Peening" or damage. (Look All Gears | (23),(24),(33),(202) at races at an angle in | & (214) bright light.) |--------------------------------------------------------- Shift Lever | * Shift Lever Housing - Non-conforming shift lever Rattle/Buzz | Assembly (135) or housing assembly (135) or Or A Ticking Noise | (136) (136). | (Too much clearance.) Can Be In All | (Remove boot (130) (If shift lever housing seal Gears And Neutral | and measure clearances is not available, use gasket (Noise May Lesson | between yoke and maker GM P/N 1052943.) if You Hold The | housing and yoke and Shift Lever) | shift lever,the axial (No service for shift lever | clearances should be housing assembly, except (The Noise Usually | 0.03 to 0.1mm an ball boot - replace as an Gets Worse At | socket that mates into assembly.) Higher Engine RPM's)| transmission should have | no up-and-down movement | when pulled and pushed on.) |--------------------------------------------------------- Neutral Rattle | * Loose Vacuum Line - Improper running engine (Clutch Engaged) | On Engine transferring irregular | pulses to transmission gear | * Disconnected Spark set, causing rattle. | Plug Wire (Will Be Louder | When The Acces- | sories Are On, As | * Out Of Tune Engine When The Air | Conditioner | * Clutch Disc Not - Clutch disc transferring Compressor | Anti-Rattle Version engine pulses to transmis-. Is Cycling) | sion 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 - Damaged or worn bearing. | (214) | * Pilot Bearing (3) - Damaged or worn bearing. | | * Countershaft Bearing/ - Worn countershaft bearings/ | Gears (201), (53) gears or any speed gear | or (54) Speed Gear bearing. | Bearing(s) (14), (17) | or (30) --------------------|--------------------------------------------------------- GEAR GRINDS/CLASH | * Proper Fluid Level - Improper/low fluid. Any Forward Gear | USE ONLY GM P/N 1052931. Grind | * Proper Type of Fluid (Automatic transmission | fluid or engine oil do | not work well with | synchronizers which | results in gear clash.) |--------------------------------------------------------- | * Refer To Clutch | Diagnosis |--------------------------------------------------------- | * Interlock Ball(s) - Two gears trying to engage | (323) at the same time. Missing/ | damaged interlock parts. | * 3-4 Shift Interlock | Pin (110) | | * Synchronizer Ring(s) - Synchronizer ring (2) bronze | (2) transfer to speed gear | clutching cone. | * Speed Gear(s) (1), | (13), (18), (22) | and (31) |--------------------------------------------------------- 1st And 2nd Gear | * 1-2 Synchronizer - 1-2 Synchronizer sleeve Grinds (Especially | Sleeve (20) (20) installed backwards When Cold) | (clutching teeth on sleeve | have different angles to | aid in cold shifts). | Worn or damaged clutching | teeth. | | * 1-2 Synchronizer - Damaged or worn sleeve (20) | Sleeve (20) and teeth or teeth of speed | Speed Gear(s)(18) gear(s) (18) or (22). | and (22) |--------------------------------------------------------- Reverse | * Reverse Synchronizer - Damaged or worn 5-R (Spin-Down Time | Parts (28) and (8) synchronizer hub/sleeve Is OK With Engine | assembly. At Hot Idle And | Vehicle At A | (A slight grind is (Have customer hold down Complete Stop) | design intent because clutch pedal for a little | reverse is not longer period of time before | synchronized.) shifting into reverse.) --------------------|--------------------------------------------------------- HOP-OUT | Slips Out Of Gear | On Acceleration | Or Deceleration | (Transmission Under | Load) | | 1st And/Or 2nd | * 1-2 Synchronizer - Damaged or worn sleeve/hub | Sleeve (20) or spline lock. | Hub (21) |--------------------------------------------------------- 3rd And/Or 4th | * Roll Pin (107) - Roll pin (107) not at 12.5mm | Height proper depth, rubbing on | front housing (200). 12.5mm | is NOT MEASURED FROM TOP OF | FORK, REFER TO SERVICE | MANUAL FOR PROCEDURE. | | * 3-4 Synchronizer - Spline lock omitted or only | Sleeve (8) partial spline lock on | (Cannot Tell Visually) sleeve. | | * 3-4 Synchronizer - Damaged or worn sleeve/hub | Sleeve (8) or Hub spline lock. | (12) |--------------------------------------------------------- 5th And/Or | * 5-R Synchronizer - Damaged or worn sleeve/hub Reverse | Sleeve (8) or Hub spline lock. | (28) |--------------------------------------------------------- (Coast ie. | * Shift Plunger - Damaged, worn or missing Not Acceleration | Assembly (242), parts. Or Deceleration) | (243),(244) or (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 | * Input Bearing - Oil drain back hole not Retainer | Retainer (205) lined up with hole in | Oil Drain Back front housing. | Hole Location | | * Seal (209) - Damaged or worn. | | * Input Shaft (1) - Damaged or worn. | | * Sealant - Lack of proper sealant. | | * Input Bearing - Cracked or damaged. | Retainer (206) | & (207) | | * Flatness of Front Housing | (200) and Input Bearing | Retainer (206) |--------------------------------------------------------- Shift Shaft/ | * Plug(s) (221) - Cocked or lack of sealant. Rails Plug(s) |--------------------------------------------------------- | Countershaft | * Countershaft Bearing - Cocked or lack of sealant. Bearing Plug | Plug (250) - Damaged bore. |--------------------------------------------------------- | Engine Rear Main | * Refer To Engine Leak Seal | Diagnosis |--------------------------------------------------------- (Leak From Front | * Oil Fill Plug (222) - Lack of sealant. Housing) | Oil Drain Plug (225) - Damaged threads. | - Loose. | | * Breather Hose Adapter - Damaged or missing. | (224) | | * Breather Assembly - Damaged or missing. | (223) | | * 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 - Loose. | (325)[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 (allowing | Bearing (318) slip yoke to move | more than seal. | | * Bolts (2WD) (304) - Loose bolts. | and (305) - Lack of sealant. | | * Rear Housing (4WD) - Loose bolts to transfer | (301) case. | - Lack of sealant. | - Mating surfaces not flat | or damaged. | | * Output Shaft Oil Seal - Worn or damaged. | (4WD) (320) | | * Back Up Lamp Switch - Lack of sealant. | (313) - Damaged threads. | - Loose. | - Leaking assembly. | * Machining Holes Ball - Improper staking. | Plug | | * Reverse Idler Shaft - Damaged, worn or missing. | O-Ring (52) | | * Reverse Idler Bolt - Lack of sealant. | (311) - Loose. | | * Boot (130) - Cracked. | - Torn. | | * O-Rings (129) & (131) - Missing or worn. | (Only On Some Models) | | * Shift Lever Housing - Inside seal torn or | (135) & (136) 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 - Internal leak. | Sensor (316) --------------------|--------------------------------------------------------- MAGNETIC PARTICLES | ON SHAFTS/GEARS | | * Oil Drain (225) and/or - Improper staking to hold | Oil Fill (222) Plug(s) magnet in place. | Magnet Missing | (Proper Magnet Retention | Should Be Checked Every | Time Transmission Is | Serviced) ------------------------------------------------------------------------------

Object Number: 79217 Size: FS

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.