Hi all. have a 99 C5 convertible automatic, and since changing out the lame 273 rear gears to 342,s I have this slight slipping in 4th gear. Never noticed it before the change. After this happens a few times it shifts very fast and hard from 1st to 2nd. Then I reset the codes and it's ok. Have a rebuilt tranny waiting to go in just in case that is what the problem is. just do not understand why it is happening after the rear gear change. any info would be great.
4th Gear Slightly Slipping
- Thread starter electraglide
- Start date
12cents
Power User
Did you change the torque converter to a higher stall? That is the only thing I can think of outside of a tranny problem.
Just a shot in the dark but could it possibly be because of more load on the drive train due to the diff change ? Although that doesn't necessarily explain why only in 4th gear......
Far from an expert, but decided to give AI a shot at it. Check out the following info, which gives a pretty good opinion of possible causes, but I really doubt the extra speed it turns, really would have a direct effect on what you are experiencing.
Because the transmission and the rear differential are bolted directly together as a single unit (the transaxle) in a C5 Corvette, changing the rear end gear ratio requires completely separating the two components. [1, 2]
If your transmission is suddenly popping or slipping out of fourth gear following this procedure, the issue is almost always a result of mechanical misalignment created during the reassembly, rather than an issue with the new gears themselves.
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To help narrow this down, could you clarify:
Because the transmission and the rear differential are bolted directly together as a single unit (the transaxle) in a C5 Corvette, changing the rear end gear ratio requires completely separating the two components. [1, 2]
If your transmission is suddenly popping or slipping out of fourth gear following this procedure, the issue is almost always a result of mechanical misalignment created during the reassembly, rather than an issue with the new gears themselves.
1. Shifter Linkage or Torque Tube Alignment
The most common culprit is that the shifter linkage or torque tube assembly was slightly disturbed or bolted back together out of alignment. [3, 4]- The Cause: When dropping the rear subframe to swap the differential, the entire drivetrain angles down. If the shift linkage rod or the shifter box itself gets pulled, twisted, or bolted back down slightly off-centre, it will restrict the mechanical throw of the shifter. [3, 5]
- The Symptom: The shifter feels like it is in fourth gear, but internally, the slider is only catching the very edge of the gear teeth. Under load or deceleration, it easily pops free. [3, 6, 7]
- The Fix: Perform a C5 Shifter Alignment Adjustment. You will need to remove the center console, place the shifter in neutral, lock it with the factory alignment pin, loosen the alignment torx bolts, let the linkage settle naturally, and then re-torque them. [4]
2. Output Shaft Endplay and Thrust Tolerances
The interface between the T56 manual transmission (or 4L60E automatic) and the differential housing is incredibly tight.[1, 8]- The Cause: When swapping rear gear ratios, the differential must be shimmed properly. If the output shaft endplay or the mating surface tolerances between the transmission and the new differential are slightly out of spec, it can exert axial (pushing/pulling) forces on the transmission main shaft. [3]
- The Symptom: Fourth gear is uniquely sensitive to this because on a T56 manual, 4th gear is a direct 1:1 drive where the input shaft and main shaft lock together. Any structural deflection or excess shaft play forces the 3-4 synchronizer slider forward or backward, kicking it out of gear. [3, 8]
3. Damaged or Worn Shift Fork Pads
This is a highly common physical issue that often triggers right around the time of heavy drivetrain service. [8]- The Cause: The factory C5 3-4 shift fork uses plastic/nylon pads. If the transmission was forced or angled roughly while being unbolted or re-mated to the differential, these brittle plastic pads can easily crack or shatter. [8, 9]
- The Symptom: Without the pads, there is excessive slop inside the shift fork. The fork cannot push the synchronizer sleeve completely over 4th gear, leaving it vulnerable to slipping out under load. [3, 5, 6, 10]
4. Drivetrain Mount or Bushing Deflection
- The Cause: To pull the rear end out, the transmission mounts must be unbolted. If the rubber/polyurethane rear mounts were reinstalled improperly, or if an old mount tore during the process, the transaxle assembly will twist excessively under torque. [3, 11]
- The Symptom: This physical twisting pulls hard against the rigid shifter linkage assembly mounted to the torque tube, physically yanking the transmission right out of fourth gear. [3]
Diagnostic Next Steps
| 1 | Check the fluid level & color | Ensure it wasn't underfilled during the differential re-marriage. |
| 2 | Test with the engine OFF | Sit in the car with the engine off. Shift into 4th. If it feels shallow or springy compared to 2nd or 6th, the linkage is physically blocked or misaligned. |
| 3 | Check for vibration | If 4th gear pops out specifically when you let off the throttle (deceleration), it points to an alignment or output shaft bearing/bushing issue. |
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To help narrow this down, could you clarify:
- Is your C5 an automatic or a 6-speed manual?
- Does it slip out under hard acceleration, or does it pop out when you let off the gas pedal?