A/T - Harsh Reverse Engagements
BULLETIN: # 024DATE: September 1990
TRANSMISSION: All
SUBJECT: Harsh Reverse Engagement
Harsh Reverse Engagement
This bulletin addresses harsh reverse engagements in many types of transmissions. Reverse usually has two components engaged. These components are an input member and a holding member. The Input member may be a direct, high-reverse or front clutch. The holding member may be a low-reverse band or low-reverse clutch. For this bulletin, we will always call the Input member the reverse input drum and we will call the holding member, the reverse holding member.
Many times the cause of a harsh engagement Into reverse is a timing problem. The reverse input drum may be engaging before the reverse holding member. If the reverse input drum applies first, the planetary gear train with all the connected rotating parts begins to spin, building momentum. When the reverse holding member applies it must stop the complete rotating mass of torque converter turbine, shafts, drums and planetary assemblies. If all of these parts come to a sudden stop, it can produce a jolt that is uncomfortable to the driver. Transmissions with heavy or large diameter rotating components could add to the harsh engagement problem. Some examples would be a transmission with a torque converter clutch creating a problem because of the extra weight of damper assembly and the clutch plate. Some transmissions, like the C6 and 3L80/THM400, have comparatively heavier drums and planetaries.
One way to help prevent a harsh engagement is to make sure the reverse holding member applies before the reverse input drum. You can do this by using band adjustments and clutch pack clearances to help regulate which applies first. Before you start any internal work, check some external factors that can influence harsh engagement.
USE THIS ORDER FOR CORRECTION:
1. Check engine tune. Does the engine idle smooth and have good vacuum? Low vacuum raises line pressure on vacuum modulator equipped transmissions causing harsh engagement.
2. Set engine idle speed to exact manual specification for that particular year and model. The faster a component is rotating, the harder it is to stop. Engine speeds as much as 100 RPM over specifications can cause harsh engagements. RPM's this slight are difficult to detect by just listening to an engine. It is best to check the RPM with a tachometer, but not the one on the dash as it is not accurate for this.
3. Check main line pressure. High pressure can cause aggressive engagement of components.
4. Work with reverse holding member clearances. (Make sure it is not too loose)
5. Work with reverse input drum clearances. (Make sure it is not too tight)
6. Some transmissions may require an additional wave plate in the reverse drum (3L80/THM400, or 4L60/THM700-R4).
BULLETIN RECAP
^ Check engine tune.
^ Check engine RPM.
^ Check pressure.
^ Tighten reverse holding member clearance.
^ Loosen reverse in put member clearance.
^ Install wave plate.
FOR ADDITIONAL INFORMATION:
ATRA: TSB 86-33, TSB 85-54 TSB 85-52, TSB 83-44