The TC clutches are partly locked up at times, and fully locked up in 4th & 5th gears.
So what you may be experiencing is more than likely TC clutch slippage or shudder in 4th gear.
Time for a SunCoast?
From 545RFE Manual:
The clutch can be engaged in second, third, fourth, and fifth (if applicable) gear ranges depending on overdrive control switch position. If the overdrive control switch is in the normal ON position, the clutch will engage
after the shift to fourth gear. If the control switch is in the OFF position, the clutch will engage after the shift to third gear.
The TCM controls the torque converter by way of internal logic software. The programming of the software provides the TCM with control over the L/R-CC Solenoid. There are four output logic states that can be applied as follows: ² No EMCC ² Partial EMCC ² Full EMCC ² Gradual-to-no EMCC
• NO EMCC Under No EMCC conditions, the L/R Solenoid is OFF. There are several conditions that can result in NO EMCC operations. No EMCC can be initiated due to a fault in the transmission or because the TCM does not see the need for EMCC under current driving conditions.
• PARTIAL EMCC Partial EMCC operation modulates the L/R Solenoid (duty cycle) to obtain partial torque converter clutch application. Partial EMCC operation is maintained until Full EMCC is called for and actuated. During Partial EMCC some slip does occur.
----- Partial EMCC will usually occur at low speeds, low load and light throttle situations.
• FULL EMCC During Full EMCC operation, the TCM increases the L/R Solenoid duty cycle to full ON after Partial EMCC control brings the engine speed within the desired slip range of transmission input speed relative to engine rpm.
• GRADUAL-TO-NO EMCC This operation is to soften the change from Full or Partial EMCC to No EMCC. This is done at midthrottle by decreasing the L/R Solenoid duty cycle.
Note: If you want a copy of the 545RFE manual, let me know, I can email it to you as I have it in a PDF document. 462 pages...
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