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Old 10-10-2015, 11:58 AM   #1
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Question tranny? 05 gmc jimmy (anybody? )

while cruzing in overdrive with no load on the motor, the rpm goes up and down about a 100 rpm and it feels like the torque converter is going in and out of lock steadily. any body have any suggestions? or is it eaven the transmission? anything else I could look at? runs great and shifts fine otherwise.

Last edited by miketo; 10-11-2015 at 09:28 AM.
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Old 10-11-2015, 10:26 AM   #2
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Re: tranny?

The Torque Converter Clutch (TCC) locks and unlocks solely on command from the PCM. The PCM commands a solenoid valve in the transmission pump body (not the valve body) which actuates the TCC hydraulics. The clutch itself is contained completely within the torque converter and is inaccessible without major disassembly.

If the solenoid is failing the PCM should have a stored diagnostic trouble code (DTC). If the TCC or another transmission clutch pack is slipping while it should be actuated there should also be a stored DTC in the PCM memory, and the Check Engine light may or may not be on.

If the TCC randomly unlocks and locks the cause could be a brake pedal switch, throttle position sensor, failing TCC solenoid/valve. or external problem. For example, random engine misfire detected by the PCM at various RPM and load conditions will force the PCM to unlock the TCC. Engine MAP values which are too high will cause a TCC unlock. If transmission line pressure is detected at a low level during operation the PCM can unlock the TCC.

Of course, all of this relies heavily upon the transmission oil level and pressure being normal, correct transmission oil (Dexron VI) that is clean and not degraded, electrical connections that are reliable, and a properly operating engine. Scan the PCM for stored or pending codes. These may offer a hint of the possible problem(s).

Fortunately, the TCC solenoid, the wiring harness, and main transmission electrical connector are all accessible by removing the trans oil pan and filter, and without removing the transmission from the vehicle, so repair might not be as costly as you might presume. If the transmission oil has been drained and refilled at regular intervals that will eliminate one potential problem.
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