CORE AS- 1767662 - Caterpillar

( 6AB, 2PD );
627E ( 6EB, 6GB );
631E ( 1AB, 1NB, 3ND );
637E ( 1FB, 1JB );
651E ( 89Z );
657E ( 90Z, 91Z );
768C ( 02X );
772B ( 64W );
776B ( 6JC ) Tractors;
657E ( 6MB, 86Z, 87Z ) Scrapers;
769C ( 01X ),
773B ( 63W ),
777B ( 4YC ),
785 ( 8GB ),
789 ( 9ZC ),
793 ( 3SJ ) TrucksIndividual Clutch Modulation (ICM) transmissions have been reported to "hunt", or shift rapidly up and down through two or three gears. The probable causes of transmission "hunting" are: 1. If the hunting is very rapid, with several shifts per second, the most likely causes are: a. The 9W1409 Control Group (actual gear switch) may not work correctly. If the control group does not correctly indicate when the selector valve has reached the desired position, overtravel may occur.b. A 6T9290 Solenoid Valve (upshift and downshift) may be leaking, not closing correctly when power is removed from the valve's coil. This can result in shifting past the desired gear.c. High transmission oil pump pressure [more than 3200 kPa (465 psi) at high idle] may result in transmission hunting (overshifting). High oil pump pressure can affect the solenoid valve's ability to deactuate rapidly. Note that oil temperature can slightly affect the oil pump pressure.2. On the 700 Family of off-highway trucks and tractors, if hunting occurs only under load, as on an upgrade, and the back-and-forth shifts occur at the rate of one every second to two seconds, the ICM transmission control may be receiving an incorrect BRAKE signal.
When the brake or retarder is ON, the ICM transmission control will allow back-and-forth upshifts and downshifts. When the brake and retarder are OFF, a time delay prevents back-and-forth shifts from occurring more often than once every 2.3 seconds, to prevent hunting. Check the BRAKE diagnostic LED on the ICM transmission control. If it remains ON when the brake and retarder are OFF, troubleshoot the air system, brake and retarder pressure switches, and wiring to locate the problem.