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| Illustration 1 | g03675356 |
Retarder control valve (1) Service brake and retarder pressure switch (rear brake circuit) (2) Service brake and retarder pressure switch (front brake circuit) | |
Some input signals to the Power Train/Chassis Electronic Control Module (ECM) during a shift can cause a hunting condition of the transmission. A hunting condition is a series of rapid, unwanted transmission upshifts, and rapid, unwanted transmission downshifts. A hunting condition is caused by ground speed changes near a transmission shift point.
The Power Train/Chassis ECM is designed to prevent a hunting condition. The Power Train/Chassis ECM will only make one turn around shift approximately every 2.3 seconds. A turn around shift is when the transmission upshifts or downshifts to the next gear position, then shifts back to the original (before the shift) gear position.
In normal operating conditions, a rapid shift may be necessary. An example of a normal operating condition is stopping the machine. If a rapid downshift is not made in this example, the engine will stop. When the service brake or the retarder is used, the service brake and retarder pressure switches will signal the Power Train/Chassis ECM that rapid shifts are necessary.
The service brake and retarder pressure switches will signal the Power Train/Chassis ECM when a higher engine rpm is necessary for an upshift or for a downshift. The service brake and retarder pressure switches also signal the Power Train/Chassis ECM to allow rapid shifts.
When the service brakes or the retarder are not used, the Power Train/Chassis ECM receives a ground through the service brake and retarder pressure switches. The Power Train/Chassis ECM will use normal engine rpm for upshifts or for downshifts. The Power Train/Chassis ECM allows only one shift back to the previous gear to occur for every 2.3 seconds.
When the service brakes or the retarder are used, the Power Train/Chassis ECM will allow rapid upshifts and rapid downshifts in order for the machine to stop quickly. The downshift point will also be raised and will maintain a higher engine rpm during downshifting. This function maximizes the amount of cooling oil that flows to the brakes during braking. The service brake and retarder pressure switches are open when either the service brakes are used or the retarder brakes are used.