Volvo VNL 780 Operator's Manual page 338

328 OPERATION
Note: The retarder is automatically disabled
when the ABS (anti-lock brake system) is
active.
The Transmission Retarder is an option with Allison HD and MD transmissions. It is
activated by an on/off switch in the auxiliary panel and is controlled by a stalk switch
mounted in front of the wiper switch. You can set 6 different levels with the stalk switch.
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