Correct answer
Follow the manufacturer’s instructions and do not use it in those conditions if prohibited.
Follow the manufacturer’s instructions for that vehicle and the available grip. The retarder acts on the driveline and can cause the driven wheels to lose grip. If the manufacturer prohibits use on wet or slippery roads, do not use it; selecting a lower level does not override that prohibition. Where use is permitted, operate it as instructed and avoid abrupt application. It is not a parking brake, and rain does not itself increase its effectiveness.
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Always use it because rain increases its effectiveness.
Use it just as on a dry road, since grip has no effect.
Follow the manufacturer’s instructions and do not use it in those conditions if prohibited.
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