In your recent article on mountain driving, you suggested drivers shouldn't use "Jake Brakes" on rainy or snowy roads. Is that true always?
There's no universal Yes or No answer to this. Caution should always be used when operating on anything other than clean dry pavement. Since an engine brake has more than enough power to lock up the wheels, it's possible that when the wheels lock up the engine will stall. Some recent model trucks have electronic safeguards that will disengage the engine brake if the wheels are about to lock up, but don't depend on it.
It a matter of traction. If the roads are very slippery, or the truck is lightly loaded, the likelihood of skidding due to wheel lockup is far greater. A situation like this will cause the drive wheels to lock up and skid, causing the trailer to push forward. This results in a jackknife where the tractor turns sideways and slams up against the side of the trailer.
Use of the engine brake on slippery roads requires a high degree of vigilance. I suggest you descend the hill slowly with the engine brake on low or medium setting. If the road appears slippery, turn it off. Of course, you'll need to be going slow enough to use your service brakes at a modest application pressure as well, as they may cause a wheel lock up too. Even ABS is no guarantee against a jackknife. The jackknife may have already begun before the ABS allows the wheels to begin free-rolling again.