Bump start a 4WD standard in 2WD or 4WD on low traction ground

Question

This is really more a hypothetical question than IRL but I'm curious as to the correct procedure in case I find myself caught in this scenario (which is a little more likely than you might think).

Case #1: Assume a 2006 Ram 1500 4.7L with manual 6 speed transmission, electronic 4WD engagement and available 4WD Lock.

Case #2: Assume a 1997 Jeep TJ 2.5L with manual 5 speed transmission and manual transfer case 4WD engagement.

Given:

• On a hypothetical 'wet grassy/muddy slope', I would be coasting downhill in neutral to gain some forward momentum then shifting to 2^nd or 3^rd before dropping the clutch and transferring forward motion to the engine¹.
• The hypothetical 'wet grassy/muddy slope' has a low enough traction coefficient that travelling up the hill would necessitate 4WD.
• The Dodge's battery is sufficiently depleted to be unable to start the vehicle while still having enough charge to maintain an electronic 4WD engagement² as it would have been driven up the hill in 4WD. It doesn't matter what stage of charge the Jeep's battery is in.

I'm assuming that in both cases, the 4WD would have to be active before the roll was initiated. The Jeep transfer case should be engaged at a standstill and the Dodge's electronic shift to 4WD takes a few seconds that you wouldn't want to be worried about when rolling down a hill.

I get that initiating a roll downhill in 4WD is going to require additional effort to gain the same forward momentum as in 2WD but conversely, I believe the added 'moving parts' will contribute a form of pseudo-flywheel effect.

Question:

If I'm trying to 'bump start' either of the above vehicles coasting down a wet grassy/muddy slope, should I be in 2WD or 4WD before dropping the clutch?

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¹ _{4WD-Low (Bull Low) doesn't really enter into these scenarios from my perspective but comments/answer responses to the contrary are welcome.}

² _{Having never personally engaged an electronic 4WD on a partially dead battery and a engine that isn't running I am only assuming that it can be done.}

Answer 1

TL DR: Use 4WD if at all possible.

You'd want to use 4WD if you can ensure it will go into and stay in 4WD prior to the maneuver. This is for a couple of good reasons:

1. You are most likely only going to get one chance at it, so make the best of it. If you are pointed downhill, depending on the situation, if you are unable to get it started on your first try due to lack of traction, you are going to be hard pressed to get it started. Make the most of your effort and get the most traction you can.
2. A RWD vehicle will have less traction on the drive axle when pointing down hill. This is just simple physics and how a vehicle behaves. If your vehicle is pointing downhill, the center of gravity will shift forward towards the nose of the vehicle, meaning the rear end will have less traction. If you are on a slippery slope and only have it in RWD (4WD not engaged), your tires are going to slip easier going downhill than you'd expect on a flat or uphill surface. Having 4WD engaged will allow you the greatest chance at success when only thinking of traction.

While you stated the battery condition would not make a difference on the Jeep (or you didn't care about the state), I'd put to you it absolutely does matter. The reason I state this is because if your battery is dead enough to where it doesn't have enough energy to excite the alternator, you won't have enough energy to fire the components to allow the vehicle to run. Those components include not only the ignition system, but such things as electric fuel pumps. Having vehicle momentum is not enough in and of itself to get a vehicle started. The reason I state this is because if you don't have enough battery power, there is a distinct possibility you will not get it started.

Bottom line, if it were me, I'd be using the 4WD (as long as it can be engaged) in the bump-starting process. It's the only way to give vehicle a fighting chance to get running.