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Discussion Starter #1
Ok so this is my first time having a 4WD capable vehicle.

When I got it, the dealer told me I should just keep it in "Auto" mode if I dont plan to actually go on a beach, or offroading, etc. He said the jeep will sense rain, snow, etc and automatically change to 4WD as needed. Hence, the "Auto" I guess.

It has snowed a bunch since getting the car and when I drive I never notice the car go into 4WD. I tried looking at the manual but it doesnt really explain well enough to me at least, what is supposed to happen. Will it only switch to 4WD if there is >5" of snow or 10" or what?

So I have at times switched the knob to snow and i can definitely feel that its going into 4WD and i get the "4WD lock" light show up on the dash but is there something Im missing with the "Auto" option or am i just dumb?
 

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To tell you the truth I have no idea how exactly the 4x4 works. The manual is very vague. I did find a document from 2017 about the Compass Select terrain system, I don't know the source of this, but I attached a screenshot.

I drive in auto unless there is pack snow on the road, then I switch to snow. I haven't had a chance to try the other modes yet.
 

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My understanding is that the vehicle is predominantly FWD up until it senses slippage at one of the corners. At that point, the system distributed power to the appropriate wheels to ensure traction. So if you leave it on auto, it'll be AWD if needed. If you pay the 4WD pick, it does just that, no switching back to FWD for mpg optimization until unlocked. The Snow, Sand, and Mud modes should just be preset calibrations of the 4WD mode, again locked.

Not sure if it's true, but I thought I read on here too that it starts off actually in 4WD, and doesn't drop to FWD only basically until you're highwsy cruising. Hence, be no burn outs at Green lights, very little torque steer, etc.
 

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As far as leaving it in Auto like your dealership recommends, I'd use my own judgement. I've only had my Compass for a little over a week, and encountered a pretty bad (6 or 7 inches) snowstorm on day 3 of ownership. There was a NOTICEABLE difference with the selector set to "Snow" while driving. Definitely much more stable. Since then, on some questionable semi-slushy/sorta clear roads, I felt the Auto selection was good enough. No trips to the beach yet, Michigan isn't ready. Lol
 

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Well let's see if I can explain this a bit. When in auto mode the Compass will use some variables to work out what is the best way to help you maintain control and momentum. However since even in certain cold snowy conditions their can be good grip it may still to chose to run in 2wd mode. While it is quick and most often seamless to go back and forth through different settings there are times there might be a lag in the computer figuring out what is best moment by moment as your driving.

This is where the Selec Terrain comes in. By selecting on of the modes the computer will lock in certain settings on the power transfer,traction control,braking and engine management that are best for what you have selected. This means the Jeep will spend less time hunting for how to manage those things constantly.

Take Snow for example. Power transfer is set to 60% front and 40% rear unless a certain amount of wheel spin is detected this setting will help take away a lot of squirreliness on the slippery road. Engine throttle response is reduced to help keep spinning down, if you have an auto transmission it will be locked into 2nd gear take off again to reduce wheel spin,and the way the computer uses the brake lock differential settings will be tuned for best dealing with snow/ice.

But in mud mode things change I think that locks the power transfer 50/50 front and rear,the system tunes itself to allow more wheel spin to help keep tire tread clear.

Left in auto mode it can still do these things it just take a bit more time to figure it all out. If you watch some videos of Jeeps equipped with Select Terrain diving off road or slippery conditions you can actually see the difference in how the system works in both auto and Select Terrain. In auto the diver holds a steady throttle even as wheel spin starts within a few seconds the system finds a traction solution and movement will start. When Select Terrain is selected for the correct situation movement often starts immediately.

So there is nothing wrong with auto mode but your going to have the best results and more relaxed/fun driving experience using the Select Terrain setting that matches your driving conditions.
 

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Discussion Starter #6
thanks for the responses. It seems to make sense a bit more now of how the actual terrain control works. I didnt really think anything was wrong but you know, its just weird driving with 2-4" of snow on the ground being in auto and not feeling anything change to help traction, etc.

Follow up question, in modes other than Auto, is there a max speed to not go over?

Obviously, if its snowing Im not gonna drive 75 down the highway but I know some friends have told me that some older vehicles you really couldnt go over like 30 or 40mph or else it would mess with the transmission and such?

Is this a thing or no?
 

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A few things to add here. It is my understanding if the temp drops below 32 it will put you in 4wd as it could be icy out. I believe if your wipers are on it will go to 4wd as well. Also in snow mode it starts off in a higher gear from a stop to keep to much power to the wheels causing you to spin the wheels when you start off. To be honest of its snowing out and the roads are slippery I would put it in snow mode. As far as speeds they use to say dont use 4wd over 55 mph but I think that was more so for the fact the speed limits were 55 back in the day. The only time you cant go fast is if you have the vehicle in 4wd low which if you dont have a Trailhawk you arent going to have 4wd low to begin with. I think the max speed in 4wd low is like 15-20 miles an hour.
 

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awesome video Mudman1! I don't know what half that crap means but those graphics are cool! thanks


You should study up on the stuff you don't understand it is both interesting technology and most helpful to understand to make the most out of your Jeep or any other vehicle using similar systems.

The power transfer units pretty much do what their name suggest. They allow power to be moved around between the front and rear axles depending on which axle has the most traction or can lock into a given ratio of power front to rear for better handling. This is also where the rear axle can be totally disconnected to allow an efficient front wheel drive mode for better fuel economy.

Power transfer is handled by a computer that uses many types of input to determine how best to keep the Jeep moving in a controlled way. These inputs are outside temperature,state of windshield wipers,pitch of vehicle,position of steering wheel, wheel speed sensors on each wheel,the driver's selected mode and maybe even a few more I missed.

The Brake lock differential you most likely understand if you know how differentials work. But basically it is almost like a reverse anti-lock brake system. It applies brakes to the spinning wheel instead of cutting brake pressure to a stopped wheel. How aggressively it does this can be adjusted by the computer but I don't know if it reaches full potential outside of rock mode if the Jeep is so equipped. But the videos I have seen even on none Trailhawks is very impressive. Basically any of the videos where a Compass keeps moving with one wheel in the air is the brake lock diffs at work.

Below is a video that I like as you can actually watch the the system working the brakes on lifted spinning wheels. You can see it constantly checking the lifted wheel to see it it has found traction. It repeats this process till it cuts the brake to that wheel and it does not spin freely. I also like this video because it shows the non-Trailhawks are still very off-road capable!

 
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