My current job is driving railroad crews to where they need to go, and half of the places I need to get them to is offroad and poorly maintained (if maintained at all). I drive the company's 2006 Chevy Suburban with 4WD.

This is a high-rainfall area, and sometimes I have to go through deep water. I am okay with this as long as I have some idea what I'm doing. I know to go very slowly.

My question: how deep can I go safely? If I had a crew member hanging out the front window to visually check the depth, how deep is where we're getting too deep? To the bottom of the doors? Or is that pushing it?

Also, there seems to be Upper and Lower 4WD buttons. I've used the upper one when I've need to, but never the lower one. What is the difference?

Any info and tips welcome.

Sue

The difference between 4WD high and low is a gearing ratio, usually. If you put it into low, the vehicle will go slower for a given level of engine revolutions, and that's a pretty basic explantion. It allows the vehicle to have more torque to run at lower speeds.

My guess is that the absolute limit for depth of water is determined by wiring. Allow your wires to get wet, and the ignition may not fire because the spark will run to ground before it gets to the spark plug. But that's a guess.

Not that I have lot's of rescue experience or anything, but from the basic physics of the situation, driving through water is deceptively dangerous. People typically have the mistaken belief that a bigger, heavier vehicle is safer when traversing water, but that's not necessarily true. A football field-sized super tanker floats just as well as a rubber raft. It's the volume of water displaced that generates the buoyancy, so the bigger the vehicle body, the more water it displaces.

People also often underestimate the power of moving water, even slowly moving water. A slow current pushing against the large exterior of a Suburban is a lot of force per square inch.

In general, I would avoid driving over flooded roads if there's any way to go around since you can't see if there are big holes or objects that could flatten your tires under the surface. But, if I were to proceed, I would not let the water reach the frame rails. Water can rise quickly--or you could drive into a depression which suddenly puts the water over the frame rails--and like I said, it really doesn't take much depth of water to generate enough buoyancy for your tires to lose traction, even in a really big SUV. At that point, if there's any sort of current, you could be pushed downstream and flipped over or sent into deeper water quite easily.

I suppose the only exception to this advice is if the interior intentionally or unintentionally filled with water. Then the vehicle wouldn't really be displacing much water and wouldn't be very buoyant. It's like that scene in the movie Dante's Peak where they're crossing the river in the Suburban and the interior fills up with water. (But they had a snorkel for the engine, too. )

Around these parts, lots of emphasis on when not to drive through water thanks to flash floods (we have what's known as the Stupid Motorist Law ... basically, you pay for the rescue if you try to cross water when it's a dumb thing to do).

From the NOAA website: "a depth of two feet will float your car." Moving water compounds things.

Here's a webpage you might find useful: A Selected Few 4 Wheel Driving Tips.

My experience with 4x4 offroad is limited but in that experience a 'low' range 4x4 often means that the front wheels are going to run at a different gear ratio than the rear...no good for pavement but can be handy for getting up a hill at any more than a snails pace.

Also, how much water you can go through has a lot to do with the design of the vehicle. How high is the air intake for the engine? Many vehicles have snorkel kits available to raise that level. How is the exaust designed? If you stall in hood deep water will water leak into the engine from the exhaust side due to the water pressure? (water in an engine creates hydrolock and can do amazing amounts of damage). How waterproof are the electronics? Will the lights go out if you swamp for example?

You'd think most modern 4x4 vehicles would have this covered...but then many modern 4x4 vehicles weren't designed to see anything but pavement despite what they're advertised to do.

I know a few guys with Jeeps who spend tons of money on countering the whole water thing. I doubt my Jeep Compass could handle more than a couple feet of water and only slowly at that...it's just a glorified station wagon after all

The problem with a flooded road is you cant tell if the road is still intact or has been eroded by current. A good rule of thumb is to keep the water line below the frame rails. As others have mentioned much deeper than that and you run the risk of water shorting out the electrical system AND getting into the vent tubes of the differentials. (the later is not an immediate danger to the vehicle but will cause some expensive repairs if not taken care of soon after a water crossing with any depth) Not to mention the buoyancy risk. I have tried crossing a body of water that was only hub high(half way up the tire) and because of the power of the current it began pushing my truck sideways. However slight it was it was still in fact moving the vehicle and I was nowhere near the frame rails in depth.water crossings if the bottom(solid or soft) and depth is unknown is a scary proposition.

In my limited experience the biggest limiting factor is where your engine gets it's air.

If your intake sucks up water the engine, at the very least, stops. Unless your fast turning it off and very lucky it's toast. If there is any chance of it having sucked up water you need to pull the spark plugs and clear the cylinders before you do anything else. Some trucks and SUVs suck air through a hose that reaches down in front of the wheel well. Which means if the water gets much above the hubs your sunk. Simply relocating the air intake so it draws higher up can make a big difference.

In my experience the wiring isn't usually all that vulnerable. Fresh water isn't all that good a conductor and because the engine compartment tends to get pretty wet anyway they are protected from shorting out fairly well. I would look at where the battery is. You don't want water to get into the battery or let the terminals short out. A sealed battery and/or moving the battery shelf up some might help. A plastic battery box might help keep splashes at bay but if you can't keep the water out you need to provide a drain for it to get out.

This is also true for tail lights and position lights. If they are truly sealed your good to go. But if not they have to have a way to drain. Using silicone dielectric grease, designed for the purpose, to keep water out of the sockets and prevent corrosion is a good idea.

Spark plug wiring are typically sealed under rubber boots with silicone dielectric grease. If your planning to ford streams you would be wise to closely inspect those spark plug wires to make sure thee are no cracks or worn spots. Intact and well sealed they shouldn't give you any trouble.

Of course a diesel would eliminate the spark plug wires as a source of potential trouble.

I'm not real clear on the effects of fording on wheel bearings. I have heard that for just occasional crossing of water over the hubs there is no need to change anything. Just make sure they get regular maintenance.

This is what I have done and so far so good. A couple of times a year I was running a small car deep enough to have water over the hubs during hurricane season. A friend who does a lot of fording streams with a jeep suggested I might switch to a special grease and maybe install some sort of bearing protector. The car kept rolling so I guess everything was okay and now I have a truck. Still something you might ask your mechanic about.

As long as you exhaust system is intact the air pressure of the exhaust gasses should keep the water out. At least well enough for short dunkings.

You might ask around to see if there are any off-road or FWD clubs around your area. They would have experience in your area and be able to look over your vehicle with a critical eye.

having seen videos on tv of folks that have attempted to drive through moving water,

and knowing how slick underwater rocks may be,

and agreeing with the folks' comments above,

and finding your comments and posts insightful and humorous and would be greatly missed - please don't drive through an unknown depth of rapidly moving water - if at all possible.

they can't be paying you enough.

and if you must go, keep the water below the body of the vehicle.

Going by GM specs for a tahoe, your suburban (basically the same setup) should be able to ford through 20" of still water at 5mph. I wouldn't attempt going through fast moving water though. Like many have said here, moving water has a lot of power, a few inches of fast moving water is enough to push a vehicle sideways.

As far as 4x4 low, what it does in a suburban is multiply your torque by about 2 through a change in gearing. Therefore, if you're trying to make it up a steep slippery hill (like a boat ramp or dirt hill offroad) it will allow your vehicle to make it up the hill without putting as much strain on the engine and transmission. This should only be used on slippery surfaces and only at a crawl. The best thing to do would be to check the owners manual, which should offer an explanation of how and when to use it. It's a bit more complex in its operation than 4x4 Auto or even 4Hi mode.

A general rule for water crossings is if you can't walk through it, you shouldn't drive through it. Even slow moving water can sweep a car downriver pretty easily.

If you plan on doing this regularly, there are precautions you can take. There are techniques such as creating a bow wave to help drive through really deep water, and there are preventative measures you can do to keep your car from getting ruined. The air intake is only a part of the problem, you also have to worry about the vents on the axles, transmission and transfer case to keep water out of those areas. Then there's the whole issue with the electrical parts. A good 4x4 book will give you a much better explanation.

If you want to go through really deep water, get a diesel

http://youtube.com/watch?v=tzGh43-leO4

and if that's still not good enough, then you'll need a submarine