Explanation Of 4 Hi Vs 4 Lo?

jbeall · December 22, 2009

Hi All,

I've got a 2003 GMC Yukon XL. I use the 4 hi setting when I'm in icy or snowy conditions and the pavement hasn't been cleared, or I'm on gravel, etc.

I've read the manual's description of what 4 lo is for ("delivers maximum torque to all four wheels"), but I'm still not clear on what it actually does. It is just a gear reduction that lowers the ration of engine revolutions to wheel revolutions, thus giving slower speeds with higher torque? Or is it more than that, e.g., is it locking the differentials to prevent wheel slippage? In which case you would only not want to use it on pavement, only in snow, ice, mud, or other conditions where the wheels can slip as you're going around a turn.

I've been searching on this and have found some general info on what 4 hi vs 4 lo can mean, but nothing specific to my vehicle--and it seems that the meaning can vary from vehicle to vehicle.

Thanks for any feedback!

-Josh

Matt_ · December 22, 2009

Hi All,

I've got a 2003 GMC Yukon XL. I use the 4 hi setting when I'm in icy or snowy conditions and the pavement hasn't been cleared, or I'm on gravel, etc.

I've read the manual's description of what 4 lo is for ("delivers maximum torque to all four wheels"), but I'm still not clear on what it actually does. It is just a gear reduction that lowers the ration of engine revolutions to wheel revolutions, thus giving slower speeds with higher torque? Or is it more than that, e.g., is it locking the differentials to prevent wheel slippage? In which case you would only not want to use it on pavement, only in snow, ice, mud, or other conditions where the wheels can slip as you're going around a turn.

I've been searching on this and have found some general info on what 4 hi vs 4 lo can mean, but nothing specific to my vehicle--and it seems that the meaning can vary from vehicle to vehicle.

Thanks for any feedback!

-Josh

^Thats what it does

Nothing else magical about it. Does not change anything in the differentials.

RaddHazzard · December 22, 2009

4Lo is for speeds below 5mph. Its a very low gear giving you maximum torque. You really should never have to use it unless youre stuck bad or need to pull someone out of a bad situation.

59ih340 · December 22, 2009

The simple way to look at it is if you have 4.10 gears in 2wd,when you put it in 4wd you have 4.10 gears front and rear.in low it doubles so you would have 8.20 gears front and rear.hi is generally 1:1 low is 2:1.The engine turns the same the changes in ratio are inside the case.You only want to use 4hi or low when there is ice snow mud etc.

GasHog369 · December 22, 2009

Once i was backing up thru about 3 feet of snow with my 03 silverado onto my side patio.... well, i bogged down and was dragging snow back .. Finally i was spinning in 4hi in reverse and wasnt going anywhere.... I put it in 4lo , gave it some gas, and it crawled out like nothing and then i was able to continue backing up the rest of the way in 4lo to get out I remember tho that when the transfer case shifts into 4lo it makes some awful noises that sounds like stuff breaking , lol . but its all good...

jbeall · December 22, 2009

Nothing else magical about it. Does not change anything in the differentials.

Some people think that it locks the differentials. Does 4 lo lock the diffs on some other vehicles and that's the reason for the confusion, or what? Or is there an option on some GM trucks/transmissions that it might lock the diffs in 4 lo?

rodram · December 22, 2009

Four wheel drive does not lock the differentials on the rear axle or front axle, in any case, in any truck, based on what I know. If the left to right differentials all lock up, you virtually cannot steer the vehicle because all four wheels are going to try to drive in a straight line at the same rotation speed.

4wheel drive (high or low) does send power to BOTH the front wheels and back wheels at the same time, although there is still the left to right differentials on the axles. This is why you don't want to drive on dry pavement in 4high or 4low because all 4 wheels are driving the truck and it will be trying to drive in a straight line and when you try to turn there is a lot of stress put on the wheels... on dry pavement your truck will buck and jump... on snow it will not be as noticeable

One difference from this is GM trucks with Auto4wd, where there is a differential between the front and back so that the truck can steer without problems... and there are cluthes so that when a wheel starts to spin some drive power will be transferred to the front so that it 'automatically' becomes a 4wd (with front to back differentials)... and when the spinning settles down then the truck 'automatically' returns to 2wd operation.

Another difference from this is AllWheelDrive (or FullTime4WD)... like on some Denalis and other vehicles... where there is always power to the front and rear wheels... obviously there is a differential front to back and left to right to permit the truck to operate and steer on any driving surface.

jbeall · December 22, 2009

One difference from this is GM trucks with Auto4wd, where there is a differential between the front and back so that the truck can steer without problems... and there are cluthes so that when a wheel starts to spin some drive power will be transferred to the front so that it 'automatically' becomes a 4wd (with front to back differentials)... and when the spinning settles down then the truck 'automatically' returns to 2wd operation.

Another difference from this is AllWheelDrive (or FullTime4WD)... like on some Denalis and other vehicles... where there is always power to the front and rear wheels... obviously there is a differential front to back and left to right to permit the truck to operate and steer on any driving surface.

My vehicle has an Auto 4WD setting. Does that mean it has a center differential? Is the center differential locked when I have it in 4 Hi, but working when I have it in Auto 4WD?

Matt_ · December 22, 2009

no you dont have a central diff.

auto mode basically just engages the clutch packs when wheel slip occurs, the same clutch that is used to engage 4wd

alwcurlz · December 23, 2009

Hi All,

I've got a 2003 GMC Yukon XL. I use the 4 hi setting when I'm in icy or snowy conditions and the pavement hasn't been cleared, or I'm on gravel, etc.

I've read the manual's description of what 4 lo is for ("delivers maximum torque to all four wheels"), but I'm still not clear on what it actually does. It is just a gear reduction that lowers the ration of engine revolutions to wheel revolutions, thus giving slower speeds with higher torque? Or is it more than that, e.g., is it locking the differentials to prevent wheel slippage? In which case you would only not want to use it on pavement, only in snow, ice, mud, or other conditions where the wheels can slip as you're going around a turn.

I've been searching on this and have found some general info on what 4 hi vs 4 lo can mean, but nothing specific to my vehicle--and it seems that the meaning can vary from vehicle to vehicle.

Thanks for any feedback!

-Josh

4 Lo is a feature not designed for everyday use. It only increases torque, essentially doubling the turn ratios. Your top speed will be about 40MPH. I've driven with 4 Lo on all surfaces without any issues other than very very aggressive pulling and grabbing while turning. The 4 Lo feature is something you would use if trying to pull something much heaver that is stuck or yourself being stuck where a slower turning tire achieves better results for traction's sake. It's easier on the transmission and drive train in general and provides more temporary pulling power. It's not something you'll use very often if at all.

Some people think that it locks the differentials. Does 4 lo lock the diffs on some other vehicles and that's the reason for the confusion, or what? Or is there an option on some GM trucks/transmissions that it might lock the diffs in 4 lo?

4 Lo does not lock anything. It is an option/feature in the transfer case that affects the rest of the drive line.

My vehicle has an Auto 4WD setting. Does that mean it has a center differential? Is the center differential locked when I have it in 4 Hi, but working when I have it in Auto 4WD?

I'll try to keep this simple.....

Simply put, your Auto 4WD is a feature which turns your 4WD on for you. It simply engages clutch packs in the transfer case when slippage/traction loss is detected via wheel sensors. Auto 4WD just makes your 4WD function more user friendly for those have varying road conditions constantly, going from bare pavement to packed snow, ice, or loose dirt/gravel. When normal 4WD is chosen, there is a lot of grabbing when making turns so the auto feature is nice.

You have only two differentials. In the front, and in the back. The front differential never locks, as would be thought in a positive traction function. However, your rear differential may have a "locker" style feature. This means the differential is an "open" differential which functions like all other typical rear differentials, that is in this case till one wheel spins and the mechanism engages the lock making it a positive traction unit.

Some people are looking way too far into this and coming up with some really confusing and odd ideas.

gmtech4 · December 24, 2009

ya think ? auto shop is not offered everywhere I guess.

every kid should take auto shop for 2 yrears

i should start a new thread about that question

alwcurlz · December 24, 2009

ya think ? auto shop is not offered everywhere I guess.

every kid should take auto shop for 2 yrears

i should start a new thread about that question