Auto 4wd Vs 2wd

[1999 Tahoe 4x4]

I have noticed that when I press the Auto 4wd button there is a slight difference in road noise compared to 2wd even in DRY HIGHWAY DRIVING conditions. I always thought that driving on the highway with Auto 4wd set would be the same than driving in 2wd except that the truck would use 4x4 as needed. But the road noise that I'm experiencing suggests different.

 

So AT REGULAR, DRY PAVEMENT HIGHWAY DRIVING CONDITONS what happens if you use Auto 4wd vs. 2wd. Does the truck perhaps "behave' in Auto 4wd mode like some other all wheel drive models?

[sno]

Seems to me that for any kind of "automatic" 4WD system to engage when needed, the front hubs (or whatever these things have now) would have to be engaged. Thus, the front axles will be turning all the time- just not getting power from the transfer case until needed. One could definitely hear and maybe even 'feel' this extra frictional drag from the rotating parts. Heck, it might even cause some axle binding/wheel hop depending on how the system is designed.

[VMax2007]

In 2wd, the transfer case disengages the front driveshaft. The front axles always turn but are engaged/disengaged with the drive shaft via an electric clutch in the front diff. So, in Auto 4wd, the transfer case is engaged and the front drive shaft is always turning. Autotrac uses input from the ABS system to detect rear wheel slippage and engages the clutch in the front diff as needed, thus providing power to the front axles. So, in Auto 4wd, the noise you are hearing is the front driveshaft turning. Make sense?

[1999 Tahoe 4x4]

In 2wd, the transfer case disengages the front driveshaft. The front axles always turn but are engaged/disengaged with the drive shaft via an electric clutch in the front diff. So, in Auto 4wd, the transfer case is engaged and the front drive shaft is always turning. Autotrac uses input from the ABS system to detect rear wheel slippage and engages the clutch in the front diff as needed, thus providing power to the front axles. So, in Auto 4wd, the noise you are hearing is the front driveshaft turning. Make sense?

Thank you, and yes, it kind of makes sense.

 

So basically Auto4wd is ALMOST the same as 4wd except the way the electronic clutch is engaged in the front differential:

 

In Auto4wd the transfer case is engaged + the electronically operated clutch in the front diff is cycled on/off automatically as needed,

while in 4wd the transfer case is engaged + the front diff clutch is always on.

And in 2dw of course neither transfer case not front diff electric clutch is engaged.

 

Did I get that right?

 

BTW: on a second note, today I threw it in 4wd on the dry highway and going straight it was a bit louder than usual, I think. I checked under the truck, no signs of a leak, so I hope this noise in 4wd will remain considered a normal operating condition.

[redvett]

Read your owners manual and it says do not drive on dry hard surfaces in 4wd only in Auto 4wd. Damage will result.

[1999 Tahoe 4x4]

Read your owners manual and it says do not drive on dry hard surfaces in 4wd only in Auto 4wd. Damage will result.

Thanks for the warning, I understand that. That is why I said in my post going straight. Otherwise never drive around in 4wd on dry pavement. However I put it in 4wd every once in a while on a straight strech of road thinking it is a good idea to get the 4x parts moving and keep them lubed. That is how I have noticed that it seemed a bit louder this last time.

[sceptre]

Read your owners manual and it says do not drive on dry hard surfaces in 4wd only in Auto 4wd. Damage will result.

Thanks for the warning, I understand that. That is why I said in my post going straight. Otherwise never drive around in 4wd on dry pavement. However I put it in 4wd every once in a while on a straight strech of road thinking it is a good idea to get the 4x parts moving and keep them lubed. That is how I have noticed that it seemed a bit louder this last time.

 

 

 

my 06 is a little noisy too.....wonder if its normal??