Why not 4wd on highways? Hums in 2wd not 4wd

[chcknugget]

Hello,
I've read that 4wd should not be used on dry pavement, why is this? Also, why can't you use it at highway speeds?

Anyways, recently I started my car up and right away something was rubbing like a tire against the frame but it was the front diff grinding at slow speeds. It stopped in 4wd. the nasty grinding is gone, but now it seems there is always a little grinding in 2wd until i push the buttons back and forth enough to totally switch out. Also, now my jimmy turns perfect in 4wd and doesn't bind nearly as much, but it might not completely be in 4wd.

This leads me to believe the front dif is partially engaged. What is the fix for this? Check the vacuum lines somewhere?

[swalt]

It is typically the vacuum switch on top of the transfer case that causes you the problem. Since you now have grinding, your front axle is damaged and the front diff must be dismantled as the gear is shearing.

[swalt]

Just did a quick search and here is exactly what you you will need to replace. (Hopefully no metal chunks got into your axle...unfortunately mine did) I found mine only did it in very cold weather which led me to believe the vacuum switch was freezing up.

The part that caused this is the transfer case switch. It didn't engage the front front halfshaft all the way and caused the shift sleeve and gear to get damaged. To repair this you need a ....New transfer case switch, new shift sleeve and the gear that the sleeve moves onto. This is a common thing on these trucks.

[Paddle_Grl]

I had the same problem with my 2000 blazer...except mine would happen when I was in cruize control on the highway..I would go to slow down and it would make the most horrible grinding sound..I would have to hit the 4wd then the 2wd and it would stop..sometimes I would actualyhave to stop and fix it....unfortunately for me...no one believed me...I said it sounded like it was "slipping" and a few months later..my transmission blew.....with the new transmission...I have never had the problem again...
Karin

[swalt]

Originally Posted by Paddle_Grl

I had the same problem with my 2000 blazer...except mine would happen when I was in cruize control on the highway..I would go to slow down and it would make the most horrible grinding sound..I would have to hit the 4wd then the 2wd and it would stop..sometimes I would actualyhave to stop and fix it....unfortunately for me...no one believed me...I said it sounded like it was "slipping" and a few months later..my transmission blew.....with the new transmission...I have never had the problem again...
Karin

I was able to make mine disapper as well by doing the same thing. Once the gear wore down enough though, it was futile. I still have the gear that was removed. I will try and post pic of it shortly.

[BlazerBoyLT98]

Originally Posted by swalt

I was able to make mine disapper as well by doing the same thing. Once the gear wore down enough though, it was futile. I still have the gear that was removed. I will try and post pic of it shortly.

That will be a sight to see!

[swalt]

Originally Posted by swalt

Just did a quick search and here is exactly what you you will need to replace. (Hopefully no metal chunks got into your axle...unfortunately mine did) I found mine only did it in very cold weather which led me to believe the vacuum switch was freezing up.

The part that caused this is the transfer case switch. It didn't engage the front front halfshaft all the way and caused the shift sleeve and gear to get damaged. To repair this you need a ....New transfer case switch, new shift sleeve and the gear that the sleeve moves onto. This is a common thing on these trucks.

As promised, here is the picture of the gear. If I had known the vacuum switch was the culprit, I would have replaced it the first sign of any problem. I fought with my aftermarket warranty company for 3 weeks on this repair. Final bill was $2200 parts and labour.

[Paddle_Grl]

wow!!! I can only imagine what mine must have looked like....

[chcknugget]

Hey guys, I thought I'd report back with an update:

The problem was a vacuum line. The line that goes from the transfer case to the front of the car (maybe the vacuum actuator) was rubbing against the front driveshaft and a hole was rubbed into the rubber. I cut out the section with the hole and inserted a coupler and everything works perfect again as it should. O yeah, I zip tied the vacuum line AWAY from the driveshaft too.