New member here with a 99 Chevy Blazer. I was in 2wd travelling Xmas day and all of a sudden on the highway i hear a bad noise whining noise that got louder and I immediately started slowing down and stopping. As I slowed down it sounded like it might be dry ring and pinion gears grinding in the front differential. I had the vehicle and my wife towed to a hotel for the night. Tow truck driver said he thought it may be the actuator not disengaging all the way. He drove it around the parking lot and put it in and out of 4wd for a while and it stopped. I didn't want to drive anymore without getting it fixed. I had it towed to a service station and they said the actuator was not engaging all the way but also it was wanting to engage in 4wd by it'self even when in 2wd. Since it was the day after Xmas and the dealers in the town were closed that Saturday, they said they did not go into the differential in case they couldn't get parts and have me stranded all weekend. They pulled the vaccumm line under the battery and plugged it. They said it should be ok to get home the 240 miles I had to go but I would have no 4wd. It drove home just fine but now I need to find out what is wrong. I have never been into the newer 4wd systems so I don't know what is there and what could be wrong. Last 4wd I worked on was a 76 blazer. Nothing electrical or vaccumm about that one. The garage suspected a gear in the differential was not lined up so it would not completely engage, but I don't know how it would try to engage it'self in the first place on the highway. Any help would be greatly appreciated. Thanks

 

I would start by reading this thread here may help you understand how works

http://www.zr2usa.com/mboard/index.php?showtopic=21989

 

That thread might not fully apply to his truck. Depends on what transfer case he has. And that is besides the point... There are plenty of tech articles related to the 4wd system here on this site! Just check out the Tech Article (DIY) section.

Do you have the 4-button or 3-button switch in the dash?

 

It is the 3 button.

 

Your problem could be a frozen front diff actuator cable. This cable should move 1/2" to 3/4" for proper engagement of the front diff. You can move this your self by first removing the battery and then the battery tray. The cable has a return spring on the opposite end so there will be some resistance, but movement should be smooth.

If you have a hand vacuum pump, you can test the actuator at this time as well. Pull a vacuum on the actuator. It should pull the cable the required 1/2" to 3/4" and hold it with around 15psi-vac. If the actuator cannot pull the cable and/or the vacuum decreases over time, the actuator is bad.

With the truck in 4wd, you can put a vacuum gauge on the vacuum line at the actuator. With the engine running, the gauge should read almost the same as the gauge reading with the vacuum gauge connected straight to the engine intake manifold.

 

I forgot to ask does your light for 4hi come on or does the front axle just try to engage?

 

The light for 4hi did NOT come on when it started making the noise and tried to engage by it'self.
The mechanic who disconnected the actuator said he did try to engage it manually and it would not stay engaged, so I assume the cable is not the issue although I plan on taking it off and making sure it operates smoothly.

 

I would put money this is what you need this
http://shop.advanceautoparts.com/web...0008_725232903___

My moms was going down the interstate her front axle engaged and it sounded like both wheel bearing and cvs went out at the same time. I put it in 4wd drive and it stopped. I jacked up the front end and bearing and cvs were good. I noticed that the front end was locked in (i turned one tire the other moved) so with it running i reached under the battery box and i could fell the acuator cable was pulled even i 2wd I unhooked the vacume line by the brake booster and it stopped. So i pulled the vacume switch on top of the transfercase it was stuck open. Put that switch in it and it worked just fine.

Her encoder motor went out like two days before this happened i fixed that the same day.

hook every thing back up if the noise goes away in 4wd i would bet its just that switch

 

If you need to dig deeper into the engagement mechanism inside the front axle, run a search for "collar" with "swartlkk" as the author like so:
https://blazerforum.com/forum/search...k&query=collar