I have a 98 Blazer LT 4x4. Tonight I was driving home (about 55mph) and everything was fine until I started to slow and noticed a rubbing noise. Kind of a hard noise to explain but I'll tell you that it gets really loud the slower I go. Its there until I come to a complete stop. Then when I start driving again, it starts and gets louder until about 55, then starts to fade away. Before I parked it, I drove up the street and went into 4 hi and it seemed to go away. Back in 2 and it came back? Any ideas what this could be? Its also very difficult to tell if its coming from the front or rear. I would have to assume its coming from the front because it seems pretty loud.

 

Check the vacuum switch on the transfer case:



If the spring loaded ball gets stuck, the switch will leak vacuum causing the front axle actuator to partially engage the clutch on the right front axle. Dead give away is when it starts making the noise, select 4WD hi, if the noise goes away, that's the problem. It's located on the transfer case inboard of the front driveshaft yoke, right behind the encoder motor. Dorman makes the part and it's under $20 and 15 minutes to replace it. Dorman part number 600-500.

 

Thanks a lot for the reply. Is there a way to check it? Just take it out and see if the ball is sticking? Or do I just replace it right away and see if it resolves the issue.

 

The easiest test is as I explained above. The best test is remove it and check the ball for free operation. There are 3 vacuum lines attached to the switch, if transmission fluid runs out of them when you disconnect them, the switch is definitely bad, and there may be fluid in the vacuum system. If that's the case, they need to be cleaned out.

 

this looks like something everyone should look into esp with slightly off vac issues that are possibly attributing to semi aggressive idle...

 

This vacuum leak will not affect engine performance. The vacuum that leaks through the switch is contained in the vacuum actuator under the battery. That's what causes the actuator to partially engage the axle clutch. Good thought though

 

well.. I was on my way to pick up the switch today (in 4hi because I didnt want that noise) and when I put it into 2 to see if it was still doing it, it was not. I kept driving it, and nothing.. noise is completely gone. Should I still do the switch. Its actually $75.00 where im from so I'd be pretty happy not having to do it. But if the problem might happen again, I should really get it changed. Any thoughts? Is this something that will occur again. Can it happen again if I just leave it in 2 hi and not switch it at all?

 

$75 for that?? you must be from Canada.

 

Haha.. Yup! Brutal isnt it.

 

If in fact the switch is what's causing the problem, (we don't know for certain yet) you can disconnect the vacuum line from the actuator under the battery and stick a bolt in the end of the vacuum line. That will prevent the actuator from engaging the clutch. Drive around with it like that for a few days, if the noise doesn't return, you can pretty much rest assured the switch is the problem and replace it. Check around online, make sure they will ship to your address. Dorman OE Solutions 600-500 - Vacuum Control Switch | O'Reilly Auto Parts