I'm new to this forum, but I've been on Wrangler Forum a lot this past year. This is a Blazer I will be working on a lot, but do not own. I'm an entry auto tech, and have the tools to do anything, so I do side work in my own garage for friends and relatives.

This Blazer has many other problems that won't cost too much, but first I'd like to figure out this noise, to tell the owner if this old girl (200k miles, '97) is worth new tires, tie rods, shocks, and possibly front half shafts and diff seals, and front/rear bearings.

The noise sounds like you're driving over rumble strips, but it's not constant. The noise speeds up with vehicle speed. Does it starting at 5 MPH, up to highway speeds. happens regardless of brake pedal and skinny pedal position.

My first thought was the front hub bearings, but then I took a tight turn and did that "hop" like a part-time 4WD system does when it's engaged. 2H light is on, and I kept going between 2H, 4H, and 4L. 2H and 4H feel the same, 4L has the obvious gear ratio change (feels like 4:1).

Next step is to take it into the shop, balance the bald-*** tires to see if it helps, and see if the front axle is engaged. Plus check the tie rods more. The wheel has left-to-right play that appears to be tie rods, maybe bearings.

With the 4WD I know it's a vacuum system and there is a diaphragm under the battery. Can anybody tell me HOW MUCH vacuum it should have, and WHEN? I assume it'll get whatever vacuum the engine puts out, but I don't have the factory service manual. Also, is this a part time or full time system, and if it's part time, what will driving in 4H on the road do to it?

 

You need to understand how the 4WD system works on the 2nd gen Blazers. Front axle engagement is the only thing that is vacuum actuated. Read here. Front axle engagement for both transfer cases is covered under the second link.


3-button controls:
http://www.rsgear.com/articles/1998_03.pdf


4-button controls (has "auto" button)
https://blazerforum.com/forum/tech-articles-diy-29/nv236-wiring-diagnostic-information-41198/

 

Hell yes, exactly what I'm looking for! The owner probably won't and I'm not making enough money off of this for it to be worth buying one... Thanks a ton. I'll look through these diagrams.

 

From your description, it sounds like you have a 3 button 4wd switch on your dash board. If so, the likely solution to your problem is a stuck transfer case vacuum switch.

It's located on top of the transfer case and will have 3 vacuum lines connected to the top of it. The switch will stick open allowing constant vacuum to be applied to the front axle, keeping it engaged.

The diaphragm under the battery should only have vacuum applied when in 4hi or 4lo.

 

It has the four button, with the "auto" option, which I believe to be the NV236 now. This would still have the diagram, correct?

I've traced a vacuum switch issue on my Wrangler but there is a vacuum motor, this one just seems to pull a cable. Same concept, but why would there be three lines? One for vacuum "supply", one to be sent to the diaphragm (in 4x4), and one to go where?

Based on the symptoms it seems like the front axle is simply stuck engaged, possibly due to a corroded shift cable. The rumble noise is appearing to be a worn prop shaft, I'll hopefully be underneath this thing soon to check out the driveshaft.

The below video is what I'll be looking for. For all intents and purposes I'm not a parts changer so I'll actually test it before throwing parts at it.

 

NV236 has an electronic vacuum solenoid above the distributor on the firewall. Has no vacuum switch present on the transfer case.


Again, the vacuum servo under the battery only locks/unlocks the front axle. Transfer case operation 2WD vs 4WD vs Auto is completely electronic through the TCCM and the encoder motor that changes the position of a shaft in he transfer case itself.

If you want to check the front axle engagement on a NV236, just jack up the passenger front wheel and leave the other front wheel on the ground. Start the engine and select 2WD. If the 2WD light on the control stops flashing, the right front wheel should turn easily and the transfer case should be in 2WD. Now select 4WD. if the 4WD light on the control stops flashing, the wheel should now be very difficult or impossible to turn and the transfer case will be moved into 4WD by the encoder motor. If you have trouble getting the vacuum servo to disengage the front axle when 2WD is selected, probably the lines are switched on the vacuum solenoid and vacuum is being trapped (only one side of the solenoid is vented). This happened to me. If you want to test for a corroded cable and mechanical parts for front axle engagement, just pull on the cable with your hand when the engine is shut off and spin the tire to see if the RF wheel locks in.

You should always keep in mind that a NV236 keeps a significant connection between front and rear through clutch plate preload in the transfer case. You will never get the transfer case to disconnect enough internally so you can spin the front driveshaft by hand.

 

Thanks for the help, it gave me good info...

Today I worked on it, replacing the tie rods and all four shocks. It rides and handles much better but the rumble is still there, so I took out the driveshaft. Nothing changed.

I was, in fact, able to turn one front tire while the other was on the ground with the engine running. So it's not stuck in four wheel drive.

I'm leaning towards bearings, axle shafts, or tires. All are bad except for one bearing that is new. Next on the list is new tires, then we'll go ahead with a bearing and the cv shafts. It sounds like part changing, but all of them are bad and it's hard to diagnose which one is so bad that it causes the issue.

I was also informed that this is a 99, not a 97. And the tires on there currently were bought from a less-reputable used tire shop. The new tires will be put on at the more reputable tire shop that I work at.

 

UPDATE: A wheel bearing (that I found was bad and actually had planned to do the weekend the bearing failed) gave out, and it ended up getting towed to my house to get one bearing and two axle shafts, as well as new brakes which were destroyed from the bad bearing. I noticed bad ball joints, which I ended up doing a few weeks later.

So, the fix seemed to be a bearing. Must have had a nice pit in the race. I would have looked but the bearing had nothing to it left.