Terrible thumping noise when in 4wheel drive

[RoadrunninRN]

Help-please!! I have a 91 Explorer :exp: that I just adore--but it's not liking me much these days!! 2 days ago--my husband was trying to drive up a moderate incline on our farm and put my truck into 4H. It went in OK, but as he drove there was a fairly steady run, then a THUMP, then a steady run, then another THUMP, etc, etc........ The thumping seems to be at a regular rhythm. This happened last year-- and my husband thought the hubs were worn, so he reversed them this fall when I bought new tires. But....lo and behold ....the THUMPing is BACK!! He thinks it may be something in the transfer case....because we smelled something that seemed to be like burning rubber when he finally got to the top of the hill. Any ideas/suggestions on what to check? Any help would be greatly appreciated!!

 

[Blee1099]

Do you have manual or auto hubs? Also how is your transfer case fluid. You should hear some noise from the front end when its in 4wd. Its very common.. Especially when you are turning since the inside wheels & outside wheels are spinning at different speeds.

 

[RoadrunninRN]

Automatic Hubs....it's not the regular humming I sometimes hear when going down the road in 4H. Not sure about the fluid inthe transfer case--as I am definitely not a mechanic. Hubby is pretty darn smart on these things...so I reckon that is part of what he plans to check this weekend. I do know that last winter I had a hard time sometimes getting it out of 4H. Even after backing up about 20-30 feet and then driving forward, at times it would still hum like in 4H, then all of a sudden a HUGE THUMP, then it was out of 4H. Hubby very rarely drives my Explorer because he has a big 2000 F250 Super Duty, so he never got the opportunity to experience my "Thumps" last year!! This was his first shot at it, and this was the first time I ever noticed the burning rubber smell.

 

[snowburns]

Automatic Hubs....it's not the regular humming I sometimes hear when going down the road in 4H.

Kinda off topic, but what is that noise?

 

[RoadrunninRN]

Normally--if I have Explorer in 4H when driving on highway in winter snow...it hums. The noise I am having probs with is this loud THUMP regularly ( I described in my original post).

 

[MrShorty]

The regular hum she hears in 4H is the sound of the front drivetrain (axleshafts, driveshaft) spinning, which don't normally spin in 2WD with the hubs unlocked.

I once had an occasional thump (more of a bang) when I was in 4H on my BII. Cause was mismatched tires. Oh, they were all 235/75/15's, but different manufacturer for the front two and the back two. Anyway, there was enough difference in tire size that it caused enough binding that it needed an occasional "bang" of some sort to relieve the tension. It also had difficulty going from 4H to 2H. Does yours have difficulty going into 2H? I've even heard of a case or two where the truck had the same tires front and rear, but one pair was much more worn than the other which caused the same kind of thing.

In short, one possibility that I see is excessive drivetrain windup. Tires are one possible cause, there are others, but might want to look into it.

 

[RoadrunninRN]

Well, if I was just going on what went on last winter-- I would lean toward the tires, but I just put new Michelin LTX AT's all the way around. So I don't think it would be the tires putting it in a bind. Obviously I am no mechanic...so I am just speculating here--is there something in the transfer case that may not be catching all the way? Or is jumpin out of gear? It is bizarre because you go for 5-10 secs--OK, then THUMP(loud-jars the truck), then go 5-10secs OK, then THUMP again, etc,etc. ??

 

[MrShorty]

Check the tire pressures, if you haven't already. Having one tire low on pressure can create enough difference to cause drivetrain binding.

Beyond that, it sounds to me like an auto hub may be going bad and slipping.

 

[RoadrunninRN]

Are the auto hubs reversible? Hubby switched them around when tires were changed, really didn't see anything obviously wrong with them at that time, and this is the same THUMP I kept getting last winter. This is SO frustrating!! Do you know if the transfer case in 89 Bronco II is the same as 91 Explorer? Or the auto hubs? We have an old 89 Bronco II that we have used for parts, wondering if I will get away with it this time......

 

[Brock94]

The hubs are reversible so that's not a problem. From what you described about them not coming unlocked after backing up 20-30 feet and then popping out suddenly, it sounds like they may be jammed up with grease and dirt. This would also explain the rythmic clunking when going into 4wd to get up the hill-- they may be trying to engage, but can't because they're full of dirt and grease.

There are a lot of threads on here about cleaning autohubs, so you might want to do a search. It also seems to be pretty common for mechanics or DIYers to mistakenly "pack" the hubs with grease when they pack the bearings-- that will cause them to jam up.

First thing I'd try if I were you is to clean out the hubs.

 

[RoadrunninRN]

Thanks a bunch!! I will pass this info on to hubby.....maybe that will be the answer, with no big expense right here before Christmas!!