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AWD transfer case issues/noise


April 24, 2015
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City, State
Bitterroots of Montana
Year, Model & Trim Level
1985 Porsche 944
I have a 98 AWD with lots of miles and a humming noise I am trying to locate.
I suspect the transfer case viscus coupling is locking up because I get the classic binding when turning, and a very loud grinding/humming sound at any speed over 10mph.
Last night, I removed the front drive shaft and took it for a spin, now I have just humming sound, which I think could be the tires. The grinding sound/feel seems to be gone.
Would a locked TC viscous coupling still make noise with the front drive shaft removed?

Here's some background:
I've rotated the tires, but three of them are worn on the inside.
Replaced both front wheel bearings, the drivers side bearings were shot, the passenger side was OK.
Checked the half shafts, and they are tight, and look newish.
Fluids in the front diff and TC looked good.
Pulled the front shaft and it is nice and tight. It is a locally rebuilt double cardan style. Driving without the front shaft, the truck no longer binds when turning. Also, when turning the tires or spinning the input shaft to the front diff with the wheels off the ground, the diff feels smooth, and I don't hear or feel anything weird.

So, now I would like to determine if the TC would still make noise without the front shaft, and I am just hearing the tires, or if the TC could still be the source of the humming sound. Anyone have experience with this?


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That sounds like other typical reports of noises etc, in their AWD Explorers.

If you do have good clean fluid in the transfer case and front diff, you are well ahead of the curve. Most people start wondering and replacing parts before they check/change fluids. Well done.

Are you sure you have a double-cardon front drive shaft, on a 98 AWD? That was originally a CV shaft, Ford stopped using the DC shaft in 96 or 97. The CV shaft is smoother, but it sounds like you've got more going on.

If the tires are not really the exact same diameter, it might be best to replace those before anything bigger like the transfer case.

If you didn't change the TC fluid, I'd do it again with the best fluid you can get. How does that DC front shaft fit, does it have some range of movement fore/aft when installed? With the DC shaft, I'd inspect the u-joints again very carefully, with it off. If you get any hint of roughness in any bearing there, I'd change them all.

With most things replaced or checked out etc, it usually ends up being the CV drive shaft. Since you don't have one, and everything done right, it may end up being the DC shaft, or the TC.

Uneven tire wear will also cause your symptoms due to the different rotational speeds of the tires forcing the transfer case to hunt for traction. Uneven tire speed is what it is trying to overcome.

I suggest first a new set of 4 tires. All 4 tires must have the same diameter, and since this varies by manufacturer, it is best to have a matching set. While you are at the tire shop, have them stick it on the rack for an alignment check, this will identify worn suspension components. Get the tires and suspension sorted first, at this time I would leave the front drive shaft off. SInce the vehicle will want to creep on an incline, carry some sort of wheel chock device for parking on an incline.

Are you sure you have a double-cardon front drive shaft, on a 98 AWD? That was originally a CV shaft, Ford stopped using the DC shaft in 96 or 97.
Hi Don, thanks for the reply. I had read that the DC shaft had been phased out, I'm guessing the CV shaft was replaced fairly recently this unit. It does have a slip yoke at the front. All the U-Joints feel good, absolutely no play in them.

I've replaced the front upper and lower ball joints and tie rods, along with the sway bar end links. All were in poor condition, which is probably why the tires wore on the inside. I then had the alignment done.

Jon - would the TC still "hunt" with the front shaft has been removed? One rear tire has wearing on the inside, the other looks fine. I put the two most similar tires on the front.

I will try some new TC fluid. Is there a friction modifier I should use?

How much wear are on those tires, compare the back two to the front two? The AWD viscous clutch only does anything when the front tire speed differs from the back tires. Normally the two drive shaft speeds are the same, thus there is no resistance in the viscous clutch to that.

When the speeds differ, then the fluid and the clutch work hard to keep the speeds the same. That's why even when turning the wheel in a parking lot, people say the AWD vehicles seem to resist or bind a little. It just feels a little different than any non AWD vehicle. That is under normal conditions, with identical tires.

If there is any slight diameter difference between the front pair, and the rear pair, then the viscous clutch resists that difference. It tries to force the speeds to match, which creates heat and will ruin the clutch.

It sound like it may be a good time to replace the tires. You can cheat a little and mix the tires as pairs which match in total diameter, but that's risky too, and not good for handling either.

The TC needs a good Mercon V, no friction modifier. It's easy to change so people should do that more often.

Hi Don,
Three of the tires look like they have ~40% tread left and each is worn more on towards the inside. The fourth tire looks to have ~70% tread, and even wear.
I took a look at the front output shaft of the transfer case, and it is the later cone style for use with the CV joint front shaft. The build date of the car is 9/97, so, a very early 98.
Going out right now to change out the TC fluid with a Mercon V fluid.

If the TC is still noisy, I will probably go ahead with my planned 4406 swap (unless the ticky lifter doesn't clear up...).

I would not put the shaft back in with that tire difference. It sounds like that 30% diameter difference could be enough to upset the TC, not necessarily kill it, but hard on it. I'd bet that the noise and feeling of the truck will be noticeably different with that mismatched tire. It's hard to conclude that by an opinion of another's description, so it's just my opinion.

The CV shafts used to be very expensive back about ten years ago and before that. People had a hard time justifying $400 for a new shaft. Now you can buy a rebuild kit for about $80, and it takes under an hour to do it. The CV shaft is great when it's right. If you can do the BW4406 swap though, that would be a good improvement. Everyone who has done it really like it.

Quick question, would I be able to spin the front output shaft off the TC (front driveshaft removed) by hand while the transmission is in park, and the rear driveshaft is still installed and the park brake is on? Right now, I can move it a few degrees back and forth, but it kinda sticks at either end of rotation.

It shouldn't move much, the front parts are used to hold the vehicle in park. That's why without the front shaft, that output shaft will turn very slowly(no tires to stop it) as the rear rolls.

Maybe I have a similar problem... sorry to hijack.

In my case, I have a low-load "wub wub wub" on/off throttle that I thought was front diff groan. Also shows up while turning. (Newly purchased 2001 AWD Mounty 5.0, 31" tires)

Bought $30 pnematic brake bleeder from harbor freight, sucked out the sludge and put the purple stuff in. Told myfelf it helped, but the mountains disagree.

I have a small clunk as well, so u-joints are on my list along with TC fluid and rear diff fluid. Should I add anything else?

To comment on above - IMO the difference in rotational speed for 1 tire having a bit more tread is nil. If you are curious, just run 30 psi in good tire, and 40 in bald ones to compensate... that is, if you belong to the "effective tire radius is based on hub-to-ground distance" club. If not, chock your bad tires well, then jack good tire almost off ground. Proceed with smokey burnout (preferably with open diff) until satisfied.

Ok, sorry. I'll go to bed now.

So, I have been driving without the front drive shaft for a few weeks (not every day), and the noise is still there. I feel like it has to be the tires, but I am not in a position to replace them at the moment. I did rotate them, though. They are M+S tires with cupping on the inside due to bad ball joints and alignment (both fixed).
Last night, I lifted the front tires off the ground, and spun them by hand while listening for noises through my stethoscope. I checked both hubs, the ends of the front diff half shafts, and the tail shaft. I do not hear anything. No bearing noise, no clunking, nothing.
I would think if a bearing in the diff was bad enough to hear and feel while driving, then you should be able to hear it through the stethoscope. The only noise I could hear is one of the rotors dragging a bit against the pads.

Would the CV joints make a humming sound when driving straight ahead? The sound doesn't really change when I turn right or left The "feel" tight when I try to rotate them by hand, or move the shafts up and down.

Is there any fluid leaking from the front diff, any dirtiness around the pinion seal or axle seals? If so, I'd get some new fluid in there soon, they can go completely dry and kill the diff.

I did check the front diff a few months ago, and it seemed clean, but I'll check it again, maybe even change the fluid. The only real leak in the area is coming from the oil pan, and that is really bad.. What do you recommend for the front diff?

Now, shouldn't I be able to hear a bad bearing in the diff with a stethoscope when turning each wheel, especially if I can hear and feel the humming and rumbling while driving?
Is it possible that the bearings in the TC would make the same noise/vibration with the front shaft off?

I might see if I can borrow some wheels/tires to try and eliminate my tires as the source.

If the tires are different in the tread wear from one pair to the other, you should be able to rotate them and tell if the sound changes or moves.

The front gear oil is easiest to change by removing the left axle and using a cheap pump to remove most of the fluid. Dropping the whole front member to get the cover off is a lot more work. To cheat and get a lot out, you can remove just the wheel and access the fill hole to run a pump hose into. I don't think that will get as much fluid out, but once you do see what the fluid looks like, then you can better know if it needs to all come out, or just some by these methods.

You can use any very good 75-110w or close to that. I like Amsoil for it's top quality Severe Gear, plus it's cheaper than the other common synthetics available, about $12 per quart, while Ford's is near to $25 per.

I am still chasing that humming grinding noise that was still present after removing the front shaft and replacing both wheel bearings.

I put the explorer up on jackstands (still no front shaft), removed all tires, and started it up and put it into gear. Then I went around to listen for noises with my new stethoscope. Here's what I found: No odd noises from the transfer case or transmission. Lots of noise from the rear hubs, and a little bit of noise from possibly one of the rear axle shaft bearings.

Next, I pulled the front CV joints, and both are very tight and smooth. They feel new to me. I then hand turned the front diff input shaft and listen with the stethoscope, and I can hear some bearing grumbling from the right side. The left of course doesn't turn without the CV, but it feels very smooth and clean.

The rear rotors are quite rusty, and the parking brake surface is very rough from rust. The shoes were dragging, and the adjustment barrels were not working. The parking brake barely worked. With the rotors removed, and running the ex in drive, everything sounded normal, except possibly the right rear axle bearing, but only through the stethoscope.

I suppose the little bit of bearing noise in the front axle could be the culprit. It would get much louder at speed and with weight on, right? Or (and?), the rear bearing could transmit the sound forward?

I've got a whole rear brake setup on order, and I guess I will go ahead and get the front axle bearings too. If the sound is still there, then I will do the rear axle shaft bearings. Guess 370k miles wears on parts...

I now have the front diff out.
There is an excessive amount of backlash, but the wear marks look good on the crown teeth.
The spider gears feel tight.
I am pretty sure that the passenger side bearing where the CV shaft slides in is bad. I don't hear any noise in the bearings inside the axle housing.

Question, should I pull the gears and re-shim the pinion? Is it fairly easy to do? Or, at 375k miles, should I buy a used diff or just run this one as is (after I replace the bearings).
Also, I know I need a slide hammer bearing puller, do they usually have an attachment to pull the intermediary shaft out as well?

Good job to get that out and find the noise, the bearing. What is the backlash as you see it? The rear 8.8's call for .009-.012" backlash, so the front D35 should be something similar. If it's not way too loose, I'd suggest not disturbing the pinion nut or shims etc. The pinion seal is important, does it look like it's leaking much? That would be a good reason to replace that seal, which requires the nut R&R(mark it against the shaft clearly beforehand).

The intermediate shaft will pull out, it's got a cir-clip on the end which kind of engages a groove in the side gear of the diff. That shaft often comes out with the CV axle, mine did this last time.

Put great grease in it after you change the bearing, and that should make it last for a long while more.

OK, decision time.
I've inspected the front diff, and it looks like the carrier bearings and pinion bearings are fairly worn, along with the side axle bearings. The result is a pretty sloppy pinion to crown interface. The good thing, the gears all look good, and there wasn't any real metal in the gear lube when I drained it. However, between the left axle bearing and the seal, there was a bit of metal shavings built up, I'm guessing from the bearing itself.

Since I don't know how to set the carrier or pinion preload and shimming, I would have to take it to a shop, and the local guys are quoting around $500 if I supply the bearings.

I know I can get a used diff for a few hundred, but getting the correct AWD version might be a little harder to find nearby.

Should I attempt to rebuild the diff myself, or start looking for a replacement.

If I go for a junkyard item, is it possible to use a ranger or non-AWD explorer dana 35 SLA diff?

You can use any front diff(D35) Explorer from the 1997 up models. I'm not sure if the Rangers to 2009ish are exactly the same, but they do take the same D35 diff. I think the only one you have to avoid if the vacuum controlled version of 1996 V6 trucks.

For the rebuild pricing of $500, I'd agree and go hunt a lower mileage unit, lesser miles and not leaking should be best. Be picky about the pinion seal being dry, you don't want to have to replace that immediately.

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You can use any front diff(D35) Explorer from the 1997 up models. I'm not sure if the Rangers to 2009ish are exactly the same, but they do take the same D35 diff. I think the only one you have to avoid if the vacuum controlled version of 1996 V6 trucks.

Don't all the non-AWD explorers have the vacuum servo stuff on the right axle shaft? Or do they control the engagement in the TC?