Hi fellas. I just picked up a 94 F150 AT 302 4wd with 80k miles on it. I am noticing a loud humming noise from the front end while driving any speed over 10mph. When I let off the brake from a stop, I can't really hear it until I reach around 10mph. It is pretty loud all the time, even at speeds of 25-30, and I don't notice it getting much louder at highway speeds. However, it is very apparent and I need to pinpoint the cause. It has fairly new Goodyear Wranglers but it does not sound like tire noise. The noise is there whether I am on or off the accelerator. I had a bad wheel bearing on my 97 Explorer, but that had a higher pitch hum to it...this sounds much lower. Tonight I am going to do the steering test for diagnosing a bad wheel bearing. I have done a little research on here and there have been quite a few possibilities mentioned. I am pretty good with mechanics, so I will be doing all the work myself. As the truck has automatic locking hubs, it seems one of the possible culprits could be the hubs not disengaging. I assume this would cause friction and make some sort of abnormal noise or create a certain amount of drag. Before I say any more, I'd like to hear some feedback from you guys. Thanks in advance

Jamie

 

If everything is perfect in your front driveline a stuck hub shouldn't make much, if any, noise. That said, a stuck hub often makes noise!

Jack up each front tire and spin it. Look in through the steering knuckel at the U-joint. It should not be turning. If it is, try spinning the tire the opposite direction (reversing is what unlocks the hubs). If either (or both) of your hubs is stuck, then the front axle and driveshaft are always spinning.

But they shouldn't make noise when they spin, so if that is the first problem, next you need to figure out what's wrong that makes the noise. Best bets for that are U-joints in the axle (there are three of them) or in the front driveshaft. It could also be an unbalanced drive shaft, bad bearings in the spindle (where the outer axle shaft spins), something in the front diff, or something in the t-case.

The lower pitch might indicate driveshaft, since it spins slower than the axle.

 

Thanks for the feedback. I will be sure to check for spinning of the axle/driveshaft with the front tires. I will also listen for noise while spinning the tires.

I wanted to add another note. I know these trucks aren't known for fuel economy, but the mpg I am getting during these first 200 miles is super low. That's another reason I was wondering if the hubs were staying locked, thinking the extra drag would cause a significant loss of fuel economy. Any thoughts?

 

There are at least a dozen moving components in the front drive line, any or all of them are suspect at this point. It's a process of elimination. Do some basic troubleshooting before asking for "what do you think it is?". Anything we suggest is pure speculation at this point and may cause your wallet to become very thin.

Stick to the basics...jack up a wheel, spin the tire, listen. No noise? go to the other side, repeat.

 

Ok fellas, here is what I checked so far. With the tranny in park and parking brake on, I jacked up one wheel at a time. Both spun freely with no movement of the axles/u-joints. The right has a little growl to it but the left sounds fine. I was thinking maybe wheel bearings, but I took it for a drive and did the steering wheel jerk test and the noise did not go away or pause at all. So I'm not sure if it's wheel bearings or not at this point. Either way, since I know my front axles aren't turning, I know my front drivetrain is not the culprit right? It is my understanding that while in 2wd, the front driveshaft, axles, differential do not move at all...is this correct?

 

You are correct, the front driveline doesn't turn when in 2WD. You've shown that your hubs are unlocking correctly. It still might be worth making sure the t.case isn't stuck in 4WD, but that's a lot less common than a hub sticking.

 

How exactly can I confirm that the transfer case isn't stuck in 4wd?

 

Just turn the front driveshaft while in 2WD with the rear tires on the ground. It should turn freely since it's disconnected from the rear driveline.

With auto hubs you'll have to jack both front wheels up to do this because as soon as you start turning the front d/s the hubs will engage.

 

Thanks man. Should I be able to spin the front driveshaft by hand?

 

You got it! With the transfer case in 2H the front output shaft will just freewheel, so the entire drivetrain forward of that should be free to turn by hand. If your auto hubs are working they will both engage (maybe not exactly at the same time) and try to turn the front wheels as well. That's why I said to jack up both front wheels, so you'll still be able to turn everything.

If the t-case is in 4H/4L then the front drivetrain will be locked to the rear drivetrain and with the rear tires on the ground you won't be turning much of anything.

 

Pull that front right hub apart and inspect the wheel bearings. You said it growls when you turn it by hand so start there.

 

Ok here is what I found tonight. I raised both front wheels and spun the front driveshaft freely until the hubs locked. The only problem is only the right tire started to spin. I saw the u-joint on the left side turning, but once it stopped and tried to spin the left tire, nothing happened. Does this mean my left hub is not engaging? Also, i spun the driveshaft both ways until it stopped. It was hard, but I broke it loose to turn it the other way. Is it normal to hit this "stopping point" when spinning the front driveshaft?

 

That all sounds pretty normal. Whichever brake is dragging least will allow that tire to spin easier than the other, so the open differential will do what it does and spin the tire that spins easiest. If you wanted to be sure you could block the right tire so it can't spin as easily (or leave it on the ground) and then only the left tire would spin.

Whenever you reverse directions with auto hubs they disengage and then reengage (unless the driveshaft isn't turning, then they disengage and stay that way). So when you turn the driveshaft back and forth you are feeling that. Keep how that works in mind if you are ever temptyed to "rock" the truck in 4WD to get unstuck. Everytime you change direction the hubs will do that. And if you get the back tires spinning before the fronts reengage you'll hammer the hubs. That's the "best" way to break auto hubs (and the best reason to replace them with manual hubs, if you plan to do that).

 

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