First off, you guys rock - what a comprehensive, helpful, jerk-free forum. I'm new to the Ford family, just picked up a 2004 Navigator at the beginning of the month, and this forum is by far the best place to start my bug hunt. I've already replaced my blend actuator thanks 100% to posts here - $32 and an hour of scraping forearms installing it, vs $250+ at a dealer.

The Navi is in fantastic shape, my mechanic says the engine (131k) runs like new, and the interior and exterior are near flawless. But, my next bug: a hum/harmonic noise that starts like clockwork at highway speeds - starts at 60 through around 70, and it's a low pulsating on/off hum. I thought it might be tire wear, maybe a balancing issue, but no vibrations, just the hum, loud enough to notice, but not screaming at me. Running through these forums makes me think it may be bearing or diff related, so I had the mechanic test it out on a lift at speed, and lo and behold he tells me it's essentially stuck in 4WD. With the switch on either 2WD or A4WD, all four wheels are spinning in gear, and manual rotation. Using the switch I can hear it click into 4H and the light goes on (haven't tried 4L - honestly with this bug I'm afraid it might get stuck there, then I'm up the creek), but without a dash light for either 2WD or A4WD, I can't really tell if it's doing anything.

So after all that, I'm wondering if anyone has advice, solutions, or at least a scientific method of squashing this bug, and getting me out of 4WD. I'm thinking maybe that harmonic is just what the truck would normally do at highway speed in 4WD, basically telling me "hey, I should be in 2WD, otherwise you shouldn't be going this fast on dry pavement", and the stuck-in-4WD bug would eliminate the harmonic.

Much appreciated!

-Steve

 

Remove fuse #115, go for a drive, and see if the problem goes away.

 

Thanks - had to wait for the ridiculous SPRING SNOW to clear to get an idea of sound. I pulled the fuse, no change, same harmonic. Though if it's stuck in AWD, cutting the power to the transfer case etc would just leave it where it was, right? Still stuck in AWD? Any ideas how to easily get it out of there?

 

Pulling fuse #115 also kills power to the front clutch on the transfer case. Pulling it is a way of determining if the t-case is electrically or mechanically locked. Sounds like yours is mechanically locked. So onto the shift motor mounted on the back of the t-case. If you remove the 3 bolts and drop the shift motor you will find a cam behind it. Turn the cam to take the t-case out of 4WD. If that does the trick then you'll need a new shift motor.

 

Awesome, thanks - will work on this ASAP!

 

If he tested it on the stand and the wheels are turning with the engine, then both the transfer case and wheel interlocks are engaging, so follow step above to determine the transfer case "stuck" scenario. If they are testing it by putting it on the lift and spinning the wheels by hand with the vehicle off, the half shafts will turn with the wheels since the interlocks rely on engine vacuum to disengage. If you start the vehicle and manually spin the wheels and they turn with the half shafts, then there is a interlock issue but not transfer case (assuming car is in park). If you can't manually spin the wheels with the engine running and in park, then there is both an interlock and transfer case engagement.

 

Update: Turns out the person who owned the Navi before me was a little confused on the vacuum system when he changed out the air suspension. Several hoses were hooked up incorrectly, several simply not hooked up. Mechanic found that the AWD was locked on because the vacuum wasn't releasing properly. Couple of quick hose changes and voila, it's out of AWD. However the hum is still there, so it looks like almost definitely a bearing on front left wheel. Will keep you posted...anyone have any idea what a new bearing is gonna run me? Oy...