I'm trying to diagnose a problem with my esof hubs on my 04 superduty. Here's what happening. When I select 4X4 at the dash the vacuum pump starts and engages the hubs. After about 4 minutes the hubs release. I still have 4X4 indicating inside. The transfer case is locked (I checked and the front shaft is locked) The hubs spin free, but WERE locked for about 3 - 4 minutes. (checked with truck in the air and front wheels off and rocking the hubs.) I have no heating or defrosting issues. Even after the hubs release, my heating, AC, defrost and vent all work correctly. I've already replaced my vacuum lines at both hubs with steel braided type.

1. Do the vacuum hubs require more suction than the ventilation system and I'm only losing some vacuum, but not enough to cause the system to be stuck in defrost?

2. If there is a leak at the hubs, wouldn't only the leaking hub disengage?

Where should I look next?

 

Check out TSB 04-18-5. I had the same problem on my '04. After removing the right hub I could tell the axle was bottomed out on the inside of the hub. Instead of replacing the hub as the tsb states, I took mine apart, cleaned and lubed it ( I think the two small metal engagement tabs needed to be synchronized). Anyway, mine has worked fine since then. Total cost- $17 for a 5/8 drillbit.
May not be your problem, but it worked for me.

 

Have you tried disconnecting the ESOF solenoid plug after switching it into 4x4?

 

Try willie's TSB first. But if they WERE working before, sounds like you have a hub problem.

With my '01, that's exactly what happened. One hub would unlock after the initial vacuum pulse. Turned out, it had broken the latch internally. Useless.

Went to Warns

 

I was thinking hubs, but they lockup and unlock at exactly the same time. The problem is that they don't stay locked. The solenoid remains engaged when selecting 4X4 and stays that way until changed back to 2 wheel drive. However, when selecting 4 wheel drive, the hubs both lock up and the solenoid engages giving me wheel drive. But after about 3 or 4 minutes, only the hubs unlock; both at the same time. They 4X4 solenoid remains engaged. So I think I'm dealing with a vacuum leak that is common to both hubs. If I select 2 wheel drive and right back to 4 wheel drive the vacuum pump starts up again and the hubs re-lock. Again only lasts a short time then the hubs unlock again. Thanks Willie I'll check out that TSB.

 

The vacuum solenoid should not be providing constant vacuum to the hubs. The solenoid should provide a vacuum pulse that locks the hubs when commanded by the computer. The hubs should remain locked at that point. They don't require constant vacuum to remain locked.

When you switch from 4x4 to 4x2, the vacuum solenoid provides a lower level vacuum pulse that should unlock the hubs.

I'd tend to suspect that, if your hubs are indeed locking, then you have a mechanical fault in the hub that is allowing them to unlock. The only other possibility is that for some reason the computer is commanding them unlocked when it shouldn't be, and that seems unlikely.

 

There is a 40-second-some-odd vacuum pulse to LOCK the hubs, and then a 20-second pulse to UNLOCK them.

If they are not staying locked, they are not latching. The system does NOT require constant vacuum to keep the hubs locked.

If they are BOTH unlocking (and verify that by reaching around back of the wheel and turning the u-joint by hand on BOTH sides) that sounds like that TSB that was mentioned above.

 

I think this makes sense. I didn't realize that the hubs actually latched. I thought that they receive a vacuum pulse to lock and then a lower level vacuum maintained to stay locked. I'll take another look at that TSB. I just find it odd that they are locking and coming unlocked by themselves pretty much at the same time. The hubs do definitely lock and unlock when switched as long as I do it within about two or three minutes. They unlock by themselves shortly after that. I was testing them with the truck on with a couple of helpers, one switching inside the cab and 2 of us rocking the wheels until they released.

 

Thats because the "4x4" dash light only has a sensor on the transfer case. The light does not indicate the status of the front hubs (if they are locked or unlocked).

 

I wonder... if you don't have enough vacuum, if they would latch? Maybe you do have a vacuum leak somewhere...

Put a vacuum gauge on the hub line and see what you get when you turn it from 4x2 to 4x4 and back.

 

If you go to the on-line manual, it has a great pin-point test procedure outlined. Very easy to follow if you read it. The link is posted on this site.

 

I realize that doesn't indicate anything with the hubs. Read the rest of my posts. I'm checking the hubs at the spindles. They lock after selecting 4X4, we checked by looking to see if the axle shafts would turn or not. The transfer case being locked was just one more thing that we checked. What I stated was that with the transfer case locks up when selecting 4X4, and the front wheel hubs both lock up at the same time. Then after 3 or 4 minutes they unlock at the hubs. Allowing the front wheels to turn freely, while the transfer case and front driveshaft remain locked.

I'm tempted to change that vacuum pump on spec. I'm suspicious of a vacuum leak near or at the pump. I hear a hiss under the hood near the pump at about the time when the hubs release.

 

When they release, the vacuum solenoid vents vacuum - so it'll hiss. I don't know where the vacuum pump is on a diesel, is it near the passenger-side fender? That's where the vacuum solenoid is.

Seriously, quick check, connect a vacuum gauge to one of the hub lines and see how much vacuum you're pulling. Autozone or whatever might loan one to you.

 

That's my plan, but I've got to wait until the weekend. Don't have any time right now. As for the pump, it's on the passenger side fender next to the battery on that side. Interesting the you mention the hiss. I get it when I select 2 wheel drive; but I also get it when that 3 or 4 minutes goes by and they release by themselves.

 

Update... Got a vacuum pump and tested my lines and hubs. I followed this procedure that I found online.
ESOF 4WD Troubleshooting
It is easier to troubleshoot the system if you have a “Mityvac” vacuum pump. Turn the ignition switch to run (you don’t have to start the truck) and the air controls to vent and the fan control to low. If the truck’s vacuum pump comes on wait until it stops running before each test (open the hood and you’ll hear it better). First you need to remove both lines (left and right vacuum lines) from the hubs. Plug the passenger’s side line and attach the vacuum pump to the driver’s side line. Switch the dash controller to 4WD high. The vacuum gauge should go to 10 in.-Hg. or higher. (It should hold the vacuum at 10 in.-Hg. or higher for 45 seconds on engage and 15 seconds on disengage.) Repeat the test plugging the drivers side and testing the passengers side. If you have the right amount of steady vacuum the problem is probably in the hubs. If you do not have the right amount of vacuum or it is bleeding off, plug the lines back on the hubs and go to the next step. Second remove the vacuum lines from the PVH (Pulse Vacuum Hublock). This is the solenoid located on the passenger side fender well behind the battery and in front of the vacuum pump. Plug the vacuum tester into the lower hose and wait for the vacuum pump to build back up and shut off. The vacuum gauge should read and hold steady at 10 in.-Hg. or more. If it does, the vacuum reservoir should be fine and not leaking, so go on to the next step. Third, plug the bottom line back to the bottom of the solenoid and hook the vacuum gauge to the top barb of the solenoid (not the vacuum line but to the solenoid itself). Try switching between 2WD and 4WD again. The gauge should read 10 in.-Hg. for 45 seconds on engage and for 15 seconds on disengage. If it doesn’t it is probably the solenoid (PVH) on the fender that needs replacing. This was my problem so this was as far as I have gone. If all these things check out, THEN you need to go to the hubs and check them for leaks with the vacuum tester. Most of the time it’s a fuse, PVH, vacuum line or vacuum pump and not the hubs.

The lines to my hubs held fine for more than 1/2 hr. Checked at the outlet line of the solenoid. So things start acting wrong at the vacuum solenoid. It seems to release the vacuum on its own. With the vacuum guage and pump on the inlet side. If I select 4X4, the hubs lock with 10 in/hg. Then the solenoid releases vacuum at about the 3 min mark without receiving a release command. (as in selecting back to 2H) The pump and resevoir both test and hold.
The only thing I don't know is what voltage sequence switches the vacuum solenoid to make sure it it receiving the right signals. Odds are it's probably the pump.