Let first start by saying that I've already done a search and read many threads on this issue so I'm not trying to beat a dead horse.

Long story short. I was out hunting a couple of weekends ago and needed 4WD, I flipped the switch and had nothing. Vehicle is a 2001 F250.

This weekend I started trying to diagnose the problem. I had to replace the passenger side hub because it was frozen, the selector dial would not budge. I hoped that was the issue but it wasn't. As a preventative measure, I also changed the rubber vacuum lines going to the steering knuckle on both sides.

I took the truck to an empty parking lot, put it in 4WD and had my wife drive it forward real slow while I walked outside and observed. What I noticed is that the transfer case is engaging because the driveshaft was turning. The other thing I noticed is that the driver side axle was turning, so that hub appears to be locking in fine.

However, the passenger side axle, the one that I replaced the auto hub on, was not turning.

My question is, since the driver side seems to be locking it doesn't appear that I have a vacuum issue because when I traced the rigid vacuum line down from the right inner engine compartment, I discovered that the line T's. If I had a vacuum leak, it would affect both sides and the driver side wouldn't be locking in.

What else should I be looking for on the passenger side? This weekend I planned on removing the bearing hub and axle and replacing the seals. Can't hurt.

 

First thing I would do is verify whether the hubs are locked or unlocked. They can get out of sync. The hub is like a pull-on pull-off switch. Pull it once it goes on, pull it again it goes off. Since you worked on one side, it is possible that the two hubs are not synchronized.

To verify manual hub function, put the ESOF switch in 2WD. Make sure the transfer case is unlocked by turning the front drive shaft. Then reach in and turn the wheel U-joint by hand. The engine can be off. You don't care about vacuum at this point. Both left and right U-joints should turn freely. If one of them is still locked, then turn the manual dial back and forth a couple of times, ending in the Auto position. If either hub is still locked, which you verify by trying to turn the U-joint, then you still have a stuck hub.

If all that checks out as okay, then you can move on to the vacuum diagnosis.

 

I was working on both sides so I know both hubs are in the auto position.

I think I've ruled out a vacuum leak in the lines since the drivers side locks in. I must either have an obstruction in the steering knuckle where the vacuum line enters, or I have a bad seal.

I'm assuming at this point unless someone else steers me in another direction.

 

I have had a hub that was stuck in the engaged position and would not unlock. So you could have a hub that is not working and one that will not unlock. Using the auto function make sure they unlock. Your driver side could be locked all the time.

 

The drivers side is locking and unlocking fine, I checked it and the hub on the passenger side is new.

 

Just to be sure, before you go any further.

Is the 4x4 light coming on on your dash?

If you put both of the hubs in the lock position, can you then switch to 4x4 and actually get 4x4?

 

The 4X4 light is coming on but I haven't tried locking both hubs in yet. I'll give it a try but I first have to remove my wheels because the damn center caps are closed. Kinda sucks in an emergency.

 

Ok, here's the latest. Last night I put the truck up on jack stands at all four corners. With the hubs in the "Auto" position I turned the switch to 4 Hi and the front wheels did not spin. The driveshaft and the axles were turning but that's it. Next, I shut the truck off and turned both hubs to the "Lock" position. I started the truck and flipped the switch to 4 Hi and bingo, all 4 wheels were turning. After removing the jack stands I proceeded to check all of the vacuum lines starting at the fenderwell next to the battery. I didn't notice any breaks or damage. Now my question is, what next? Tonight I'm going to pick up a vacuum gauge to check for vacuum at the steering knuckles. If I don't get any vacuum there, does that mean that the vacuum switch next to the battery is bad? Anything else I should check?

 

If you can, get a vacuum gauge/ hand pump. You can do what you plan, check for vacuum pulses at the hub. With the pump, you can manually pull a vacuum at different places and see if it holds.

Once you get the gauge, check vacuum at the pump, at the solenoid, etc. If you have good vacuum holding at the solenoid, and no vacuum pulses after it, then you have likely found your problem.

The issue is, though, that it is very common to have enough of a vacuum leak after the solenoid that it simply can't pull enough vacuum to activate the hubs. The leak can be in the lines, but if they are all looking good, it may be that it is in the hubs themselves.

 

Since I have a gas motor doesn't it just have a vacuum switch vice a vacuum pump. I thought just the diesels had a pump?

 

Yes, that's correct. I tend to think diesel, because that's what I have.

The hand pump/vacuum gauge is still a good diagnostic tool. You can, for example, hook the hand pump directly to the hub nipple with a short piece of hose and pull a vacuum on it. Not only should it lock, it should hold the vacuum pretty well. If it does those things, you've eliminated that hub as a problem, etc.

 

Well, after lots of troubleshooting I got the 4WD fixed on Saturday. I had to end up replacing the PVH solenoid.

I bought a vacuum gauge ($23 well spent) and with the old solenoid in place I checked the amount of vacuum at each steering knuckle and discovered that each was only getting 5.5 inHG of vacuum which fell below the required minimum.

After replacing the solenoid I rechecked the vacuum and bingo, problem discovered. With the new solenoid in place I am now getting 19 inHG of vacuum.

Once I got everything buttoned up I put the truck up on jack stands and gave her a try. I was quite thankful when I threw the swith to 4 Hi and all 4 wheels began to spin.

I'm so glad I didn't have to tear down each axle to replace seals. Been there, done that, don't feel like doing it again anytime soon.

I'm back in business.

 

Good to hear.