Hey guys, on a recent trip I realized my 4X4 didn't engage with the hubs in 'auto' position. I had to lock them and then the dash switch would work and engage/dis-engage the 4X4( why was this the case)?? I know jack about 4X4 so I need a little help on how to figure out my problem. Does anyone have any ideas on where to start? I looked in the tech folder but couldn't find anything?!??!

Thanks in advance and as always rep will be given.

 

The dash switch causes an electronic motor on your transfercase to shift the transfercase in and out of 4hi, 4low, and 2wheel drive.

It also activates the vacuum motor and switches vacuum to the ESOF hubs which lets them engage when in the auto position.

So if you hubs are gummed up or you have a vacuum leak or malfunction your hubs will not lock in but your transfercase will still go into 4hi or 4low. This is why your 4 wheel drive worked once you manually locked in your hubs.

 

First question, is your 4x4 light coming on. If so, you don't have a transfer case shifting problem.

The vacuum system is actually pretty simple. On the passenger side firewall is an electric vacuum pump. There is a vacuum line coming off of it that goes over to a vacuum reservoir. Also connected to the reservoir is a T fitting with two lines from it. One line goes forward into the cab to the heater controls. The other line enters a loom and goes to the PVH (Pulse Vacuum Hublock) solenoid. This solenoid controls vacuum to the automatic hub locks.

The first tests I would do are pretty simple. With the key in the RUN position, unplug the line at the pump. The pump should start running immediately. Put your finger over the nipple and the pump should stop immediately. This verifies your pump is good.

Once you've done that, I'd hook the pump back up, leave the key in RUN and watch the system for awhile. Maybe 10 to 15 minutes. The pump should run long enough to pull a vacuum in the reservoir and all the lines and then shut off. If the system holds and the pump doesn't come back on, then you've essentially verified that the reservoir, the lines going forward to the heater controls, and the lines going to the PVH solenoid are all good and not leaking.

If the pump cycles back on regularly, then you have a leak somewhere in those lines or the reservoir and you need to start tracing them out to find it. Start at the reservoir, disconnect a line on the pump side, plug it and see that the pump pulls down a vacuum and holds it. Reconnect that line and move down to the next connection and do the same thing. Continue this procedure until you've traced down all the lines. If you get to a point where the pump continues to run, or cycles on and off, then you've found a leak in the last section of line you've tested.

If all of the lines up to the PVH solenoid test good, then you're left with the solenoid itself, the lines from the solenoid to the hubs, or the hub seals. This is where a good hand vacuum pump with a gauge comes in handy. You can attach the pump at the line going down to the hubs, pump up about 14 PSI of vacuum and see if it holds. If it does, then the lines and the hub seals are good. If it doesn't, then start working down towards the hubs, isolating lines to see if they hold vacuum. It is common to find cracked lines going down to the hubs this way. If you get all the way to the hubs, attach your pump to the vacuum nipple on the back of the hub, pump up about 14 PSI and see if it holds. If it doesn't, then that hubs seals are bad and need replacing.

If, in the end, all of your lines and hubs hold vacuum properly, you are pretty much left with the solenoid itself. Attach your vacuum gauge to the line after the solenoid and have someone switch to 4WD. The solenoid should apply about 14-15 PSI for up to 60 seconds. Then have them switch back to 2WD. The solenoid should apply about 7 PSI for about 60 seconds. If neither of these happens then your solenoid is likely bad.

You may want to check the center pin of the solenoid wiring connector for power. There should be battery power there with the key in RUN.

 

Here is a .PDF file from the service manual with a great deal of information on how the 4WD system works. It gives the amount of vacuum you should see for a lock pulse and an unlock pulse (less than I thought) and other information.

Also, should you find that your vacuum pump, lines and solenoid are all working and there are no leaks, you are left with gunked up hubs. There is a good hub cleaning and regreasing procedure here.

Welcome to guzzle's 4x4 Autolock Hub Lubrication Maintenance Web Page

P.S. I wonder if one of the mods would like to put this stuff in the tech folder. It seems like I'm typing something like it once or twice a week now.

 

Awesome thanks! Rep coming your way.

 

Yep I do get the light so I'll go though the rest of what you posted. Thanks for taking the time to do that, I really appreciate it and as usual you're a life saver. Rep coming your way.

 

Thanks, dchamberlain exactly what I was looking for. You guys are life savers!

 

I saw a physical hole in my vacuum line when I replaced the ball joints a while ago. Are the vacuum lines easy to run back to the solenoid? Do I buy the part from Ford, or is there a certain size / length of vacuum line that I can pick up?

 

Yes easy

Any auto parts store will have it