Well to start off some background info i suppose:
Vehicle: 2002 F350 Crew Cab Lariat 4x4 automatic w/ ESOF (Electronic Shift on the Fly)
Miles: 167k

Ok now to my problem...So just recently I've started having some problems with my 4wheel drive. I used it last weekend and it worked fine then on thursday the 4x4 would not engage (and light on dash would not come on). Neither 4high or 4Low would work, either by manually getting out of the truck and locking the hubs in or using the ESOF. I did some searching under the hood and found that a vacuum line coming from the Pulse Vacuum Hub Lock Solenoid had come apart at a connection point. I reconnected them and the 4x4 worked for about a day and then quit again. I checked that connection again and it was fine as were all others along the vacuum lines. I know that when there is a vacuum leak the heater will default to the defrost posistion. When I turn the ESOF switch to 4high or low the heater does not do this, so i guess no vacuum leak (please correct me if im wrong). Saturday I went into the gas station to get some food and left the truck running (with it in 2wd and hubs on auto ESOF switch on 2wd), when I came back out, 5 min later, the 4x4 light was on and the truck was in 4high (mind you the hubs were still on auto and the ESOF switch still set on 2wd). I switched the 4x4 switch on and off a few times before the light finally went out and the transfer case disengaged. This kind of led me to believe that my 4x4 switch could be going out...im not sure but before I go dumping any money into the truck i was just curious if anyone had any ideas what my actual problem could be??? Are there any tests I can run on other components to rule them out? Thanks for the help in advance guys!

Run Down of Problems:
1) <NOBR id=itxt_nobr_3_0 style="FONT-WEIGHT: normal; FONT-SIZE: 100%; COLOR: darkgreen">4x4</NOBR>high or low will not engage manually or by using ESOF switch (4x4 light does not come on)
2) 4x4 Sometimes switches itself on with the hubs in <NOBR id=itxt_nobr_4_0 style="FONT-WEIGHT: normal; FONT-SIZE: 100%; COLOR: darkgreen">auto</NOBR> and ESOF switch set at 2wd (4x4 light will come on)
3) Often when the 4x4 is engaged it will not disengage
4) All vacuum lines connected
5) No vacuum leaks (I think, because heater will not switch to its default, defrost posistion, when the ESOF switch is set on 4x4)
6) There's snow on the ground, the 4x4 wont work and I need a beer!! haha

any help would be great thanks again guys!
-Jim

 

On the back of the switch, the middle wire I think, you should have @ 6V in 2WD, 2.8V in 4H and something lower in 4L, @ 1V I think. I'm pulling these values from memory, maybe someone can check a manual and get them for sure.

Also, check your relays. Pull the 22 pin harness off the 4WD control module. Ground the Brown/White wire in, I think pin 10 (it's right next to the Orange/Blue) and you should hear the relays click, this switches the transfer case to 2H. Then ground Or/Bl and it should click and the transfer case should go to 2L. If this passes, you may have problems with either your 4WD control module or the gearmotor encoder assembly. Be sure to ground Br/Wh again to go back to 2H. You'll need a scanner for further testing if none of this does it. If you can find one and pull the GEM or 4WD codes, that'll help a lot in narowing it down.

 

There are also a couple of transfer case relays. Outlined in red, bottom right corner, of this Power Distribution wiring diagram for a 2002. Also notice just above that is a "Four-wheel drive control module". I am not sure but these relays may be mounted behind the radio. Just something else that may need checking. I have no idea where the 4wd control module is.

What baffles me is why it shifts to 4wd with the switch in 2wd. Could it be the 4wd control mudule?

Edit: Good stuff bcg. Just 3 things, 2002 has a VSM and not a GEM, 2002 VSMs have been known to have problems and where is the 4wd control module?

 

My '02 has a GEM. The 4wd control module is behind the glovebox. The relays are mounted against the firewall by the master cylinder.

 

Thanks for the info bcg. I like to learn about the later models. The ESOF relays on my E99 are here.

 

Great help guys, i'll have to check them out this evening. Thanks, i'll let you know how it turns out

 

After you trace out your electrical, do yourself a favor and upgrade to the Warn manual hubs and block of your vacum source going to them.

 

alright guys...this is what i've found out. I tested the voltage on the back of the switch and everything seemed pretty close to the values you gave me (i had 5.85V in 2wd, 2.58V in 4h, 1.00V in 4L). So i obviously have power to the back of the switch, leaving me to believe that the 4 wheel drive module is ok, am i wrong on this? I have yet to pull off the 22 pin harness, although i have looked for it behind the glove back, is it mounted on the firewall behind the glove box or in the upper right? Also my relays look a little different from your picture...I have two sets of relays by the master cylinder, both look identical unlike yours. Are the 4x4 relays the two nearest the master cylinder or the two nearest the wheel well and firewall...thanks again for the help guys. I'll try to get a pic of my relays on here soon...

 

Do you have any codes in the GEM or 4WD module? Correct voltage to the switch doesn't necessarily guarantee a good module. If you can get a code, it'll really help point you in the right direction faster but you're going to need a scanner, not a cheapo code reader.

The 4WD module is behind the glovebox to the right (looking forward). It'll have 2 plugs in it.

The 2 relays are the ones closest to the firewall by the master cylinder and they are identical.

Again, if you can get a code, I can get you a troubleshooting flowchart that will be a lot more helpful.

 

02s have a vsm......Ask me how I know....Mine went bad and had to replace it...Im not sure if the 4*4 is run by the vsm.I want to say no but Im not sure,Im wrong alot..LOL..BCG,are you sure you have a GEM and not a VSM?Pretty sure all 02s have VSM..

 

Well, the codes come up as GEM and I have to connect to the GEM system to look at those values, so...

I could be wrong also, though. I just know what the scanner is telling me.

 

alright, i'll see if i know anyone with a scanner and try to pull a code for you guys.

 

If you are in CO I'm down in the springs and have an AE. We could meet up and see if there are any codes..

 

Yeah, i live just north of thornton. This weather is hell gettin around right now too haha but i make it. give me a call or txt and we can set somethin up that'd be great. I appreciate it. (303)495-8269 [Jim]

 

When I was stuck in Co with my 4x4 out, I filled my bed with snow to help get better traction. I'm sure you know to do that but just thought I'd mention it anyway.