4x4 wont engage (yes i searched)

[94slowbra]

so to start off yes i have searched and couldnt find how to test and find out which part isnt working. my a/c and heater all work fine, both the 4x4 switched on and without. i can change where the air comes out with no problem. also i can hear my vacuum pump coming on. when i undo the two vac lines that run to the solenoid i can hear the air for a sec then it fades away. is the pump not pulling enough? with the line connected to the solenoid and in 4x4 i can undo the vac line that leads into the knuckle and i feel/hear nothing. i even tried bypassing the solenoid and got nothing coming out of the vac line by the knuckle. when hubs are locked im golden so transfer case, transfer case motor and drive shaft are all good.
how much vac is needed to engage the 4x4 and how can i test that?
is the vac pump and all or nothing thing or could i just not be pulling enough?
does bypassing the solenoid and still getting nothing eliminate it as the culprit?
any help would be appreciated. thanks

[ZMANN]

sounds like a vacuum issue

the solenoid does this

for 4x4 and back to 2x4 it sends two different siginals commanded from th PCM/ECM after 30 seconds no vacuum will be present the job has been done commands ,, the time and HG vary

any vac guage will work to test even your finger should suffice to see if it holds the tube to your skin

but have the truck running with someone in it to switch to 4 hi while you watch the gauge it should move to about 10 psi ,,


the fact that u state u went direct passing the solenoid and got nothing plus u don't hear the pump run when u pull that has me suspicious


the hubs themselves u can suck test to see if they hold psi

[smokeum99ta]

I'm pretty sure it's a vacuum leak. I had the same issue last week. I ended up going to a shop that hooked up a smoke machine to the vacuum lines going to the hubs and found smoke pouring out the inner seals on both sides. Replaced both seals and all is well. The smoke machine is a awesome source to trouble shoot vacuum lines. I'd try that first before buying random parts.

[94slowbra]

Originally Posted by smokeum99ta

I'm pretty sure it's a vacuum leak. I had the same issue last week. I ended up going to a shop that hooked up a smoke machine to the vacuum lines going to the hubs and found smoke pouring out the inner seals on both sides. Replaced both seals and all is well. The smoke machine is a awesome source to trouble shoot vacuum lines. I'd try that first before buying random parts.

again im open to suggestions but if it were the hubs i would have vac pull in the lines that i removed from the knuckles right? i have nothing there so i believe my problem lies before the hubs.

[ZMANN]

yes you would have checked for vac at the knuckles but like I mentioned that vacuum is only present for a brief time when commanded ( switch activated ) then it's gone
so you would have to be fast to check

this system is so simple you don't need a smoke machine

[94slowbra]

Originally Posted by ZMANN

sounds like a vacuum issue

the solenoid does this

for 4x4 and back to 2x4 it sends two different siginals commanded from th PCM/ECM after 30 seconds no vacuum will be present the job has been done commands ,, the time and HG vary
i found a pretty good file that explains hor the esof system works so now i have a much better understanding. you are right the system produces a high vac for a few seconds to activate the 4x4 and then its gone and produces a lower vac to return to 2wd. i thought it was a constant pull. this will have to be a 2 man job since its time sensitive
any vac guage will work to test even your finger should suffice to see if it holds the tube to your skin
i did this this am and it does hold to my finger. so know there is vac coming from the pump to the canister and to the appropraite lines. how much? not sure ill get a vac gauge this weekend to see. apparently to activate 4x4 it needs to pull about 10 psi or so.

but have the truck running with someone in it to switch to 4 hi while you watch the gauge it should move to about 10 psi ,,


the fact that u state u went direct passing the solenoid and got nothing plus u don't hear the pump run when u pull that has me suspicious
correction: i do hear the pump running. if i unplug the two lines that lead to the 4x4 solenoid i can hear the air pulling then after about 7-10 secs i cant hear it. then i plug it back and let it rebuild pressure and then i can hear it again if i remove it. i guess the pump has to rebuild pressure.


the hubs themselves u can suck test to see if they hold psi
i will buy a vac pump this weekend and see if they hold.

so far i believe my vac pump to be working and vac line to be good (when i switch to 4x4 i can still control my vents to come out where ever i want). i will test solenoid and go down from there (lines and hub seals). what a pain in the ***.

[94slowbra]

ok so i think i narrowed it down to the passenger side hub seals. i can pull 20 psi of vac and hold steady on the driver side at the knuckle but cant get past 5-7 on the passenger and its steadily drops.
few more questions:
can just one front tire engage in 4x4? i plugged the passenger side line and tried to engage 4x4 with all tires in the air and it didnt engage. drive shaft would spin but again no front tire spin.
what are the part numbers for the o-rings/seals i need to repair the front hubs?

[jwsland]

How easy do your hubs turn manually? My entire system has been checked and my only issue is my hubs are almost ceased. The 4x4 will not engage unless I get out and manually turn the hubs to "lock" with a pliers. Just haven't bought new hubs yet, don't even wanna know the price!

[94slowbra]

very easy. ive owned the truck for a little over 3 yrs now and have only used 4x4 a hand full of times, just this last time i noticed it wasnt working. i guess i could just get out and lock the hubs manually right now until i decide to replace the seals/o-rings. just aggravating i guess.
again if anyone has the part numbers for the seals i need to replace in the front that would be great. thanks

[dhanna6powerstroke]

Originally Posted by 94slowbra

very easy. ive owned the truck for a little over 3 yrs now and have only used 4x4 a hand full of times, just this last time i noticed it wasnt working. i guess i could just get out and lock the hubs manually right now until i decide to replace the seals/o-rings. just aggravating i guess.
again if anyone has the part numbers for the seals i need to replace in the front that would be great. thanks

Here ya go. I know it says 99-03 but it lists others in the link.
Ford 7.3 Diesel Drive Train, Transmission and Axle Parts