4 Wheel Drive Help ;;; PLEASE
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New User
Joined: Mar 2008
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4 Wheel Drive Help ;;; PLEASE
Good Morning to ALL. I just got my 98 F-150 back from the garage . I had a Motor swap done. Ok, well last night, I needed to put her in 4wd and it did not connect. The Light indicator came on saying it was in 4wd, but needless to say, the back wheels were spinning in the snow, and the front wheels just sat there. I NEED HELP. Any direction as to where I can Start Looking? Fuse? maybe he forgot to connect something? I dont know and that is why I am here.. PLEASE HELP. Thank you ALL
Senior User
Joined: Feb 2008
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From: Colorado
Check the 4x4 activator on the front axle. Could have been unplugged accidently. Check the fuse at the power distribution box located under the hood on the driver side up towards the firewall. It is a rectangle looking black box. The fuse is one of the square Maxi fuses. Should be in the second row. It's a 30 amp fuse. There is also a fuse in the cab in the fuse box located by the steering column. It should be in the upper right corner of the panel. It's a 10 amp fuse.
New User
Joined: Jun 2007
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See if you have the model that has the 4WD "motor" (really a pneumatic diaphram). If so there are 2 pneumatic line coming from either side, one on each side. They route along the passenger side fender well, by the exhaust manifold (bad design, I know), to 2 pneumatic valves on the passenger side firewall. Check to see if all is intact/operational.
Lead Driver
Joined: Sep 2006
Posts: 7,907
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This is one of those times that not knowing how the total system works makes it hard to explain on a forum.
Here is the major sections you need to be awhere of.
1. the dash switch signals the GEM module.
2.The Gem operates the tranfer case motor, the relays that route vacuum to the front engage motor.
3. these actions report back to the Gem that lights the dash lamp when it sees all the signals.
A failure that can't be detected within the design is where you are at.
So you can see it will take running down just where it fails.
In doing the motor swap, something has been left unplugged/hooked up, assuming it worked before the work was done.
There are two electrical relays on the firewall in the battery area that have vacuum lines attached. Are they hooked up and to the correct units in the right way? The Gem can't tell at that point if there is a problem.
It's not as easy as just a fuse.
Good luck.
Here is the major sections you need to be awhere of.
1. the dash switch signals the GEM module.
2.The Gem operates the tranfer case motor, the relays that route vacuum to the front engage motor.
3. these actions report back to the Gem that lights the dash lamp when it sees all the signals.
A failure that can't be detected within the design is where you are at.
So you can see it will take running down just where it fails.
In doing the motor swap, something has been left unplugged/hooked up, assuming it worked before the work was done.
There are two electrical relays on the firewall in the battery area that have vacuum lines attached. Are they hooked up and to the correct units in the right way? The Gem can't tell at that point if there is a problem.
It's not as easy as just a fuse.
Good luck.
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New User
Joined: Mar 2008
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The 4wd worked very well before this. Never ever had a problem with it. Here is where i stand as of right now. Took the truck back to the mechanic. ( He seems clueless about the 4wd on this system ) anyhow, we traced the Vac lines, and have good vac pressure to the front axle. He didnt take off the cover to the activator. When i pull the shifter into 4wd, I hear 2 clicks under the dash with about a 3 second delay in between the 2 clicks. about 7 seconds later i get another click. The Light indicater in the instruement panel lights up when i put it in 4wd showing that the vehicle should be in 4wd, but it is not. Is there somewhere i can go to get a schematic of the vac lines and the gem module locations? As of right now, I am getting ready to go out and check the fuses ( as suggested above ) and pull the activator cover. Is it Possible the 2 electrical relays by the battery could be backwards?. I really appreciate the help. Thanks
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New User
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Ok, I think I got it.. Did all of the above, still can't find the GEM module, but never the less, took a crow bar, pushed the activator in manually, then jumped in the cab and put her in 4wd. Ok then I pulled it out. Backed up, pulled forward, put her in 4wd again, backed up again very slowly and she SLAMMED in pretty hard on the drivers side. The passenger side seemed to go right in this time. Went back and forth a several times, switching her in and out of 4wd. She seems to going good as of right now. I REALLY appreciate all of the help i received here. This is one of the best and informative sights I have visited. Thanks again: GO PITTSBURG HAHA HA HA HA
Lead Driver
Joined: Sep 2006
Posts: 7,907
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Normally if you listen close you can hear the front vacuum actuator 'clunk in' hard if everything is loose and not binding.
Good luck.
Good luck.
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New User
Joined: Jan 2009
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Added question to the thread.
I am having the same problem. I got good vac pressure going to the actuator on both lines, but when I put the truck in 4wd the truck will not engage. If I push the actuator in manually the 4WD will engage and work until I put it back into 2WD. It will properly disengage, but it will not re-engage until I crawl under the truck and do it all again. Is it possible for the vacuum servo to go bad? I've never seen one actually go bad, but I would like to get some kind of answer before spending 100+ on a new part.
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Yes it is possible for the actuator to be bad. Test it with a Mighty Vac hand pump to see if you can operate it both ways. Good luck! 