Hi All,

My Dad's four wheel drive is not working....the transfer case seems to be working (High and Low) properly but the front end is not locking. I haven't really had a chance to look in detail of check my manual but I did notice a plug that is not connected to anything. It is hanging right around the front of the front driveshaft. It is orange/redish in color and looks like it is the female end of three prong connector. Any ideas of where this should terminate and if this may possibly be the cause would be appreciated. Thanks for all of your help.

jkacker

 

The 98 Expedition should still have the center axle disconnect in the front. Engage 4x4Hi with vehicle in park. Jack and support the front end so both wheels clear the ground. Try to turn the front drive shaft. It should not. Depending on answer will decide what diagnostic route to follow.

JMC

 

The transfer case is engaging but the front end is not....it turns freely. Your help is much appreciated.

 

By the front drive shaft I mean the shaft coming out of the transfer case and going to the front differential. The two shafts coming out of the front differential and going to the wheels are called half shafts. So, does the front drive shaft turn when the key is ON the transfer case in 4Hi, transmission in park and the front wheels off the ground?

JMC

 

Sorry, I should have been more specific.....with all four wheels on the ground I can not turn the front driveshaft. But the front end (half-shafts) are not locked in. The front wheels spin freely when in the air. It looks like the vacuum lines are hooked up to the front diff okay but I don't have a guage to actually measure the vacuum. If the vacuum tests okay what do I check next? Any suggestions? Thanks in advance.

 

The plug hanging near the front diff needs to be plugged into the front axle... there is a sensor there that tells the GEM that the center-axle disconnect has locked. It will not try to drive the front wheels if the CAD has not locked.

Now, once that switch is plugged back in, you'll want to try the test JMC mentioned, with the vehicle idling in park. That'll narrow it down to either a t-case issue or front diff issue, but I suspect that once plugged back in (if the switch is working) the system should work again.

-Joe

 

Everything is plugged back in (it was non-related)....the T-Case is working and engaging properly. The front end is not. There is vacuum to the actuator on the front diff and I can lock the front end in by manually moving the shift lever on the diff but it will not stay locked in. Therefore, I'm thinking it is the actuator/shift motor on the front diff. Any thoughts/comments are appreciated.

 

OK, now that we know that, you need to look at the vacuum solenoids that actuate it up on the firewall. The two solenoids are about an inch and a half around and 2 inches tall. The GEM grounds one or the other of them, never both, so you should only have vacuum coming out of one of tehm at a time, never both at the same time. What commonly happens is that one of them will stick, giving you vacuum on neither side or both sides of the CAD when you engage 4wd (depending on which one is stuck).

Troubleshooting is simple: Disconnect the vacuum lines coming out of them. With the truck idling, you should have vacuum on one of them. Shift to 4wd, and you should have vacuum on the other one. Switch back, and so should the vacuum. If one of them sticks, there's the problem.

Now, if they are working properly, you'll need to reconnect the solenoids and check for the same thing at the CAD on the axle end of the tubes. It's not uncommon for one of the tubes to crack or break. A small piece of tubing is all that's needed to fix it. Just slip the tube over the break with a pair of hose clamps and all should be well again.

Good luck! Keep us posted...

-Joe

 

The solenoid closest to the fender should have vacuum when 4x4 is engaged. It should be a blue line. They fade and can sort of look green too. The other line is red and can fade to pink but there is no way to mix them up. One last thing is that the solenoids will not work if the key is not at the ON position. need current to energize the relays.

JMC

 

I finally went out and bought a vacuum gauge......pulled the connections at the firewall/solenoids and did not have vacuum on either line. I'm assuming the lower connection is the vacuum feed and supplies vacuum at all times then the solenoids switch accordingly. Is that correct? If so, what should I check next? Could it be a bad check valve somewhere? Thanks in advance.

 

I'm having the same issues with my 97 but it only seems to happen after I have driven the truck some. If I try to put it in 4x4 when cold it does..............

 

If you don't have any vacuum into or out of the solenoids, you'll need to trace the vacuum lines back from there... I'm not sure where they feed from offhand.... I'll see what I can dig up from work tomorrow.

-Joe

 

I really apprciate your assistance.....I couldn't really tell where the feeds were coming from but didn't dig into it all that much. I wanted to do a bit more research first. Thanks again.

 

Just checked.... the schematics don't show squat beyond the sketch of the vacuum schematic. It appears that the vacuum to the 4x4 solenoids comes from a tee fed from the manifold. Unfortunately I can't help much beyond that... never looked at mine. Vacuum somehow has to get from the doenstream side of the intake manifold to the solenoids. I suspect it's right there near the back of the engine, somewhere on the upper intake, but I don't know for sure. Never looked that closely on the vacuum source for the solenoids.

-Joe

 

Thanks Joe.....a shop is trying to track down those feeds right now.

Anyone know exactly where the vacuum to the solenoids feed from?

Thanks in advance.