Hey guys. New to the forum. Pretty active in the lightning forums under another Username but first time on here. I have a daily 1998 5.4 Expedition EB 4x4. It has the four position selector for the 4x4 (2WD, A4WD, 4HIGH, and 4LOW). 2HIGH, 4HIGH, and 4LOW all work exactly as they should with no issues. The A4WD however doesn't. For it, the light comes on on the dash but no matter how I spin it, I get no front wheel assist. Its the weirdest thing. Any ideas? My assumptions are these... Apparently the shift selector is working properly because the light comes on and also because the case works in 4high and 4low which are further along the selection range than A4WD. Anyway, thanks in advance for any info.


Neil

 

The switch contact could be broken for the 2WD position only.
How do you know the 4WD is powering the front axle when in 4lo and 4hi?

 

I know the 4hi and 4lo are working because it binds on gravel and I've used each to get myself unstuck. That and the lights come on. If it wasn't working, i would probably still be stuck from last winter.

 

A damaged switch doesn't seem too likely as they're pretty reliable. When I check the FSM later today, I'll give you the specs for testing it.

A MUCH more likely suspect is the 4wd solenoids, specifically the AXLE DISENGAGE solenoid. and its lines. They are hard plastic and get brittle with time. The rubber boots crack and leak as well. I suspect this is why my Expy sometimes misbehaves in the same manner. Planning on redoing the lines with silicone hose, same stuff I used on my twin turbo RX7.

 

That'd be awesome. The switch isn't broken. I truly believe its either GEM related or something in the TCase.

 

I forgot to get the specs, but I know from memory that between the input terminal and output terminal, each of the 4 possible switch positions has a different amount of resistance so the GEM can uniquely identify each position and trigger the solenoids properly to engage/disengage where appropriate. A similar system is used on steering wheel audio buttons, I call it an Impedance Tree.

I don't know the figures offhand, but they made it so it's nearly impossible to confuse the GEM. Will get those tomorrow when I get a chance to. Pulling 12hr days at work is draining me, but the paycheck will be nice.

 

Nice. I see. Well, my GEM got wet at one point. Wondering if somehow that is causing some lingering problems. Grasping at straws really.

 

With the key ON, try manually triggering the disengage solenoid. Look at both solenoid connectors, you'll see one wire that's the same color on both. This is ignition switched 12v, ignore it for now. Go for the OTHER wire and ground it for a second or two. This bypasses the GEM and will force the front axle to disengage.

This is assuming that the vacuum lines and solenoids are not leaking from age. Again, silicone vacuum hose and proper routing/retention earns its weight in gold here, same way it did on my RX-7 which has just over FIFTY FEET OF VACUUM HOSE.

To get an idea of what it's like and why it drives people nuts diagnosing a failure, Google "FD RX-7 vacuum diagram".