Greetings! I have a 1997 F250 - light duty - or medium duty not sure what is correct, I refer to it as a heavy half. 5.4 l engine, 3.73 limited slip, 4 wheel drive (ISOF), auto tranny w/OD, and ..... AIR SUSPENSION. I ordered this truck from Provo Ford and received it in March of 1997. It now has 220,000 miles,

Here's my problem. The 4 wheel drive system is not working. While out on a Forest Service road last October, I put it into 4wd high. When I returned to the paved highway, I turned the 4wd selector to 2 high. But, the 4x4 light on the dash did not go out. I could feel it wasn't in 4wd.

I took the truck to a 4wd drive shop and they diagnosed the problem as a bad GEM module. Ford doesn't make a replacement module so they started looking for one on the interweb. The first one he received was clearly wrong, the second one he got did not work. I retrieved it in late February with the original GEM module installed.

I took it to my local Ford dealer, and they declined to work on the truck. Their policy is they do not work on any vehicle over 20 years old. So much for standing behind your product, but I sort of get it. I took it to another shop, they fiddled with it, but they didn't really want to work on it either.

I brought it home and removed the GEM module. I found one on Evilbay that very closely matched the GEM I took out. I installed it. IT DOES NOT WORK. All other functions in the truck work - windows, doors, wipers, etc etc., but it doesn't do anything for the 4wd.

Question: Could a replacement GEM module need to be reprogrammed? I read that it needs to have the correct VIN programmed into it and the the tire size. Could this be it?

I also tested the 2wd and 4wd solenoids. I have do have input vacuum, both have power, but the 4wd drive solenoid seems to be the one getting the signal from the GEM to be in the 4wd mode. Also, the 4x4 light on the dash is on - with the new GEM installed.

Sure would appreciate some help, I've been messing with this since October of last year!

 

Well, I'm not convinced the GEM is your issue.

Your post says the light doesn't go out and you can tell it's not in 4wd....does it go into 4wd when commanded? I'm guessing not but want to clarify.

Watch the shift fork or better yet check vacuum at the lines on the front diff. Does it reverse for 2wd and 4wd?

 

Let me provide a few more details regarding the GEM module.

When the 4wd quit working, the original GEM module would click 6 times when the key / engine was turned on. When it first stopped working, I turned the switch from 4wd high to 2wd high and noticed the 4x4 light did not shut off. However, I did not feel the front wheels trying to grip when I turned onto pavement. After several days, the 4wd light did go off on its own. It was pretty clear to me the 4wd switch no longer worked, neither the light on the dash worked did not come on and there was no sound from the transfer case shifting (into low if selected). At that time I had never heard of a GEM let alone the role it played in the 4wd system.

Ok - when I got a new (used) GEM module and installed it, the first thing I noticed was that it did not click on startup. Nor did the 4wd drive light illuminate. Because I'm a glutton for punishment, I've switched the new GEM and reinstalled the original GEM, it clicked 6 times and the 4wd light stayed on.

I decided to switch back to the new GEM. Again, no clicking, but now the 4x4 light is on. The selector switch doesn't do a thing.

Today - to try to answer your questions -

No - the 4wd does not engage when commanded ( turning the selector ****).

I checked the vacuum lines to the solenoids. The constant vacuum to each solenoid seemed week, putting my finger over the end, I could barely feel vacuum. I checked further down toward the source - and there is ok vacuum. I checked lines and connections and replaced a dry rotted hose with some new vacuum line, I'm just not sure why it seems reduced going into the solenoids. I also removed the vacuum storage tanks behind the battery, pulled vacuum with a mityvac and both held vacuum.

I looked at the vacuum shift motor and fork assembly. Since I don't think the solenoids are getting the correct signal from the GEM and the vacuum seems questionable, I really don't think I'm getting proper vacuum to the shift motor. I did note the position of the fork and if I understand it's position, it was more in the 4wd drive position (compressed into the vacuum chamber). I may go back tomorrow and put my vacuum pump & gauge on it to see if it will hold vacuum.

I will look at the vacuum source tomorrow to see if there is vacuum problem / leak behind the engine.

 

I also am not convinced that the GEM is the issue - yet.🤑

I think it is the shift motor on the back of the transfer case that rotates a thingy inside the transfer case and when it contacts the contact, that is the GEM’s clue to turn on the 4x4 shift light.

Perhaps your shift motor is kinda sluggish and not rotating to make and break the contacts.
You could pull the shift motor out of the TC, and with it wired up, see if it rotates when someone cycles between 4 and 2 wheel drive.

If it doesn’t rotate on command, perhaps the selector switch on the dash is not providing the correct resistances so that the GEM can command the correct rotation.

 

P.S. Is that ISOF manual shift on the fly or Electronic shift on the fly?

 

If you didn’t get a code for a bad transfer case motor, what codes did you get?

And I didn’t realize that the F250 had vacuum front hubs.

Hopefully a real expert will come along shortly..........

 

Electronic (ESOF)

 

No codes - recently - but that may be due to having the battery disconnected everytime I've been under the dash. I do keep checking and if one pops up, I'll share it.

Jim

 

Ok - thanks for the replies. UPDATE - today's digging.

I vacuum checked the shift motor. I disconnected the vacuum lines and connected the mityVAC. The shift motor holds vacuum both ways and the fork moves smoothly through it's full range. I noted that the fork moved with about 5 or so mm/hg.

I tested the vacuum source at each solenoid - as I was unsure that I was getting good vacuum. The source to each was around 10 mm/hg, plenty vacuum. I checked the "output" vacuum on the 4wd drive solenoid (as the 4x4 light is still on and the 4wd solenoid is getting the signal to lock in the hubs). That vacuum was right round 10 mm/hg.

So - with the vacuum lines disconnected at the shift motor - engine running - vacuum line connected to the 4wd solenoid - knowing I've got good vacuum signal at the solenoid - I found I had 0, zero, nada, vacuum at the shift motor. Go figure. So, I concluded the vacuum lines form the solenoids to the shift motor are no good.

I started digging - and the hard lines are so brittle. Now I'm trying to figure out how to fix both lines. I got some tubing from the local NAPA - and will see what I can do with that tomorrow.

Still - operating the shift **** does nothing. But, I figure I'll sort out the front axle vacuum issues first, then move on.

I do want to look at the transfer case shift motor next. But, like I said, I don't get a code. Any ideas on how to test on the motor - pass them on.

Jim

 

I think we're getting closer to the problem.

Made new vacuum hard lines and now have 15 mm/hg to the front differential motor.

Today I tackled the transfer case shift motor. I found a video on youtube on how to bench test the motor. I found a couple of things. First - found the motor's sensing circuits where not all working correctly - i.e., had no continuity on one of the pins in the plug - between the pin and the common. Interesting test, tells a lot.

I tried to put 12 volts to the motor leads and nothing happened. Took things apart - and the motor armature was kinda stuck. Hmmm, two problems found. I got the motor to turn, and put it back in. Surprise, the 4x4 dash light finally went out! And, the since it went out, the 2wd vacuum solenoid started to provide vacuum to the front shift motor. Way cool. But, the 4wd switch still does nothing. I'm betting its the circuitry in the motor - so ---- I've ordered a new transfer case shift motor.

Stay tuned - maybe it isn't the GEM module - after all.

 

things happen in sequence... you turn the switch to 4 x 4 and the transfer case motor shifts... at the END of the stroke it hits a switch.... That tells the dash light to turn on... also signals the solenoids to send vacuum / on / off / to the front axle.. If you dont hit the SWITCH on the transfer case, the light never goes out, and you dont get vacuum at the front end.

 

Steve et al:

I'm starting to understand how the sequence works, you encapsulated it very well. I can't wait to get the new shift motor this week and get it installed. I'll be a true believer if the four wheel drive system works as it should.

One thing I've done is to go through the plastic vacuum lines and replace the ones that broke when I was digging around. Lot's of testing with the vacuum gage and I'm pretty sure all of that is working better than it had been.

I know I wouldn't have gotten this far without the Forum and Youtube. Too bad the "professional" shops couldn't have figured this out.

Thanks