My 97 F150 Lariat has had intermediate 4x4 issues for a while. I pulled the shift motor and cleaned the plug and thought all was good. Now a few months later, the 4x4 seems to be completely dead.
Using the wiring diagram, I did the following.
Removed and tested the selector switch and even disassembled and cleaned the contacts. The resistance fits the diagram and I get a voltage drop in each position from 5 volts when I select from Off to 4WDH to 4WDL.
I removed and tested the transfer case relay and proved the coils were good and the contacts worked. (see picture below for location)
I then reinstalled the relay and took terminal 9 to ground and found the transfer case went into 4WD Low then took the terminal 8 to ground and it went back to 2WD. Not certain how to make it go into 4WD Hi by doing this. But this proved the relay worked and that the Shift Motor worked. And that the indicator lights worked.
I located but didn't pursue the Transfer Case Electric Clutch relay, since I don't think it would be in the way, but I might need to be corrected. See below for what I think is the location of that relay.

Any opinions? Can the GEM be tested? It looks like a pain to get to and remove. All other functions on the truck work, like horn, windows, locks, wipers. I think the GEM controls some of these as well. So I'm not certain the GEM is the problem.

Since finding the above relays was tough, I thought I'd share where they are on the 1997 and show how to access them easily. The dash parts was easy to remove, so if you're hanging upside down trying to access them, you're working too hard. Also each relay is clipped to a steel tongue, that you simply have figure where the lever is to slide them off.

 

Can't help but thanks for the photos!

 

If 4H and 4L come on but no front engagement, the issue is beyond where you were working.
The inner most vacuum solenoid on the firewall behind the battery may be stuck and not passing vacuum to the front axle actuator.
Or the line to the actuator or the actuator may be faulty.
Good luck.

 

The whole system acts dead. I figured I blew a fuse, but all were good and I back probed the plugs they went to and found power. So I took the Transfer Case coil ground side to ground with a probe and then heard the Shift Motor engage and the 4W Low light lit.

I wondering if an interlock like the speed sensor might have me locked out. I'm thinking if I can find it I'll pull its plug hoping that tells the GEM I'm at 0 speed and not over 55 mph.

I also want to revisit that Selector Switch and find a resistor of some value close and and jumper that into the switches plug and see if it works. Logic points toward that switch, but I can't prove it's failed.

 

The selector switch 'request' the GEM to operate dash relay. This powers the case motor to first position..
Speed sensor has no control over this for 4H.
The case motor closes signal leads to the firewall inner most solenoid to pass engine vacuum (as long as the motor is running) to the front axle actuator that links the driver side CV axle.
The solenoid completed circuit causes the GEM to light the 4H lamp. No lamp / no Gem signal to do so.
.
4L only;
The motor has to be running, shifter in neutral and foot on brake to engage 4L.
.
Initial diagnostics;
Setting with motor running, switch to 4H.
The 4x4 light should come on with 4H on and shifter in any position.
You should be able to hear all the action and dash relay click..
If not, the case motor is has not moved so you have an initial request failure.
Does the dash relay attempt to operate repeatedly?
If no, the GEM has not addressed it.
If yes the case motor has not operated. The relay clicks indicate the GEM has tried multiple times, then stops.
If no action when switch is operated the Gem is not acting, the **** on the switch is slipping etc.
Power is missing or there is a harness issue.
These major parts of the system operation is all you need to place the section where the failure is located.
The Case electric clutch has no part in all this except to 'sync' gears for a no grind engagement while the vehicle is moving, for 4H engagement on the run.
Good luck.
.

 

Blue Grass, I've read your response several times and from it I'm now thinking the position switches on the Transfer Case Motor may be the problem. For a while I've heard clicking from the Transfer Case Shift relay. Sometimes it would engage the 4WD sometimes it would just stop. After removing the Transfer Case Motor a few weeks ago and checking all the switch contacts and motor operation, I cleaned the plug and the problem seemed corrected.....except I still got some relay clicking, but the 4WD would engage. Then last week, no clicking, no engaging, no nothing, it seemed dead. So I checked all the fuses and that power was going to all the components. And proved the motor would work by taking one of the coils on the Transfer Case Shift Relay to ground and heard the relay click, the motor below running then the 4WD Low and 4x4 light lit. I verified by driving that the transfer case was in low. But doubt the hubs locked, since I bypassed most of the interlocks.

I think from what you said, the GEM needs the motors position to work the rest of the circuit, otherwise it won't send any outputs to the Shift Relay or Hub solinoids. Is that statement correct?

 

If you had the case motor out, you saw the circuit board with it's wiper contacts.
These are the contacts that operate the firewall solenoid enabling it to pass vacuum to the axle actuator and signal the GEM to light the dash lamp.
There are no front Hubs that work an in/out function.
There is a Sleeve in the carrier extension housing the Actuator slides to engage both axle halves at the same time.
It's all done with the left side axle lockup 'after' the case motor has engaged, driving the front drive shaft.
.
Be awhere that a condition could be present where the 4H or 4L light comes on but the front axle does not lock.
That can be the firewall solenoid stuck, it's vacuum lines, the Actuator or the axle hardware.
The GEM cannot see any fault past the firewall solenoid, only the solenoid 'winding' is part of the monitor circuit.
You get to involved in detail reporting.
You only need to see where the chain of events function stops and work on the issue at that point..
Good luck.

 

[QUOTE=You get to involved in detail reporting.
You only need to see where the chain of events function stops and work on the issue at that point..Good luck.[/QUOTE]
Blue Grass I see what you mean about too much detail. So let's start again. When I take the 4WD selector out of 2WD to 4WD High or Low I get nothing. No relay clicking, nothing.
So what should I check next?

I have verified the transfer case motor will run, when the interlocks are bypassed.

 

When you hear nothing, the GEM is not able to operate the dash relay to power the case motor.
That is the basic issue you have. Nothing else will work beyond that point.
Either the Gem has a supply fuse open.
The Gem is faulty.
The dash switch is faulty.
Don't look anywhere else..
.
The dash relays must have a power source in order for the GEM to supply a ground to them for operation and cut power through to the case motor.
The Gem must be able to work.
The switch must be able to request the GEM to act.
This is the bottom line.
.
To complete the action, if the case motor was operated to it's first position and you tuned back to 2wd, the GEM operates the 'second' dash relay to 'reverse voltage polarity' and runs the case motor backward to normal 2wd position. It does the same action going from 4L to 4H and back to 2wd plus switches vacuum direction to the front actuator to unlock the front axle.
It is a slick electro mechanical operation with built in timing and sequence of operation.
Good luck.

 

Bluegrass, I guess I over thought this one! I checked any fuse in the manual that had GEM by it, #'s 6,8, 15, 20. All of them were good, But afterwards I started the truck just for fun and now the 4WD functions normally. Not sure which of the fuses controls the 4x4 part. But I'm assuming the prongs were tarnished and not allowing power to the GEM.
Guess, I need to have a fuse prong cleaning party.
At least I hope I've found it. Thanks again for your help.
Josh

 

The biggest benefit is you learned how the system works.
This is what is required for anyone desiring to maintain and repair there own vehicle.
Know how systems work, then trouble shoot, then do the repairs.
Works every time.
The number of people who come to the boards for help with no background makes it very difficult to steer them through the learning curve and fix the trouble.
The biggest problem today is that nearly everyone has there nose on an IPAD of some sort every available moment.
They actually do not lean anything but just look for the quick answer to things.
It's like having a book and not knowing what is it.
When I go anywhere, all I see is people looking at these things.
In the stores, while they are supposed to be working, walking across the street.
In restaurants every member of the family has there noses in one. They hardly even talk to each other.
As you guessed it, I don't own one.
Good luck to you.

 

The 4x4 High and Low continue to work after cleaning any fuse that had GEM by it in the owners manual. But I did learn a bit more. It seems fuses 6 and 15 affect the 4x4 function. After removing them one at a time, my original problem would come back.

Each of the little 5 amp fuses had tarnish on the legs and I figure the GEM takes very low amperage, maybe even Milli Amps to operate, so I'm guessing the resistance caused by the tarnish was enough on either of the 2 fuses to cause the problem.

I truly went all around the barn to go into the door that was right in front of me. But not understanding the GEM's ringleader function really made it tough. Thankfully Bluegrass gave me the information to proceed in a more logical method.
Josh

 

how do you bypass the interlocks??on the transfer case???

 

There are no interlocks.
Where did you get that from?