2001 Ford F150 5.4l 4x4 super crew
Lots of how to's around but they always seem to miss something

Most details start like this

1. The dash switch signals the GEM module located behind the dash fuse panel.
2. The Gem operates the transfer case motor to position.
3. The motor circuit board returns signals back to the GEM about it's position through the muti wire harness.
4. The firewall relays are operated sending engine vacuum to the vacuum actuator on the front axle to engage the front drive.
5. For 4L, the GEM needs to see signals from the brake switch and the neutral selector in neutral position while the vehicle is not moving. In addition it changes the transmission shift points in the PCM.
.BUT

What and How does the 4x4 relay module come into play
4WD Transfer Case Relay Control Module F75B-14B194-BC
This is NOT the GEM

My Issue is the 4x4 will try and switch and those replays will click , but it will not fully engage UNLESS
the dash is very hot from the sun beating on it, then it works great
So is the Transfer Case relay bad or the GEM
The Transfer relay is under the dash top center and subject to the heat more than the GEM

 

What, exactly, do you mean by "engage"? An electrical relay is just a solenoid operated electrical switch. It is either closed, circuit completed, or open, circuit incomplete. You can test it with a meter. Almost zero resistance when closed, infinite resistance when open.

If you're hearing a click that would be the solenoid moving the contacts of the switch in the relay. Seems like your problem is probably somewhere else.

 

What, exactly, do you mean by "engage"?
I mean it does not go into 4x4
I order to test it I need to remove the dash

 

So, the transfer case does not go in to 4x4? The 4x4 light doesn't light? Or the hubs don't lock and you get no drive from the front wheels? How did you end up looking at the electrical relay in the dash? You haven't really described what the problem is, except for something doesn't engage. Looks like you've assumed that the sun affects the relays but maybe it's affecting something else.

Here's a good video.

 

So, the transfer case does not go in to 4x4? NO
The 4x4 light doesn't light? NO
Hubs dont Lock.
Heat effects something most likely a loose solder joint
My posting was about no one ever talks about the transfer case relays, in this case I believe they are the issue

PS
Both of those vacuum transfer solenoids have been replaced

 

Well, we used to play with replacing the relay modules once in a while just to see basically what they did
The 4x4 module is what decides if you can shift it or not
It needs to know the TC position before it allows any shift
That is a failsafe so the truck does not hit 4 low at 50 mph and throw the occupants out
The 4x4 motor itself with the failing contacts is always the issue
That is why you will find little info on the relay module
They are never the fault has been my experience
Good luck
Replace the shift motor with a new ford one and the problem will be solved
I have used new Spicer ones and had no problems with them (cheaper)
I would not go aftermarket nor the contact plate repair

 

Remember the dash getting very hot is what allows it to work.
Turn on the AC for a while and it stops working
As the motor is under the vehicle, the action of the sun heating up the dash should not have an effect on the motor
unless I am missing something

 

If it really is a hot/cold problem it could be any electrical connection, including grounds. You need a wiring diagram and a meter.

 

Transfer Case Relay Control Module
Pulled Dash cap
replaced module
BOOM
Problem resolved

 

Good work. I thought that you were assuming bad relays, like you said above.

Here's an old post with a similar problem.

https://www.ford-trucks.com/forums/8...-location.html